Download The Pleistocene and the Origins of Human Culture

The Pleistocene and the Origins of Human Culture:
Built for Speed
Peter J. Richerson
Department of Environmental Science and Policy
University of California
Davis, California USA 95616
[email protected]
Robert Boyd
Department of Anthropology
University of California
Los Angeles, California USA 90024
[email protected]
Abstract. A number of authors have advanced the argument that the onset Pleistocene
climate fluctuation is responsible for the evolution of human anatomy and cognition. This
hypothesis is in contrast to the common idea that humans represent a revolutionary
breakthrough rather than a conventional adaptation to a particular ecological niche. The
Pleistocene hypothesis is, as proposed, not wen specified. How did Pleistocene
fluctuations specifically favor the particular adaptations that characterize humans? The
revolutionary breakthrough hypothesis is similarly weak. Our large brain and all it can do
does seem responsible for our present dominance of the earth. If so, what has prevented
many animal lineages in the remote past from evolving large brains? Theoretical models
of the cultural evolutionary process suggest some answers to these questions. Learning,
including social learning, is rather generally a useful adaptation in variable environments.
The progressive brain enlargement in many mammalian lineages during the last few
million years suggests that climatic deterioration has had the general effect predicted by
the Pleistocene hypothesis. Increased dependence on simple social learning was a
preadaptation to the evolution of a capacity for complex traditions. The evolution of a
costly capacity to acquire complex traditions is inhibited because, initially, complex
traditions will be rare. Having the capacity to learn things that are far too complex to
invent for oneself is not useful until traditions are common, but traditions cannot become
common before the capacity to acquire them is common. This problem may explain why
many animals became more sophisticated learners in the Pleistocene, but why only the
human lineage found its way around the not-favored-when-rare barrier, and thus evolved
the capacity for complex cultural traditions.
Version 1.1. February, 1998. For presentation at 5th Biannual Symposium on the Science
of Behavior: Behavior, Evolution, and Culture. February 23, 1998, University of
Guadalajara, Mexico. Please do not cite without permission. Comments welcome!
Introduction
The evolution of humans is a major event in the Earth's biotic history. This statement is
not just anthropocentric hubris. Never before has a single species of organism dominated
the planet to the extent that we do. Even before the development of food plant
production, hunting and gathering peoples penetrated to every continent except
Antarctica. Most accounts of human origins take the adaptive superiority of our species
for granted. If so, the interesting question is what pre-adaptive breakthrough that led to
the transcendent, fully human, hyper-adaptive complex of characters-tool making,
language, complex social organization, and the like. According to Landau (1984)
accounts of human origins read like heroic fables. The identical title, The Ascent of Man,
that Bronowski (1973) gave his popular account and Pilbeam (1972) his textbook
illustrate the pattern. This form is a consequence of the assumption of transcendent
adaptive superiority.
More recently, following Alexander (1974) and Wilson (1975), human sociobiologists
have pursued a different line of research. They assume that humans, in the words of
Foley (1987), are just "another unique species." In the theory of evolution by natural
selection, the driving force of change is adaptation to local environments. These scholars
are suspicious of taking human adaptive superiority for granted or of granting human
culture and its products special status. Rather than being a history of a breakthrough to a
new adaptive plane, human evolution is more likely a history of adaptation to local
environments that happens to have resulted in our current ecological dominance by
accident.
The deterioration of the Earth's climate since the Miocene, leading to the Pleistocene ice
age during the last 2 million years, is a major event in the history of the planet's physical
environment. Over the last 6 million years, the climate has gotten colder, drier, and more
variable. Geology records several other glacial episodes, but the last was the PermoCarboniferous ice ages that ended 250 million years ago (Lamb, 1977:296). Perhaps the
ongoing evolution of the hominid lineage was driven by the ongoing changes in the
Earth's climate. DeMenocal (1996) and Potts (1996) give impressive evidence in favor of
this hypothesis.
In this paper, we argue that the specific mechanism by which humans mastered the
Pleistocene is our capacity to evolve adaptations to the variation of Plio-Pleistocene
environments via cultural traditions. The results of theoretical modeling of cultural
evolution suggest that the basic adaptive function of social learning is an enhanced ability
to respond to temporal and spatial variations in the environment. Many animal lineages
seem to have taken advantage of the potential of social learning. In many respects, human
culture is nothing more than a straight-forward adaptation to climatic deterioration.
Humans do differ from proto-cultural animals in having the ability to evolve complex,
multi-part traditions that must evolve cumulatively, normally over many generations.
Technology and social organization furnish many examples of complex traditions that
evolve by descent with modification like complex organic adaptations, albeit at a faster
rate. Other animals show non-existent to marginal abilities to acquire complex traditions.
Whether complex traditions truly represent a breakthrough to a qualitatively superior
system of adaptation or only an especially clever mechanism to adapt to the Pleistocene is
a moot question.
Plio-Pleistocene Climate Deterioration
In the quarter-century since Shackelton and Opdyke (1973) published the first detailed
ocean sediment core spanning the entire Pleistocene, our understanding of this episode
has deepened remarkably. Using a variety of proxy measures of past temperature,
rainfall, ice volume, and the like, mostly from cores of ocean sediments, lake sediments,
and ice caps, paleoclimatologists have constructed a stunning picture of climate
deterioration. Lamb (1977), Schneider and Londer (I 984), and Dawson (1992) give
accessible reviews of the methods used.
Since the mid-Miocene, about 14 million years ago, the Earth's temperature has dropped
several degrees. The causes of the drop are not well understood, but are probably the
result of basic geological processes (Partridge, et al., 1995). The arrangement of the
continents is probably important. Continental drift has produced a situation with
Antarctica in the South Polar region, insulated from warm ocean currents by the circumAntarctic currents. Similarly, the Arctic Ocean is closely surrounded by land masses,
insulating it from the penetration of warm ocean currents to the north polar region. Ice
cover at high latitudes reflects much sunlight back to space, significantly lowering the
Earth's total heat income. Slowing sea-floor spreading rates over the whole Tertiary
period may have lowered the output of CO2 from the Earth's interior. A lower
concentration of this important greenhouse gas in the atmosphere win result in a lower
global temperature.
The amplitude of fluctuations in rainfall and temperature increased as mean temperature
dropped. Figure 1 from Opdyke (1995) shows schematically how the envelope of
temperature fluctuations has increased step-wise since the Miocene, as measured in
Deep-Sea Drilling Project cores. As time series analysts say, the climate record of the
past few million years is highly non-stationary. Not only does the climate vary, but the
statistics that describe the variation-the variance and patterns of autocorrelation--change
with time.
The pattern of fluctuation in climate is very complex. Much of the variation seems to
arise from an enhanced sensitivity to radiation changes caused by periodic variations in
the Earth's orbit. The radiation income in high northern latitudes has a 20% range of
variation due to these effects. Milankovitch (1941) developed this hypothesis in its
modem form. Broecker and Denton (1990) give a good introductory discussion to this
and other aspects of the physics of climate variation. The eccentricity of the Earth's orbit
varies on a 95,800 year time scale, the inclination of its axis with a periodicity of 41,000
years, and the precession of the equinoxes with a periodicity of 21,700 years. The
magnitude of the direct forcing of climate by these cycles is out of phase in the Northern
and Southern Hemispheres, yet the cycles of ice growth and decay are in phase. The
Milankovitch cycles reach far back into the earth's history, but the ice age began in
earnest only two million years ago. As the deterioration has proceeded, different cycles
have dominated the pattern. The 21,700 year cycle dominated the early part of the period,
the 41,000 year cycle between about 3 and 1 million years ago, and the 95,800 year cycle
from 1 million years ago to the present (de Menocal and Bloemendal, 1995). Thus,
complexities of the response of the atmosphere-ocean-ice sheet system must somehow
amplify and coordinate the effects of the orbital periodicities. Alternatively, the
Milankovitch theory may be incorrect (Broecker, 1992; Brownlee, 1995). Most likely, the
present disposition of the continents and oceans affects ocean currents and wind patterns
in such a way as to make the global climate very sensitive to small fluctuations in
insolation. For example, the Arctic Ocean is sufficiently isolated from the warm Atlantic
and Pacific oceans that it is frozen. Its ice cover reflects the sunlight falling on it,
substantially preventing it from storing heat in summer. This in turn means that the high
Northern latitudes can build ice sheets, which reflect still more sunlight, leading to other
chilling. The small forcing from the Milankovitch cycles in high northern latitudes can
thus be amplified by the growth and wasting of continental ice in the north.
The exact driving mechanisms of the late Cenozoic climate system are still unknown. The
most influential hypothesis is that of Broecker et al. (1985). The ocean-atmosphere
coupling includes elements that are affected by the deep circulation of the ocean. Under
the current climatic regime, the North Atlantic near Greenland is the source of much of
the bottom water for the World Ocean. A subsurface current of warm, rather saline, water
moves north in the Atlantic and upwells during the winter near Greenland. This water
then loses an immense amount of heat to the atmosphere, becomes heavier, and sinks to
become the North Atlantic Deep Water. NADW eventually flows out the South Atlantic
to the Indian and Pacific Oceans where it reaches the surface in upwelling zones. Thus,
there is a great ocean conveyor concentrating immense amounts of heat in the North
Atlantic and moderating the climate of the surrounding land masses, especially NW
Eurasia. (When the poles were not occupied by or surrounded by continents, similar
conveyors presumably kept the poles ice free.) Among other things, the conveyor
circulation efficiently draws greenhouse gases, especially CO2, into the ocean abyss. This
circulation apparently shuts down during glacial conditions, probably because glacial
melt water flowing from the Eastern North American ice sheet makes the ocean surface
water too fresh and hence too light to sink even when chilled to maximum density. Once
in a glacial mode, the growth of reflective ice sheets and the dust from the dry climates of
the glacial period further chill the earth. Modeling work indicates that at least parts of
Broecker's scenario are plausible (Manabe and Stouffer, 1995).
For the last 120,000 years, data is available from ice cores taken from the deep ice sheets
of Greenland and Antarctica with time resolution as high as few decades. During the last
glacial (65,000-12,000 years before present, Emiliani oxygen isotope stages 2-4), the
climate was highly variable on time scales of centuries to millennia (GRIP, 1993;
Lehman, 1993; Ditlevsen, et al., 1996), as can be seen in figure 2. Even when the climate
is in the grip of the ice, there were brief excursions of about a thousand years duration,
called Dansgaard-Oeschger events, in which the climate briefly reached interglacial or
near interglacial warmth. The Dansgaard-Oeschger events themselves come in clumps in
which the most extreme warm spikes are just preceded by the appearance of ice-rafted
debris in North Atlantic sediments, called Heinrich events. The most highly touted
hypothesis to explain the Dansgaard-Oeschger-Heinrich excursions is the cyclic
production of glacial melt waters that affect the NADW conveyor, wind patterns over the
North Atlantic, and other major regulators of global climate (Lehman, 1993).
The last Interglacial (65,000-130,000 years before present, oxygen isotope stage 5) may
also have been highly variable on the millennial time scale. Interpretation of the deeper
portions of the Greenland ice cores is controversial because of the possibility that ice
from colder and warmer periods has been folded by ice movement to create false
fluctuations (Grootes, et al., 1993). Nevertheless, many lower-resolution records of the
last interglacial also suggest that it was frequently punctuated by episodes of near-glacial
cold (e.g. Lamb 1977: 333). As figure 2 shows, the Holocene interglacial period of the
last I 0,000 years (oxygen isotope stage 1) has been extraordinarily tranquil. The
resolution of the ice core data is extraordinary. Near the top of the GRIP core the
resolution to seasons is possible, and at 0 1,000 years before present resolution is
estimated to by 1O years (Ditlevsen, et al. 1996). High resolution data do not yet exist
prior to the last Interglacial to indicate just how unusual the Holocene is compared to the
whole Pleistocene record. The possibility that Holocene climate stability might be easily
tipped into a regime of much greater variability by relatively weak forcing is cause for
considerable worry regarding human caused increases in carbon dioxide and other
greenhouse gasses (Broecker, 1997).
High latitudes chilled much more than the equator as the climate cooled during the latter
part of the Cenozoic. New habitats arose, such as periglacial tundras. Other habitats,
especially arid and semiarid deserts and grasslands replaced forests over large areas from
equatorial to temperate latitudes. The spatial as well as the temporal variability of the
earth thus increased considerably as the Plio-Pleistocene deterioration advanced.
Culture As An Adaptation To Variable Environments
Simple Models of Social Learning
It seems intuitively as if social learning should be especially useful as a means of coping
with spatially or temporally varying environments. Social learning, Eke ordinary
individual learning, is a mechanism of phenotypic adaptation. Phenotypic adaptations, by
acquiring some information about local environmental conditions, allow organisms to
adjust their anatomy and behavior contingently in situations where environmental
conditions vary on too small a scale to adapt by genetic adaptations. Presumably, systems
of phenotypic adaptation have costs. In the case of learning, an individual will have to
expend time and energy in learning, incur some risks in trials that may be associated with
large errors, and support the neurological machinery necessary to learn. Social learning
can economize on the trial and error part of learning; if kids learn from mom, they can
avoid repeating her mistakes. Learning requires sensory organs to obtain information
from the environment, some sort of decision-making system to evaluate the outcome of
trials, and a memory system to store the results. Social learning may also economize on
evaluation costs; what worked for Mom will probably work for her kids. At least, she,,
because of inclusive fitness considerations, has only limited reasons to deliberately
deceive them.
Sophisticated learning systems will require larger sensory and nervous systems.
Considering only the energetic costs of maintaining the nervous tissue, the incremental
costs are quite large (Eisenberg, 1981: 235-6). Martin (1981) reports that mammalian
brains vary over about a 25-fold range, controlling for body size. Human brains are about
5 times as large as the brains of average mammals of our body weight. Modem ungulates
and carnivores have average brains. Average living mammals in turn have brains about 5
times as large as those of the smallest brained mammals, such as insectivores and many
marsupials. Aiello and Wheeler (1995) report that human brains account for 16% of our
basal metabolism. Thus average mammals will have to allocate only about 3% of basal
metabolism to their brains, and some get by with less than 1%. Total metabolism runs
about 1.8 times that of resting metabolism, mostly because of the mobilization of large
masses of otherwise low-metabolic-rate skeletal muscle during exercise. How nervous
system metabolic rate varies with "exercise" is poorly understood. Even disregarding
mental exercise, humans must expend something like 9% of their total metabolism on
their brain versus a little more than 1% for average animals and well under one for the
least smart mammals. These differences are large enough to generate significant
evolutionary tradeoffs. Aiello and Wheeler hypothesize that one major tradeoff in the
human case is that we sacrificed gastrointestinal tissue, which is a little more energyintensive even than nervous tissue. This sacrifice, in turn requires the human diet to focus
on higher quality foods to compensate for inefficient assimilation. Note that we have left
out of this calculation other significant costs of big brains such as increased difficulty at
birth, greater vulnerability to head trauma, and the time and trouble necessary to fill it
with usable information. The overhead cost of our large brain must be on the order of s =
-.2 (thinking here of the common expression of fitness differentials in terms of departures
from unity for a reference fitness, here a mammal with an average encephalization).
Selection differentials of under 1% can drive evolution right along, so brains will not
become large without correspondingly large adaptive payoffs. How do humans foot our
large-brain overhead bill? We are still a long ways from being able to construct a
complete analysis of these payoffs.
Economists and population biologists common study simple, heuristic mathematical
models to school their intuitions about complex processes. In this spirit, we have
constructed very basic models of the individual/social learning process along the
following fines: Suppose that the individual learning process is the primitive state.
Virtually all animals show at least rudimentary abilities to learn. When animals like birds
and mammals come to have extended maternal care, they have the opportunity to learn
socially. In our models, we assume that individuals have two sources of information, their
own experience and the vicarious experience of individuals with whom they are in social
contact. In two different kinds of models, one based on quantitative characters (Boyd and
Richerson, 1985: Ch. 4), and another based on discrete characters (Boyd and Richerson,
Boyd and Richerson, 1989), we asked how evolution might optimize the relative
dependence on the two sources of information.
In the discrete character model, we assumed that there were two behaviors (e.g. forage
collectively or solitarily) and two environments, (e.g. wet and dry). There is a fitness
benefit for behaving correctly (forage collectively if environment dry, forage alone if
wet). Individuals collect some more or less error-prone information about the state of the
environment by individual learning. They can also opt to imitate another individual. The
degree of dependence on social versus individual learning was controlled by a
confidence-interval-like learning threshold, d, that we assume would be set by selection
at an optimal point. While growing up individuals gain some idea of the state of the
environment. This information will not lie on average, but in a noisy environment, even if
the environment is dry on average, some individuals will experience an unusual run of
rainy years. They are vulnerable to mistakenly deciding that the state of the environment
is wet. The confidence parameter d tells individuals how seriously to take their noisy
samples. If d is large, individuals look for definitive evidence that the environment really
is in the wet or dry state, say entirely quite wet or quite dry during their formative years.
If they do not see such evidence, and most will not, they imitate someone, say Mom, at
random. Figure 3 illustrates graphically how the decision rule works.
If d is small enough, any information from personal experience is deemed definitive, and
reamers go with the main chance as dictated by their personal experience. In a spatially or
temporally varying world, some mixture learning and social learning are generally
advantageous. In a noisy world, an evolving population is tending to integrate the
experiences of many individuals. One can be saved from the perils of small number
statistics by trusting a sample of the population, even a sample of 1, over the noisy data
from the environment. On the other hand, in a variable environment, the individuals one
might learn from: (1) may have gotten caught in an environment switch, (2) might have
migrated from nearby environment in the other state, (3) might have unluckily gotten
seemingly definitive information that the environment is in the state that it is not. If your
personal experience is pretty definitive, it is liable to be the better guess. The optimal
confidence rule (value of d) depends upon the nature of the environmental variation and
the quality of the evidence available from personal experience. If the environment
fluctuates too rapidly in time, or if the spatial habitat mosaic is so tight that individuals
often migrate to a patch different from their parents, individuals should depend entirely
on their own experience. In such a world, mom's advice is useless, and there is nothing
but to trust to the main chance of personal experience. As the statistical resemblance
between parental generation and offspring generation increases, it is safer to depend upon
imitation and demand ever more definitive personal evidence before breaking with
tradition. In a world where the environment hardly ever really changes, but which is
rather noisy, a combination of natural selection and rather conservative reliance on own
experience will result in a population in which most individuals are doing the right thing.
Social learning becomes quite trustworthy. Figure 4 shows how the relative dependence
on social versus own experience should evolve in different environmental situations.
The quantitative character model is similar in spirit. We imagined one continuous
character (frequency of foraging alone versus cooperatively) and a continuum of
environments from wet to dry. In any given environment at any one point in time or
space, there is an optimal mix of solitary and cooperative foraging. Bayesian
considerations suggest that individuals should use a weighted average of social learning
and own experience to determine how to behave. The optimal weighting parameter in this
model behaves qualitatively just as d does in the discrete character model. It is
comforting that two models with a rather different structure give the same results.
We used the quantitative model to run a sort of mathematical tournament comparing the
fitness advantages of using a genes-plus-individual learning to a system of social learning
plus individual learning. Suppose that there is some cost to being able to learn socially.
Under what circumstances might there be a fitness advantage to adding social learning to
a repertoire where genes represent the wisdom of evolutionary history and individual
learning bears the sole weight of running up the phenotypic fine tuning? A typical
example of the results is shown in figure 5. The social learning system is a potential
advantage over a wide range of conditions. The advantage of social learning is especially
large when the environmental variance (VH) is high and the degree of autocorrelation is
high. Under the parameter values we chose for illustration at least, the dependence on
social learning is often fairly high, on the order an 75% dependence on social models and
a 25% dependence on individual learning. At very high autocorrelations, environments
become so slowly changing that genes can track perfectly well, and the advantage of
social learning disappears.
Think of it like this: Social learning is a form of Lamarckian inheritance. It couples a
mechanism of phenotypic flexibility, learning, to a scheme for transmitting the results of
past learning via social learning. A system of social learning evolves in response to the
Lamarckian pressure of learning as well as the pressure of natural selection. We can the
Lamarckian force in these models "guided variation" as it acts a lot like an adaptively
nonrandom form of mutation. Guided variation causes a population's behavior to track
environmental change in time or space more quickly than genes, lacking the Lamarckian
property, can. These results nicely support the intuitive argument. Without any further
argument, you can see the temptation to attribute the evolution of cultural systems to the
onset of Plio-Pleistocene climatic deterioration. The pattern of increasing environmental
variation, but variation organized into autocorrelated patterns on time scales from a few
generations to many, is just the thing to give social learning an adaptive advantage.
Alan Rogers (1989) threw a dash of skepticism onto this argument. In his model, a
population composed of two types, social learners and individual learners. The
population lives in a variable environment whose state can be in either one of two
regimes (e.g. wet or dry). Social learning short-cuts the costly trials of individual
learning, and hence in is potentially an adaptive advantage compared to individual
learning. Indeed, when social learners are rare they do have an advantage. They
practically always copy an individual learner, getting the same, usually adaptive,
behavior at lower cost. The trouble is, in Rogers' model, they keep on increasing in
frequency until the chances of a social learner copying another social learner becomes
high. As the chain connecting copiers to reamers increases, there is an increasing chance
that the environment has changed and that the social learner will copy the wrong trait.
The only equilibrium in the system is when the fitness of social reamers falls to the same
level as reamers. At equilibrium the mean fitness of the population is exactly the same as
a population of individual reamers. Through a sort of parasitic system the potential
benefits of social learning are dissipated by excessive copying. This effect is robust with
respect to several generalizations of the model (Boyd and Richerson, 1995).
The reason that in our models social learning does increase the mean fitness of
populations in autocorrelated variable environments is that our individuals capture the
benefits of both types of learning, whereas Rogers' individuals capture only one or the
other. Nevertheless there is an element of the Rogers result in our models too. In an
unpublished study, we replicated a classic argument from patent economics. In a situation
where the results of a valuable innovation can diffuse to a larger population, the total
benefits of the innovation can greatly exceed the private benefits to the innovator. In our
models, individuals take risks when they learn, but the costs born of this risk-taking help
keep the population current with a changing environment. The optimal amount of
individual learning is higher from the group than from the individual point of view.
Absent group selection, the Rogers effect will "under supply" individual learning,
perhaps often to the point where little if any adaptive advantage in the sense of increasing
the mean fitness of the population.
We have studied a number of other models in which the rules of social learning are more
sophisticated than the copying of a random member of the population like Mom (Boyd
and Richerson, 1985: chs. 5-7). For example, a socially learning individual might use
several adults as models. If they exhibit two or more behaviors, the social learner might
try each out and retain the one with the highest rewards. Most behaviors current in a
population are probably better than the trials that individual reamers can attempt on their
own. Plagiarism is easier than originality. Gathering a number of plausible initial guesses
about the right behavior and using one's own experience to choose the best among them
has advantages similar to the guided variation process discussed above. We call the series
of forces on cultural evolution that result from non-random social learning "biased
transmission."
Another interesting pattern in human cultural evolution is our tendency to evolve
boundaries between cultures that have the effect of subdividing human populations into
semi-isolated sub-populations. Ethnic groups are a common example. Often, such groups
are specialized to exploit particular habitats or economic roles (Barth, 1969). Models
show that cultural badges-different language, dress, religious practice-can evolve to erect
barriers to the free flow of ideas in spatially heterogeneous environments (Boyd and
Richerson, 1987). Ethnic groups thus form the cultural analogs of reproductively isolated
species. The main difference is that the barriers are much more permeable and the rate of
evolution of culture is much higher than that of genes. Human cultural niche shifting is
faster than that of animals that adapt mainly by organic evolution. Using this
"psuedospeciation7' mechanism, Late Pleistocene humans developed such a diverse array
of subsistence economies that our species spread to the ends of the habitable earth
(Bettinger, 1991:203-5). Humans may not be the only species that uses this mechanism.
Many birds learn their songs by imitating adults, creating local song traditions. Females
may prefer to mate with males that sing the songs their fathers sang. Nottebohm (I 975)
argued that these dialects subdivide local populations and allow the frequency of locally
adapted genes to increase by restricting gene flow between groups.
Social Learning A Response to Variability Selection
Richard Potts (1996: 231-238) argues that the fluctuating climates of the Plio-Pleistocene
have imposed a regime of "variability selection7' on the Earth's biota. He suggests (p. 23
7) that under variability selection "genetic variations favor open programs of behavior
that vary and extend the adaptive possibilities of the individual. These are conserved in
the gene pool over time because of the inconsistency in the short-term effects of natural
selection. Organisms eventually build up an inheritance system that enables them to
buffer larger and larger disturbances in the factors governing survival and successful
reproduction." This is an interesting supposition, though lacking in detail about
mechanisms. We suggest that social learning is one such candidate mechanism. OdlingSmee (this symposium) traces out in some detail how the genetic and social transmission
might coevolve under an extended regime of variability selection.
Testing this idea is a formidable challenge. Certainly not every lineage on the earth
responded to the onset of extreme glacial fluctuations by evolving social learning.
Further, humans are the only species to respond to the ice age by evolving the very
complex forms of social learning usually given the term "culture." The unique importance
of social learning in humans and the spectacular ecological success we become using
culture is an embarrassment the hypothesis. How can an environmental event that
affected the entire earth account for the evolution of one species' peculiar adaptation? The
skeptic might ask, if social learning is an adaptation to the Pleistocene, why aren't many
species capable of human-like feats of social learning? Is there any evidence that social
learning has anything to do with climatic deterioration? How does social learning fit into
a pattern of responses to variability selection if indeed this concept is useful?
General Evolutionary Responses to Plio-Pleistocene Climate Deterioration
Evolutionists have known the general outline of the Plio-Pleistocene climatic
deterioration for a long time, and its evolutionary consequences are the subject of much
classical work. For example, Stebbins and Major (1965), and Raven and Axelrod (1978),
discuss the evolution of the California flora in response to climate change. The evolution
of social learning is not open to plants. Plants show a variety of responses to the onset of
cooler, drier, summer drought conditions of the novel Mediterranean climate that
accompanied the Pleistocene. Trees responded mainly by range changes of existing
species, not by Darwinian evolution. As a result, California has a number of
"paleoendemics," mostly trees, that once had a much wider distribution and are now
restricted to narrow niches. Redwoods are the most famous example. Many of
California's most widespread tree species, for example our evergreen oaks, evolved in the
Sierra Madre mountains of Mexico and invaded California with the onset of the semiarid
climate. Other elements of the vegetation, including many shrub and herb genera, have
radiated under the new conditions to form flocks of "neoendemics." Wild lilac and
lupine, represented by a few dozen closely related species, are examples. Such groups
respond to changing environments with niche shifts via speciation. It might be thought
that this pattern is the result of evolutionary conservatism on the part of long-lived
species like trees and greater flexibility on the part of species with short generations.
However, some patterns do not conform to this expectation. Beetle species from the early
Pleistocene can almost all be referred to modem species (Coope, 1979), whereas
mammals have been evolving quite rapidly, with an average species duration of only a
million years or so (Stanley, 1979). No simple patterns of response to climate
deterioration have so far emerged from studies of selected sets of species.
As far as we are aware, no one has proposed that plants and beetles have responded to
climatic deterioration by any special set of evolutionary mechanisms. Rather, the
conservative species like trees and beetles have responded by dramatic niche-chasing
range shifts as climate changes. Coope (1979) describes a species of beetle from a
Pleistocene cold period fauna in Britain that was thought to be one of the few without a
modem population until one was found in a remote location it Tibet. Clark et al. (1998)
review evidence suggesting that large, long-lived trees like beeches and oaks can move
surprisingly rapidly in response to climate change. Rates of 150-500 m/yr are necessary
to account for rates of range expansion characteristic of the end of the last glaciation.
Trees could thus respond even to events on the time scale of the Dansgaard-Oeschger
events. The rapid to and fro movements of the large woody vegetation created
Pleistocene forests with diverse, probably non-equilibrium, mixtures of species that do
not normally co-occur today. Other kinds of populations seem simply to have gone
extinct or undergone niche changing speciation in response to climate change.
Elizabeth Vrba (1995) has proposed a bold hypothesis about the impact of Plio-
Pleistocene climatic fluctuations on mammalian evolutionary patterns. She argues that
the fluctuating climate should generate waves of extinctions and speciations. Consider a
warm climate that is turning cold impacting a lineage that is generally cool adapted. In
the warm climate, such a lineage will only persist as fragmented alpine relicts. Under
such fragmentation, new species will arise, but many old ones will become extinct. As
the climate cools again, formerly fragmented populations, now new species, will expand
their ranges and become temporary dominants. Under very cold conditions,
fragmentation will occur again, with a new wave of extinctions and an incipient radiation.
DeMenocal and Bloemendal (1995; see also De Menocal, 1995) argue patterns of
hominid evolution in the Plio-Pleistocene map onto the stepwise changes in the
variability in the climate record shown in Figure 1. The evidence for Vrba's hypothesis is
mixed (see other chapters in Vrba et al., 1995), but there is no doubt that ordinary
Darwinian evolution can be rapid on the geological time scale (Carroll, 1997: Ch. 3).
Thus, many lineages have apparently adapted to climate deterioration without any
mechanisms outside the limits of conventional ecological succession (niche chasing),
and/or adaptive phyletic evolution and speciation (niche shifting).
Brain Size Evolution in the Neogene
Mammals are the one group that shows clear signs of responding to variability selection
with Potts' hypothesized increased behavioral flexibility. Harry Jerison's (1973) classic
treatment of the evolution of brain size documents a major trend towards increasing brain
size in many mammalian lineages that persists right up to the present. Figure 6
summarizes his data. The data are presented in the form of cumulative frequency
distributions of encephalization quotients of carnivores and ungulates over the whole
CenozoicEra. The sample includes:
(1) archaic creodont carnivores (an extinct order),
(2) archaic ungulates and carnivores from the extinct orders Condylartha and Amblypoda
from the Paleogene (65-22.5 million yrs before present),
(3) members of the still extant ungulate and carnivore orders from the Neogene (22.5-2.5
million years before present), and
(4) a selection of living species of ungulates and carnivores.
The time trends illustrated by the figure are complex. There is a progressive increase in
average encephalization throughout the Cenozoic. However, there is an interesting
tipping of the cumulative curves to the north east through time as well. Many relatively
small-brained mammals persist to the present even in orders where some species have
gotten rather large brains. The diversity of brain size increases toward the present.
Mammals continue to be under strong selection pressure to minimize brain size, and
those that find an effective way to cope with climatic deterioration by niche chasing or
organic niche shifting do so. Other lineages evolve the means to exploit the temporal and
spatial variability of the environment by using behavioral flexibility instead. These last,
we suppose, pay for the cost of encephalization by exploiting the ephemeral niches that
niche chasing and niche shift' species leave under-exploited.
Humans merely anchor the tail of the recently much-stretched distribution of brain sizes
in mammals. We are the largest brained member of the largest brained mammalian order,
sharing our position on the frontier with a few cetaceans (porpoises). This is comforting
to a Darwinian hypothesis. Large gaps between species are hard to account for by the
processes of organic evolution. That we are part of a larger trend suggests that a large
scale, general selective process such as we propose is really operating. Nevertheless,
there is some evidence that human culture is more than just a more sophisticated form of
typical animal social learning. More on this vexing issue below.
Note that biggest shift per unit time by far is the shift from Neogene to modem species. In
the 2.5 million years from the late Pliocene to the end of the Pleistocene, encephalization
increases were somewhat larger than the steps from Archaic to Paleogene and Paleogene
to Neogene, each of which represent tens of millions of years of evolution.
Nevertheless, the Plio-Pleistocene leap in brain size is part of a trend that reaches back
before the beginning of the Cenozoic. Since detailed records of climate variability are so
far only published for the last 6-7 million years (Figure 1), it is not possible to say
whether a record of climate deterioration accompanies the earlier increases in
encephalization. Based upon the long-term trend of encephalization, our hypothesis
predicts that climatic variability in the early and middle Cenozoic must have exceeded
that of the Mesozoic when encephalization was still more modest. The degree of
deterioration in these earlier episodes deterioration should have been less dramatic that
the Pleistocene, but still appreciable. Further work on deep sea cores should provide a test
as for back as there is oceanic crust to examine.
Social Learning Versus Individual Learning?
Increases in brain size could signal adaptation to variable environments via either
individual or social learning. The mathematical models suggest that that the two systems
work together. There should be an optimal balance dictated by the spatio-temporal
structure of the variability selection imposed. Given the tight constraints imposed on
brains, we would expect to find a tradeoff between social and individual learning
abilities. Those species that exploit the most variable niches should emphasize individual
learning while those that five in more highly autocorrelated environments should devote
more of their nervous systems to social learning.
Following this line of reasoning, Lefebvre and Giraldeau (1996) conducted experiments
on two species of pigeons, one social and opportunistic and the other more conservative
and less social. The social and opportunistic species, they reasoned, should be able to
learn socially more easily than the more conservative species, and the conservative
species should be better individual reamers. They also reviewed the data of other
investigators comparing social and individual learning in a small sample of songbird
species. Surprisingly, the prediction fails. Species that are good social reamers are also
good individual reamers. Is this counter-intuitive finding a product of a very small,
potentially misleading, sample?
Perhaps not. It may be that individual and social learning are not strongly competing
processes and might even be synergistic. Jerison (1973) argued that the expansion of the
neocortex, which accounts for most of the tissue involved in encephalization trends, is
devoted to "maps' of the environment. Animals with more detailed maps need to acquire
the information to fill them out. Both social and individual learning will help do so.
Perhaps the information evaluating neural circuits used in social and individual learning
are also partly shared. Once animals become social the potential for social learning arises.
If the marginal benefit of reducing the neural circuits unique to individual learning are
modest, social species enlarging their maps to take advantage of social learning may
come under selection to improve individual learning as well. If the two systems share the
overhead of maintaining the memory storage system and much of the machinery for
evaluating the results of experience, the marginal benefits in quality or rate of
information gain may be large relative to the cost of more specialized nervous tissue. If
members of the social group tend to be kin, investments in individual learning may also
be favored because sharing the results by social learning will increase inclusive fitness.
The hypothesis that the tradeoff between social and individual learning may be very
marginal resonates with the mechanisms of social learning found in most wen-studied
cases of social learning. Galef (1988, 1996), Laland et al. (1993), and Heyes and Dawson
(1990) argue that the most common forms of social Teaming result from very simple
mechanisms that piggyback on individual learning. In social species, naive animals
follow more experienced parents, nestmates, or flock members as they traverse the
environment. The experienced animals select highly non-random paths through the
environment. They thus expose naive individuals to a highly selected set of stimuli that
form the basis for acquisition of behaviors by ordinary mechanisms of reinforcement.
Social experience acts, essentially, to speed up and make less random the individual
Teaming process, requiring little additional, specialized, mental capacity. Social
Teaming, by making individual Teaming more accurate without requiring much new
neural machinery, tips the selective balance between the high cost of brain tissue and
advantages of flexibility in favor of more flexibility. As the quality of information stored
on a map increases, it makes sense to enlarge the scale of maps to take advantage of that
fact. Eventually, diminishing returns to map accuracy will limit brain size.
Eisenberg's (1981: Ch 23) review of a large set of data on the encephalization of living
suggests that high encephalization is associated with longer times of association with
parents, late sexual maturity, extreme iteroparity, and long potential life-span. These life
cycle attributes all seem to favor social learning. It may be that large brains cannot be
supported in the absence of the opportunity to learn socially from parents at least, and
that investments in social Teaming must be amortized over a long life. Even marginally
social species may come under selection for behaviors that enhance social Teaming, as in
the well known case of mother housecats to bring partially disabled prey to their kittens
for practice of killing behavior (Caro and Hauser, 1992).
If the relationship between social and individual learning is as tight as this evidence
suggests, then we can expect to find social learning in many if not most social species.
Indeed, the best studied example of social Teaming is the food choice system of norway
rats (Galef, 1996). This species, with an encephalization quotient of about 0.4 (0.8 using
Eisenberg's, 1981:499, re-estimated allometry relationship), is among those that have
participated only modestly in the Cenozoic encephalization trend (Jerison, 1973: 212,
218). Lefebvre and Palameta (1988) provide a long list of animals in which social
learning has been more or less convincingly documented. Recently, Dugatkin (1996) and
Laland and Williams (1997) have demonstrated social learning in guppies.
The idea that brain size, social Teaming and individual Teaming are all tied to a rather
generalized environment mapping system is contrary to the attractive, widely held, idea
that brains are collections of highly specialized modules (Fodor, 1983). Tooby and
Cosmides (1989) argue that modular specialization of brain function is to be expected on
general theoretical grounds. Lefebvre and Giraldeau's result is perhaps more congenial to
the connectionist hypothesis holding that much brain tissue functions as a rather
generalized pattern recognition device. We have neither the space nor the competence to
review this issue in detail. Tononi et al. (1994) note that, at the neurological level brains
are complex just because they are at once extensively modular and richly integrated.
Thus, individual Teaming capacities may be positively correlated with capacities for
social teaming because they are mostly sub-served by the same modules and integrating
circuits.
Aspects of the social learning system in animals do show signs of adaptive specialization.
For example, Terkel (1996) and Chou (1989, personal communication) obtained evidence
from laboratory studies of black rats that the main mode of social learning is from mother
to pups. This is quite unlike the situation in the case of norway rats, where Galef (1988,
1996) and coworkers have shown quite conclusively that mothers have no special
influence on pups. In the black rat, socially learned behaviors seem to be fixed after a
juvenile learning period, whereas norway rats continually update their diet preferences
(the best-studied trait) based upon individually acquired and social cues. Black rats seem
to be adapted to more slowly changing and norway rats more rapidly changing
environments. Terkel studied a rat population that has adapted to open pinecones in an
exotic pine plantation in Israel, a novel and short-lived niche by most standards, but one
that will persist for many rat generations. Norway rats are the classic rats of garbage
dumps, where the sorts of foods available change on a weekly basis. Interestingly, in
recent decades, norway rats have been expanding at the expense of black rats (e.g.
Bentley, 1964). Its seems possible that modem garbage dumps present a much more
varied resource for rats than traditional ones, and that the spread of norway rats reflects
their better adaptation to human modernity. The theory we have described suggests that
selection on social reamers might well tune the social learning system to match the
statistical properties of the environmental variation in the specific niche the animal
occupies. This very thin bit of data suggests that the hypothesis is worth pursuing.
In the human case, we have at least one highly specialized social learning systen-4
language. On the other hand, we readily learn to make a living using a spectacular array
of techniques. As the famous language learnability argument of Chomsky shows, a
completely general learning machine cannot work (Pinker, 1994: Ch 9). A finite learner
must have a nervous system that in effect makes many assumptions about the
environment in order even make the most basic map of its environment. For example,
primates have a visual system imposes order on nerve impulses coming from the rods and
cones to produce a fairly veridical image of objects in the world (Spelke, 1990). It win
not do just to have a flood of impulses flowing from a large array of sensory cell. There
must be built in expectations about what sorts of objects are out there to sense. For
example, the visual system assumes that a set of spatially contiguous points in the visual
field that have a similar color, defined border, and coherent movement is a solid object.
This innate physics correctly recognizes a rolling ball as a solid object, though it
misidentifies clouds as such. On the other hand, the adaptive reason to have learning and
social learning is the flexibility to adapt to unforeseen contingencies. Experience teaches
us that the solidity of clouds is an illusion.
Perhaps the neocortex of the brain is an adaptation like the beak of birds. The basic beak
is nothing more than a moderately complex, functionally integrated, forceps-like device
with multiple functions. It is usually a food acquisition, handling, and processing organ,
fighting weapon, environment probe, and grooming tool, all in one. Nevertheless, despite
great commonality of form and function, bird beaks are endlessly stretched, bent,
thickened, widened, deepened. and sharpened to support the diverse niches birds occupy.
Only occasionally are new parts, like the pouch of pelicans, added to create a new
adaptation. It strikes us that the modularity-connectionist debate does not exhaust the
possible models for the relationship between form and function in brains.
We wish to underline how little we know about the adaptive tradeoffs in brain design.
Neurophysiologists, cognitive scientists, and behavioral ecologists each have something
to contribute to the puzzle of how some species can support large brains. However,
collaborations between these disciplines to tackle this question have been lacking.
Human Versus Other Animals' Culture
The human species position at the tail of the distribution of late Cenozoic encephalization
admits of the hypothesis that our system of social learning is merely a hypertrophied
version of a common animal system. However, two lines of evidence suggest that there is
more to the story.
First, human cultural traditions are often very complex. Subsistence systems, artistic
productions, languages, and the like are so complex that no one individual ever could or
did invent them. Rather, they are built up over many generations by the incremental,
marginal modifications of many innovators (e.g. Basalla 1988). We are utterly dependent
on learning such complex traditions to function normally. Think of some relatively
simple item like a hunting spear. The maker has to know how to make the stone tools to
prepare the shaft, how to knap fine-grained stone to make a good point, how to prepare
stout adhesives and fiber to mount the point, what wood makes a spear of the right
strength and weight to be useful how long and stout to make the shaft for the intended
purpose (throwing, stabbing, multipurpose), and so forth. Few of us could make more
than a crude approximation of a (late) Stone Age spear, though we could easily learn by
being taught, or even just by observing an expert spear-maker. Most animal culture
appears to be much simpler. Terkel's study of the manner in which it roof rats open pine
cones showed that individuals cannot normally learn to open a pine cone in a way that
leads to a net energy gain. However, there is only one trick involved in the successful
technique, and it is likely that a single individual innovated the trait in the beginning.
This difference in the complexity of socially learned behaviors is mirrored in a major
difference in mode of social learning. As we saw above, much social learning seems to be
dependent mostly on the same techniques used in individual learning. Experimental
psychologists have devoted much effort to trying to settle the question of whether nonhuman animals can learn by "true imitation" or not (Galef, 1988). True imitation is
learning a behavior by seeing it done. True imitation is presumably more complex
cognitively that merely using conspecifics' behavior as a source of cues to stimuli that it
might be interesting to experience. Although there are some rather good experiments
indicating some capacity for true imitation in many socially learning species (Heyes,
1994; Zentall, 1996; Moore, 1996), head-to-head comparisons of children's and
chimpanzee's abilities to imitate show that children begin to exceed chimpanzees'
capabilities at about 3 years of age (Whiten and Custance, 1996; Tomasello, 1996). There
is still considerable doubt about the significance of imitation in wild chimpanzees.
Tomasello is inclined to think that even in this species, there is remarkably little
indication that apes can ape, although human reared chimpanzees do show considerably
imitation. Whiten and Custance, on the other hand, argue that the marginal abilities
observed under impoverished captive conditions are likely to underestimate abilities in
the wild. McGrew (1992) reviews the evidence from chimpanzee material culture
suggesting that chimpanzee tools are as complex as the simplest know toolkits of
humans. Perhaps some chimpanzee tools, especially the hammer-and-anvil nut cracking
system found in some West African populations, is a cumulative tradition that is made up
of at least two independent inventions. Rehabilitating pet orangutans exhibit very
impressive imitations of human behavioral routines, though they are not known to have
any sign of imitatively acquired behaviors in the wild (Russon and Galdikas, 1993).
Parrots seem to have acquired a quite respectable but little understood capacity for
imitation (Moore, 1996). On the other hand, monkeys show scant signs of abilities to
imitate, even Capuchin monkeys, which have a higher encephalization quotient than even
the Great Apes (Fragaszy and Visalberghi, 1996; Eisenberg, 1981, 499). Thus, the lesson
to date from comparative studies of social learning suggests that very simple mechanisms
of social enhancement of cues is much more common and more important than imitation,
even in our close relatives and other highly encephalized species.
It may be that the ability to imitate others freely requires that individuals have a theory of
mind (Premak and Woodruff, 1978; Cheney and Seyfarth, 1990: Ch. 8). Without the
ability to model the intentions of other individuals, it may be difficult to translate
observations of another's acts into subjective terms so that one can replicate the act. As a
consequence of an inability to "see" the elements that go to make up complex behaviors
when they are performed by others, most social reamers cannot imitate. They must
relearn most elements of the behavior for themselves, with the social part of learning
restricted to being exposed to the stimuli that tend to elicit the correct behavior in the end.
Thus the complexity of proto-cultural traditions is much less than in the case of humans.
Even rather encephalized animals such as the monkeys, seem to use the evaluative
machinery that is used in individual learning to evaluate socially supplied cues, but to
have invested little specialized machinery to imitate.
Although we expect that social learning systems in non-human animals will prove to be
common and varied in nature, the human ability to evolve complex cultural traditions
appears to be unique, or at least uniquely hypertrophied. This generates the basic problem
of accounting for human minds. Humans have apparently penetrated a "cognitive niche"
(Tooby and DeVore, 1987) which made us a rather successful species under Pleistocene
conditions. Using complex traditions, we successfully occupied niches from the tropics to
the glaciers, penetrating to all but the harshest environments (Klein, 1989). In the
Holocene, the development of food plant production has made us the Earth's dominant
organism. If human traditional culture is a successful adaptation for us, why haven't other
species evolved similar capacities? There are a number of possible reasons. It could be
that we have simply won an evolutionary footrace to be the first animal to occupy the
cognitive niche. The final evolutionary innovations that permitted complex traditions
were rather late. The Upper Paleolithic Transition in Europe, the settlement of Australia
(requiring tolerably sophisticated boats), and other signs of the final modernization of
human cognition occur within the last 100,000 thousand years (Stringer and Gamble,
1993). It might be that upright posture, freeing the hands to make and carry artifacts, was
necessary to make complex traditions useful. Our species may have been the only one
with this or some other non-cognitive preadaptation necessary to permit the evolution a
capacity for complex culture.
Why Is Complex Culture Rare?
There is also a potential evolutionary impediment to the evolution of a capacity for
complex traditions. We show elsewhere that, under some sensible assumptions, a
capacity for complex cumulative culture cannot increase when rare (Boyd and Richerson,
1996). The mathematical result is quite intuitive. Suppose that to acquire a complex
tradition efficient imitation is required. Suppose that efficient imitation requires
considerable costly, or complex, cognitive machinery, such as a theory-of-mind/imitation
module. In such a case, there will be a coevolutionary failure of capacity for complex
traditions to evolve. The capacity is a great fitness advantage, but only if there are
cultural traditions to take advantage of But, obviously, there cannot be complex traditions
without the cognitive machinery necessary to support them. A rare individual who a
mutation coding for an enlarged capacity to imitate will find no complex traditions to
learn, and will be handicapped by an investment in nervous tissue that cannot function.
The hypothesis depends upon there being a certain lumpiness in the evolution of the
mind. If even a small amount of imitation requires an expensive or complex bit of mental
machinery, or if the initial step in the evolution of complex traits does not result in
particularly useful ones, then there will be no smooth evolutionary path from simple
social learning to complex culture.
If such an impediment to the evolution of complex traditions existed, evolution must have
traveled a round-about path to achieve get the frequency of the capacity high enough to
begin to bring it under positive selection for its tradition-supporting function. Some have
suggested that primate intelligence was originally an adaptation to manage a complex
social fife (Humphrey, 1976; Whiten and Byrne, 1988, Kummer et al., 1997). Perhaps in
our lineage, the complexities of managing the sexual division of labor or some similar
social problem favored the evolution of a theory-of-mind capacity. Such a capacity might
then incidentally make efficient imitation possible, launching the evolution of complex
traditions that could drive the evolution of still more sophisticated imitation. This sort of
stickiness in the evolutionary processes is presumably what gives evolution its commonly
contingent, historical character (Boyd and Richerson, 1992).
There is some evidence supporting this hypothesis. Even among cognitively modem
humans, the maintenance of complex traditions is not unproblematic. The Tasmanian
toolkit shrank in size and sophistication after their isolation from Australia by the
Holocene rise in sea level. Diamond (1978) argues that this may have resulted from a sort
of cultural drift. In a small population, complex skills will occasionally be lost by
accident. With few people to reinvent them and no possibility of reacquiring them by
diffusion, the Tasmanians were helpless to prevent an erosion of their more complex
cultural traditions.
Even given a capacity for complex traditions, the number of participants in a cultural
system may be critical to the complexity that can be maintained. A few rare,
unsophisticated imitators would presumably have a much harder time launching or
maintaining a complex tradition than a larger group that can pool many piecemeal
innovations. Diamond (1997) expanded this hypothesis to the continental scale. He
argues that Eurasian cultural sophistication grew more rapidly than traditions on other
continents because Eurasia was the largest, most populous continent, and because
innovations could readily spread east and west in similar ecological zones. The Americas
are not only smaller continents, but are relatively narrow and oriented on a north-south
axis. Innovations will tend to spread more slowly across than along ecological zones.
It is interesting that the manufacture of complex compound tools like a stone-tipped spear
appears quite late in the evolution of hominids. Such tools are associated with other signs
of a more complex imitative mental life, such as art and stylistic variation in utilitarian
objects (Klein, 1989: 369, 379-83; Donald, 1991). Late archaic hominid populations,
such as Neanderthal peoples from Europe, had a much less complex technology than the
anatomical modems that followed them, despite having slightly larger brains (Stringer
and Gamble, 1993). Perhaps McGrew is correct that chimpanzee tool kits nearly reach
the complexity of the simplest example among modem humans. Perhaps Tomasello is
correct that imitation is an ancient latent potential of ape cognition that is manifest in
chimpanzee and orang behavior given human models. Perhaps most or all hominid brain
enlargement throughout the Pleistocene was largely the product of selection forces other
than those exerted by the opportunities afforded by complex traditions. Perhaps hominids
as modem as Neanderthals stir used their large brains largely for individual learning and
the correlated ability to learn socially using the simple stimulus enhancement
mechanisms quite widely used in social animals, not imitation.
There is no evidence that the past 100,000 years of the Pleistocene environment has
deteriorated significantly since the shift from dominance of the 41,000 year Milankovitch
cycle to tie dominance of the 98,500 cycle about a million years ago. The ice core data do
not go back beyond the last interglacial (oxygen isotope stage 5). Thus, it is not possible
to say if phenomena such as the Dansgaard-Oeschger events are restricted to the last
glacial or whether they are more ancient. Records from old African lakes may one day
test directly whether hominid evolutionary events of the last glacial cycle could have
been driven directly by climate or not. On present evidence, a cognitive niche for a
sophisticated, imitative social learner would seem to have existed for at least a million
years before the adaptation appeared.
Perhaps human cognition came under direct selection for the capacity to imitate only
within the last 100,000 years or so as some preadapted populations began to develop
complex traditions, finessing the maladaptive-when-rare problem. The anomalously old,
complex, Howieson's Poort artifacts (Klein, 1989: 308), and the recent discovery of a
sophisticated bone points dating back 90,000 years in Central Africa (Brooks et al. 1995),
suggest that some populations in Africa began creating complex traditions as early as the
last interglacial. Anatomically modem populations also date back to perhaps 90,000 years
ago. However, in the Near East, anatomical modems are clearly associated with artifacts
identical to those made by Neanderthal populations that sometimes succeeded them in the
same and nearby sites (Klein, 1989: 303-305). The critical evolutionary steps to the
imitative capacity must have occurred in a sub-population of anatomical modems,
probably living somewhere in Africa something like 90,000-50,000 years ago. The latest
Neanderthal Chatelperronian industry in Europe does show signs of the diffusion of
complex elements from anatomically modem populations that were just entering Europe
(Klein, 1989: 335-336). Neanderthals seem capable of acquiring traditions more complex
than they ever produced without such a stimulus. Much as with apes, they seem to have
had a cryptic capacity for imitation larger than they routinely used. One gets the
impression that apes and archaic hominids are near and yet so far from the
imitative/complex tradition adaptation during the whole Pleistocene Epoch.
It is even possible that there has been no significant coevolution of human minds for
imitative capacities at all. At the very least, the paleoanthropological evidence rather
strongly suggests that nearly modem human minds evolved by preadaptation while
making artifacts that are much less sophisticated than those made after some sort of
Upper Paleolithic "revolution." At most, the period of coevolution of human minds to
reach the full range of complex traditions would appear to be bracketed by the slightly
modernized Howiesons Poort industry after 100,000 thousand years ago and the intrusion
of anatomically modern humans making the Aurignacian industry into Europe 35-40,000
years ago (Klein, 1989: Ch.6-7).
On the present evidence, the character of climate variability has been unchanged since I
million years ago. Thus, it is tempting to think that Homo erectus and archaic Homo
sapiens spent several hundred thousand years adapted to living under conditions of
extreme environmental fluctuation, using a system of relatively simple technological
adaptations represented by the Acheulean stone tool tradition. This tool tradition and its
close relatives occur from about 1.5 million years ago in Africa to about 200,000 years
ago (Klein, 1989: Ch.4). Indeed, considerable brain enlargement occurred in the
transition from Homo erectus to archaic H. sapiens, without any evidence of change in
the archeological assemblage at all (Klein, 1989: Ch. 5). It is hard to understand from the
paleoanthropological record as it stands how archaic hominids were paying the costs
ofbrain enlargement, as conservative as technology apparently was. Relatively lately and
relatively suddenly, culture became much more complex, as if some final preadaptive
breakthrough permitted the emergence imitation. To judge from brain size, the main
reliable clue, the advanced cognitive capacity preceded the first complex traditions by a
few tens of thousands of years at least. Perhaps there was a final cognitive modernization
not reflected in brain size under the influence of coevolution with complex traditions. It
would seem surprising if this were not so, but the evidence is unimpressive. It does seem
to be the case that the encoding and decoding of speech occurs at such a rapid rate that
specialized structures must be involved (Friederici, 1996). The gross anatomy of the
vocal tract of modem humans also seems rather clearly to be specialized for speech
(Lieberman, 1984). The extant data at least permit the hypothesis that the ability to
acquire complex traditions by imitation is wholly a preadaptation, and encourage the
hypothesis that it is largely so.
Donald's (1991) admirably well-specified scenario for the origins of human cultural
complexity involves a stage of sophisticated motor mimicry preceding and laying the
neurological basis for language. It is plausible, as Donald shows, that quite complex
behavior can be acquired by mimicry in the absence of language. 19th Century accounts
of the abilities of deaf-mutes to acquire many sorts of useful economic and social skills
without language suggests that they could easily learn to make a serviceable stone-tipped
spear by observation, without any linguistic aids.
However, since major increases in tool complexity postdate anatomically modern skulls,
it seems possible that this argument needs to be reversed to account for the final origin of
complex traditions. Perhaps Achuelean and Mousterian level tools were made with only
minimal imitative capacities. On the one hand, Acheulean tools show rather stereotyped
features suggesting a more sophisticated form of social learning that mere stimulus
enhancement. On the other, the stereotyped pattern remains very similar across large
geographic distance and great spans of time. There is no indication that stone tool
traditions of the middle Pleistocene were capable of sensitive adaptation to local
environment. It is as if the "cultural" patterning of Acheulean artifacts was in fact innate.
Thus, if anatomically modem skulls signal the development of modem linguistic
capacities, that change would be roughly contemporaneous with the modest shift in tool
complexity from the Acheulean to the Mousterian industry. Some Mousterian
populations (anatomical modems) evolved the capacity for complex language systems,
while others (Neanderthals) did not. The Upper Paleolithic artifact revolution in this
scenario would follow language, not antedate it.
The key step might have been the evolution of language to manage some aspect of social
behavior, for example the emergence of a stable division of labor. It would presumably
require complex negotiations between men and women to manage such a social system.
Ditto for the multi-family cooperative, risk-sharing hunting and gathering band of
ethnographic fame. It is plausible that small steps in the increase of communication
complexity would allow the coevolution of a language capacity and a cultural linguistic
system. The evolution of language might not have the evolutionary impediment to the
evolution of complexity, at least not so severe a one. We are used to thinking that
language is the ultimate human cultural achievement. The evidence that much of the
complexity of language is innate (Pinker, 1994) suggests that this system is actually more
primitive than other systems that appear to have a much smaller innate component, such
as post-Acheulean technology. If children have enough innate information to largely
create a new language in one generation (Bickerton, 1984), this system may not really
depend upon the existence of complex traditions, although it produces them as a
byproduct.
The linguistic system perhaps made the first complex technical traditions possible by
making it easy to express and memorize cultural principles verbally. In this way, a culture
which initially become complex using the oral mode of transmission may have selected
for more facile mimetic capabilities for those common learning tasks where a picture is
worth a thousand words.
Whatever the exact sequence of events, there is independent evidence from human
paleodemography that supports the idea that some sort of rare evolutionary accident was
necessary to achieve a functional capacity for complex cultural traditions. Rogers and
Harpending (1992) argue that human mitochondrial DNA records evidence of a human
population explosion between 33,000 and 150,000 years ago. In small populations,
genetic drift limits the diversity of mitochondrial genomes in the population. If a small
population suddenly expands, it will begin to preserve more variation. The age of
diversification of mitochondrial DNA lineages can be estimated from the number of
mismatches separating different lineages. If a population suddenly expands, a large
number of new mitochondrial lineages will arise more or simultaneously and will be
preserved against loss by drift. As these lineages accumulate more mismatches over time
they generate a clock that allows us to estimate the time of the original population
explosion. Although confidence intervals are wide, the data suggest that the population
ancestral to all modem humans was quite small until something like 60,000 years ago,
when it expanded suddenly by a factor of more than 100 fold. The pre-expansion
population was between 1,000 and 7,000 breeding females. At the time of the final
modernization of the human mind, we were most likely a rare and, given the nature of the
Pleistocene, endangered species. At our point of origin, our numbers were probably much
smaller than those of the archaic populations that we replaced when our population
exploded. The date of the explosion is consistent with the dates of the appearance of more
sophisticated tools, though so far only the aforementioned replacement of Neanderthals
by modem Homo sapiens in Europe around 35,000 years ago is well document. Exactly
what quirk of the evolutionary process led to the final modernization is likely to prove
very difficult to recover. Our rarity at the crucial time does suggest that only after the
capacity for complex culture was in place, and complex traditions in fact began to evolve,
did the main adaptive advantage of the cognitive change become apparent. The crucial
cognitive innovations leading to the capacity for complex culture would seem to have
occurred in a small population that was barely paying the overhead for their large brain. It
is easy to imagine such a population winking out in under the severe, variable conditions
of the last Glacial before the complex culture innovation was fully functional. If it had,
who knows how many more glacial cycles would have passed with big-brained hominids
poised on the threshold of the complex culture adaptation without crossing it.
It is thus quite plausible that the transition to complex culture required crossing an
evolutionary divide. It is also plausible that the crossing was a result of a long history of
preadaptation rather than directly adaptation for complex traditions. How else are we to
account for the near stasis of material culture (or protoculture) during long spans of time
when brain enlargement was significant?
Humans in the HoloceneA final issue concerns us. Since the climate of the Holocene has
been unusually tranquil, why have not humans and other animals begun to re-evolve
smaller brains? Kurt Vonnegut in his novel Galapagos invites us to imagine a time in the
future when the troublesomely large human brain has shrunk considerably. Several
possible reasons come to mind. Perhaps there hasn't been enough time for the shrinkage
process to proceed appreciably. Domestic animals under artificial selection, however,
have considerably smaller brains, scaled for body weight, than their wild progenitors. A
few thousand years is certainly sufficient time for shrinkage to occur if selection is
strong. Perhaps the modest fluctuations of the Holocene are at the right time scale to
maintain selection for encephalization. As Lamb (1977) describes, the Holocene
fluctuations are troublesome enough. The Pleistocene fluctuations are much more
spectacular to be sure, but perhaps they are at sufficiently long time scales as to be
mainly irrelevant.
The most promising hypothesis we think is that humans themselves have become a major
source of perturbation in the Holocene. The development of food plant production set off
a demographic revolution in many parts of the world. Even in areas of the world like
Western North America where participation in agriculture was rare, subsistence and
social organizational innovations led to considerable human population increase and
movement (Bettinger and Baumhoff, 1982). In essence, the end of the Pleistocene
ushered in an intense competitive arms race between human populations where the
winners were typically those with the more sophisticated traditions. The penetration of
humans to virtually all of the World's habitats ensures that other mammals have an
especially wily predator and competitor to deal with. Human dynamism in the Holocene
seems easily sufficient to maintain selection for complex cognitive capacities in ourselves
and other mammals. Should the quite Holocene climate persist for long enough, it is
interesting to contemplate whether Vonnegut's scenario would obtain in the end.
Conclusion
Humans are a difficult species. The Darwinian project is committed to bringing us into
the same basic explanatory framework as all other organisms. In his M notebook on
August 16 1838, while he was in full cry in pursuit of his first formulation of natural
selection, Darwin wrote "Origin of man now proved.--Metaphysics must flourish.-He
who understand baboon would do more toward metaphysics than Locke" (Barrett, 1974:
281). Darwin realized that failing to account for human behavior left a dangerous gap in
his theory through which critics could and did try to attack his whole theory (Richerson
and Boyd, in press). When, in the Descent of Man, Darwin reluctantly undertook to
explain our species, a hostile critic in the Quarterly Review, whom Darwin believed to be
his persistent critic St. George Mivart, took advantage of perceived problems in the
human story to attack the whole ediface. He gloated, it "offers a good opportunity for
reviewing his whole position7' (and rejecting it, Anonymous, 197 1). If the gap between
humans and our animal ancestors is too large, it is difficult to explain how ordinary
evolutionary processes could explain our origins. If one species escapes the net of
evolution by natural selection, the whole of so comprehensive a theory is brought into
question. On the other hand, if there is not some "great gap" between our ape ancestors
and ourselves it is difficult to account for our runaway success. Our nearest living
ancestors are forest dwelling creatures with modest ranges and abundances. Our more
immediate fossil ancestors began to extend their ranges beyond Africa a million or so
years ago. Even then, the range of Homo erectus grade hominids was not strikingly
greater than that of some large carnivores like lions and pumas. However, by latest
Paleolithic times, Homo sapiens sapiens, using toolkits of great sophistication, penetrated
to the furthermost reaches of the Old World. With the waning of the ice at the end of the
Pleistocene we exploded into the New World. In the Holocene, human societies have
become very complex, rivaling in this regard the eusocial insects.
In the early 20th Century, the Mendelian revolution eventually ended biologists' interest in
the inheritance of acquired variation. The emerging social sciences sought disciplinary
autonomy from biology (and from each other). As a result, for many decades only a few
scattered specialists and individual thinkers worked on humans in the framework outlined
by Darwin (Campbell, 1965, 1975). The important works on the topic in mid-century, for
example Dobzhansky (1962: 18-22), in his well respected Mankind Evolving, faced an
unbridgeable disciplinary gap. In the short theoretical section of his book cited above, he
acknowledged the autonomy claims of the social sciences with statements like "In
producing the genetic basis for culture, biological evolution has transcended itself-it has
produced the superorganic." However, in adjacent passages he hewed to his Darwinian
roots: "The fact which must be stressed, because it has been missed or misrepresented, is
that the biological and cultural evolutions are parts of the same natural process." In 1962
there was neither the theory nor the empirical work to make sense out of these seemingly
contradictory statements. The scientific community was content to five with such
ambiguities, a few controversy provoking figures like Konrad Lorenz aside.
In the late 20th Century, evolutionary biologists became much more interested the
evolution of behavior. W.D. Hamilton's (1964) famous papers on inclusive fitness
launched the theoretical study of social evolution, and a growing body of empirical
ethologists began conducting theoretically relevant investigations. Richard Alexander's
(1974) review paper and Edward Wilson's (1975) treatise Sociobiology left no doubt that
evolutionary biologists were going to apply this theory to humans with little or no respect
for claims of disciplinary autonomy based on superorganismic claims for cultural
processes. At the same time, the population geneticists Lucca Cavalli-Sforza and Marcus
Feldman (1973) initiated the study of mathematical models of cultural evolution. Other
threads important to the problem of understanding humans in a comparative framework
were picked up in that period. For example, Bennett Galef (1977) began his important
work on social learning in norway rats, and the first deep-ocean cores began to reveal the
true dimensions of late Cenozoic climate deterioration. Advances in paleoanthropology in
recent years have likewise been spectacular.
After 25 years of relatively intensive work, there are still many important questions
outstanding. This essay is an attempt to fink the results from theoretical modeling the
evolutionary properties of culture with the main relevant empirical work to produce a
synthetic hypothesis to explain the origin of human culture. We make no strong c@ for
its particular postulates. Our frequent repetition of the word "perhaps" and its cognates
emphasizes our belief that many important questions remain to be answered. We believe
much more strongly that our scenario has the right general ingredients to be a successful
explanation. It takes a stab at addressing the adaptive economics of large brains and
connecting these considerations to the environmental changes that must have driven the
encephalization trends of the Cenozoic. It takes a stab at explaining how humans, so
lately derived from ancestors with relatively modest systems of social learning, could at
once be spectacularly successful using culture but yet not have a crowd of competitors for
the complex-culture cognitive niche.
The hypothesis makes at least a few predictions that are testable. If it is true, the pattern
of climatic deterioration of the Cenozoic should predict the pattern of mammalian
encephalization. The correlation is good back 6 million years, and it appears that the
stepwise climate deterioration is mirrored in events in hominid evolution (deMenocal and
Bloemendal, 1995). It should also hold for the more modest brain size increases of the
middle and early Cenozoic. If there is a problem getting the evolution of a capacity for
complex culture started, we should expect that apes will show pre-adaptations for
culture. That is, they should have cognitive capacities that are homologous to those that
we use to support complex culture, but which are subsidized, if not entirely supported by,
other functions. Chimps and orangs have some capacity for imitation that is apparently
little if at all used in the wild. What function does it serve? Similarly, if parrots and other
animals do prove to have capacities for imitation that might support complex cultural
traditions, they should serve other functions.
There are a number of important questions about which information is currently scanty.
The evolutionary cognitive economics of brains is, despite the cognitive revolution,
poorly understood. Is it really true that the incremental cost of improving simple social
learning at a given level of individual learning is small? Is it true that even relatively
rudimentary imitation requires costly cognitive machinery, making it impossible for
selection to favor true imitation incrementally, beginning with rather simple traditions
and working smoothly towards complexity on the human scale? Is it true that capacities
for learning and social learning are closely tuned to environmental variation? The
empirical support for our hypothesis comes from climate proxy data that are very far
from measuring variations relevant to the fitness of particular organisms. Only the
beautifully detailed ice core data permits us to look at variation on the generational time
scale. Beyond about I 00,000 years ago, the ice core record disappears and we have only
the coarser scale of deep-sea cores. Much more will be revealed in the future, for
example from the very long ice core currently being drilled in Antarctica, or from cores
from deep, old lakes like Lake Tanganyika.
The challenge of the origin of the human mind is the same as it was in Darwin's day.
Humans must have evolved by the same basic processes as other organisms yet we are
highly unusual in our mode of adaptation by cultural traditions and in our ecological
success. Quite likely, the climatic deterioration of the late Cenozoic, especially of the
Pleistocene, played a key role in the evolution of culture. Capacities for social learning
expanded as a means of adapting to the highly variable environment of the ice age,
probably in many mammalian lineages. However, only our species went on to evolve the
capacity to acquire complex cultural traditions by imitative social learning. The events
leading up to the Upper Paleolithic revolution in cultural complexity remain obscure, but
the preadaptive breakthrough hypothesis certainly cannot be ruled out.
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Figures
Figure 1. Simplification of the oxygen isotope record for the last 7 million years in
marine sediments. The oxygen isotope record, based on samples of foraminiferan shells,
is a proxy measure of volume of ice locked up in continental glaciers. Greater
concentrations of the heavy 180 isotope indicate cold, high ice conditions. Thus, the
climate over the last 7 million years has gotten cooler on average and very much more
variable. A significant increase in variability occurred just after 6 million years ago, and
again in the middle Pliocene. Another sharp deterioration occurred in the middle
Pleistocene. (From Opdyke, 1995.)
Figure 2. The oxygen isotope paleoclimate proxy from the Greenland Ice Core Project
core drilled nearly 3,000 m to bedrock on a nearly stationary part of the Greenland ice
cap. Ice depleted in the heavy isotope of oxygen (more negative values) indicates that
large volumes of fresh water, depleted in 180, are stored in the continental glaciers. Note
the very sharp peaks and troughs during the last cold period (Marine Isotope Stages 24).
These are the Dansgaard-Oeschger and Heinrich fluctuations. The replicate GISP2 core
agrees remarkably well with the GRIP core back to MIS 5c. The deeper part of the
record, MIS 5e and 6, may be disturbed by ice flow (Grootes, et al., 1993). Note that the
last I 0,000 years have been very much less variable that the other parts of the record.
From GRIP (I 973).
Information Available to Individual (x)
Figure 3. The effect of the learning threshold (d) on the probability of acquiring the best
behavior by individual learning or tradition. The curve shows the probability of obtaining
a given estimate of the x of the average difference in yield between two environments
from a small sample of years of experience of a young forager, assuming that the
environment really is in a certain state, wet in this case. The task of the young forager is
to decide what to do. If experience seems to show that the environment is indeed rather
wet (x > d) the forager opts to forage alone. If experience seems to indicate that the
environment is dry (x < -d), our forager incorrectly opts to forage cooperatively. If
experience is ambiguous (-d < x < d), the young forager follows tradition (adopts mom's
behavior). The width of the curve is a measure of the quality of information available
from individual learning. In the curve illustrated, individual learning is fairly error prone,
and selection is likely to favor setting wide values of d so as to avoid the chance of
making an error based on noisy personal experience. However, if the environment is
changing rapidly enough, it may be better for young foragers to depend on their own
experience in spite of the risks because the risk that Mom is out of touch is also great.
(From Boyd and Richerson, 1989.)
Figure 4. The values of d, the learning threshold and L, the fi7action of young foragers
acquiring their foraging mode by social teaming, as a function of reliability of personal
experience (S) and the amount environmental variability fi7om generation to generation
(m). The * indicate that these are the evolutionary equilibrium values of L and d, those
that maximize fitness. Note that for environments that are harder to figure out (S large),
the best thing to do is to rely more on social learning (d* and L* increase). Contrariwise,
as the real change in the environment increases from generation to generation (m
increases), it is best to trust more in own experience even at the risk happening to get the
wrong answer by chance. (From Boyd and Richerson, 1989.)
Figure 5. Contour plot of the differences in the fitness of populations using culture or
social Teaming versus genetic transmission to convey information from the older to the
younger generation. Both populations use the same individual teaming rule, the only
difference is that the cultural population has the inheritance of acquired variation to that
both Teaming and natural selection drive behavior in an adaptive direction. The cultural
system is assumed also to have a higher random error rate. Here (a) measures the amount
of social Teaming, R the environmental autocorrelation (the degree to which children's
environments resemble those of their parents), and VH the amplitude of the
environmental variation. The exact shape of the topography depends upon variables not
pictured here, but the qualitative results hold for a wide range of those parameters.
Cultural transmission is favored whenever the resemblance of parental to offspring
environments is sufficiently high. It is especially favored when the environmental change
is quite large but fairly slow on the generational time scale. In the situation where the
advantage of the cultural system is maximal, the dependence on social Teaming is
substantial, around a = 0.75. Note that if R gets large enough, genes win again because in
a stable environment the higher "mutation" rate of social Teaming favors the more exact
system of transmission. The late Tertiary onset of fluctuating but autocorrelated
environments is, we suggest, highly conducive to a greater reliance on social Teaming.
(From Boyd and Richerson, 1985: 127.)
Figure 6. Cumulative frequency distributions of encephalization quotients in fossil and
recent ungulates and carnivores. Encephalization coefficients measure brain size
corrected for body weight. (From Jerison 1973: 311.)
Humans survived ice age by sheltering in 'Garden of Eden', claim scientists
By NIALL FIRTH
UPDATED: 18:26 EST, 27 July 2010
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The last humans on Earth may have survived an ice age by retreating to a small patch of
land nicknamed 'the garden of Eden'.
The strip of land on Africa's southern coast - around 240 miles east of Cape Town became the only place that remained habitable during the devastating ice age, scientists
claim.
The sudden change in temperature wiped out many species elsewhere around 195,000
years ago.
Researchers believe this could account for the fact that humans have less genetic
diversity than other species.
Read more: http://www.dailymail.co.uk/sciencetech/article-1297765/Last-humans-Earthsurvived-Ice-Age-sheltering-Garden-Eden-claim-scientists.html#ixzz2OTt7saar
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Some scientists even believe that the human race's population may have fallen to just a
few hundred individuals who managed to survive in one location.
Professor Curtis Marean, of the Institute of Human Origins at Arizona State University
discovered ancient human artifacts in the isolated caves around an area known as
Pinnacle Point, South Africa.
'Shortly after Homo sapiens first evolved, the harsh climate conditions nearly
extinguished our species,' said Professor Marean.
'Recent finds suggest the small population that gave rise to all humans alive today
survived by exploiting a unique combination of resources along the southern coast of
Africa.'
Humans would have been able to survive because of rich vegetation that was available in
the area.
The sea would have also been a good source of food as currents carrying nutrients would
have passed by the shore, bringing with them a plentiful supply of fish, the team will say
in a new research paper.
Professor Marean said the caves contain archaeological remains going back at least
164,000 years.
Professor Chris Stringer, a human origins expert at the Natural History Museum in
London, said he agreed with Professor Marean's views on the early evolution of
intelligence.
But he said he was not convinced by the argument that one band of humans were the
origin of modern man.
'However, I no longer think that there was ever a single small population of humans in
one region of Africa from whom we are all uniquely descended. We know, for example,
that there were early modern humans in Ethiopia 160,000 years ago and others in
Morocco, and populations like those may also have contributed to our ancestry.'
Many researchers believe that modern humans are thought to have evolved about 195,000
years ago in East Africa, and within 50,000 years had spread to other parts of the
continent.
It is thought that 70,000 years ago a dry period caused Red Sea levels to fall and the gap
across its mouth to shrink from 18 miles to eight miles.
A tribe of as few as 200 period took advantage of this and crossed to Arabia.
Last year Professor Morean's team announced that they believed stone age blacksmiths
mastered the use of fire to make tools at Pinnacle Point.
Knowing how to use fire may have helped the early humans who left Africa 50,000 to
60,000 years ago to cope with colder conditions in Europe.
It may also have given them a big advantage over the resident Neanderthals they
encountered.
By 35,000 years ago, the Neanderthals, a sub-species of humans whose own origins were
in Africa, were mostly extinct.
Professor Curtis Marean, , said: 'The command of fire, documented by our study of heat
treatment, provides us with a potential explanation for the rapid migration of these
Africans across glacial Eurasia.
'They were masters of fire and heat and stone, a crucial advantage as these tropical people
penetrated the cold lands of the Neanderthal.'
Read more: http://www.dailymail.co.uk/sciencetech/article-1297765/Last-humans-Earthsurvived-Ice-Age-sheltering-Garden-Eden-claim-scientists.html#ixzz2OTt5crB5
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The Human Paradox
Several data points affirm that the human species has been around for about 200,000
years. Along the Omo River in Ethiopia, two partial human skulls were unearthed which
dated to around 196,000 years ago. 1 Three partial human skulls were uncovered at
Herto, Ethiopia, which dated to 160,000 years ago. This find was especially important
because one of the skulls is nearly complete. Its distinctly human face has even been
preserved. 2 Besides fossils, mitochondrial DNA indicates that we arose some 200,000
years ago in Africa. 3
Interestingly, we had the same brain size back then as we do today. Yet, for the vast
majority of the time our species has been alive, we have been living like
animals. Civilization came late in the game, and only accounts for about 2.75% of the
time Homo sapiens has existed as a species.
The first civilization on earth was ancient Sumer in modern day southern Iraq. Its
emergence was sudden and unprecedented. Sumerian civilization seems to have been
born overnight. Pottery, the wheel, division of labor, organized religion, government,
warfare, manufacture of tools, written language, agriculture, city building, fortification –
all the critical elements of human civilization were innovated within the fourth
millennium BCE. This happened along the Euphrates River. Interestingly, Genesis
specifically states that the Garden of Eden was by the Euphrates River. 4
Prior to Sumerian civilization, humans had been hunter-gatherers for 200,000
years. They traveled in small family clans. They had no written language. They had
primitive tools. They had no kings, no governments, and no organized
religion. Nowhere on the entire earth did any humans generate anything like
civilization. These were humans like us. We are not talking about
Neanderthals. Neanderthals were a separate species, and DNA analysis demonstrates that
they were not our ancestors. 5 We are not talking about ape-people either. Ape-people,
that is the genus Australopithecus, went extinct about 1.8 million years ago, long before
we arrived. Neither are we discussing Homo erectus, whose name does not necessarily
imply that primitive hominids practiced same-sex stimulation. We are strictly discussing
humans, Homo sapiens, the same species we are today.For 200,000 years, humans like us
lived like animals! They were just like us anatomically, and in terms of brain size.Then
suddenly, for no apparent reason, we became civilized.
Why, after 200,000 years of living like animals did we abruptly decide to start living
like we do today? Why did civilization come about?
Naturalistic evolutionists have an answer – the end of the most recent glacial period
of the present ice ages. Earth has experienced ice ages for about three million years. Not
all that time has been spent in cold temperatures. Rather, there have been about 20 cycles
of cold snaps and warm spells. 6 The cold snaps are called "glacial" periods. The warm
spells are called "interglacial" periods. About ten thousand years ago, the last glacial
period came to an end. Earth’s weather became much more pleasant. We entered into
one of the interglacial cycles. Fertile earth began to emerge in places that had once been
covered with ice and snow. People came out of their caves and discovered how to plant
crops.
Previously, during the glacial period, the growing seasons had been much shorter,
which retarded the development of agriculture. But now, the warm summer months
lasted long enough to grow and harvest food. Agricultural production became more
attractive than hunting and gathering. Instead of freezing in a tent, people could build a
house and live there indefinitely. Instead of battling wild animals with spears, people
could raise domesticated livestock. Instead of climbing trees to pick bananas, people
could reach down to dig up a potato. The agricultural life was much better.
The new way of obtaining food demanded a new society. People could no longer
live in small tribes. They needed to build cities and forts to defend their food stores from
thieves. They needed people to administer the cities. Kings were inaugurated for this
purpose. They needed priests to appease the gods so that bad weather did not destroy
their crops. Certain aspects of religion were invented for this purpose. They needed
some way to till the ground and harvest their crops. New tools were invented for that
purpose.
The end of the last glacial period of the ice ages made agriculture
possible. Agriculture made civilization possible. Civilization made technology possible,
and that is why we stopped living like animals.
That's the naturalists' explanation. But is this explanation adequate? One problem
with the view presented above is that the emergence of civilization really doesn’t
correspond with the end of the last glacial period as they claim. Civilization first began
about 3,500 BCE. But the last glacial period of the ice age had already ended some 4,500
years before in 8,000 BCE. Since 8,000 BCE, the average temperature of the earth has
not varied any more than two degrees Celsius. 7 Temperatures have been enormously
stable. The warm climate we have today is the same climate humans enjoyed in 8,000
BCE. Variations from the norm have been comparatively minimal. Did it really take
humankind a full 4,500 years to realize that agricultural civilization was the wave of the
future? And if so, if they only progressed toward civilization gradually over 4,500 years,
then why did all major elements of civilization emerge so suddenly in 3,500 BCE?
Another problem is that there have been other interglacial periods during humanity’s
existence. About 125,000 years ago, an interglacial period started that was similar to the
one we are currently living in. It afforded early humans about 10,000 years of warm
weather like ours before the earth was battered by another cold snap. 8 The humans who
were living then had all the same opportunities that we did. They had the same mild
climates we enjoy today. They basked in warm temperatures comparable to ours
today. Most importantly, they were humans just like we are today, with the same
anatomy, same brain size, and presumably the same intellectual capability. Why didn’t
they create civilization? In just 5,500 years, civilization has brought us from jungle
bunnies to computer geeks, from barbarians to cell phone junkies, from nomads to moon
walkers. 125,000 years ago, humans had a 10,000 year window to accomplish everything
we accomplished in just 5,500 years. Why didn’t they? If civilization was caused by the
end of an ice age, then why didn’t the last interglacial 125,000 years ago cause the
formation of human civilization back then? Moreover, there was a third interglacial
about 185,000 to 215,000 years ago, about the same time our species first arrived on the
scene. So humanity has been on the earth long enough to enjoy three long periods of
very agreeable weather, and yet civilization only managed to take root the third time
around. If civilization was made possible by an interglacial warm spell, then why didn’t
civilization emerge the other two times we enjoyed a respite from the ice ages?
If the end of the most recent glacial period did not clear the way for civilization, what
did? Is it possible that there was a change in our intelligence without a change in our
biology? Is it possible we suddenly got smarter without a change in our brain size? And
what might cause such a sudden change in intelligence?
Besides our intelligence, there is something else about humankind that makes us
unique. Above all other species, humans feel a need for justice, empathy, and love for
others. For 540 million years, complex life forms have been killing and eating each other
with no sense of remorse, and without concern for their ecological impact. Humans are
the first to question the morality of such a system.
So there are two sparks in the human mind that separate us from other animals –
intelligence and a moral conscience. Yet neither can be satisfactorily explained by the
naturalists, for our intelligence came about long after our brain reached its current size,
and our moral conscience defies the brutality necessary to succeed as a species under the
rules of survival of the fittest. So what caused us to gain intelligence and a moral
conscience?
Could it be – God? Did God impart intelligence and love upon us poor apes? For
200,000 years, we lived as animals. Then, civilization suddenly began about the same
time Adam and Eve are said to have been created. Perhaps the injection of Adam and
Eve's super-human God-created DNA into the gene pool of the cave-people is what made
the difference.
However, the idea of an Almighty Creator raises a host of difficult questions.
The God Paradox
According to traditional theology, the Almighty Creator God sees everything, knows
everything, created everything, and can do anything. This God is the God who created
earthquakes and tornadoes,
hurricanes and wildfires, sharks and mosquitoes, black widows and rattlesnakes, and
lions and tigers and bears, who rip out the necks of other animals and eat them
alive. And this God desires a better world, as the Prophet says:
"The wolf shall dwell with the lamb, and the leopard shall rest with the young goat,
and the calf and the lion and fatted livestock together, and a child shall lead them. The
cow and the bear shall graze together… the lion shall eat straw like a cow, toddlers will
play in snakes' dens… they shall not hurt nor destroy in all my holy mountain." 9
If God wanted non-violent animals, then why didn't he just create them non-violent
to begin with? Theologians have a ready answer: God did create them non-violent, but
the devil was thrown to earth, and therefore it is the devil who turned the animals to
violence. God wants us to see the consequences of the devil's rebellion, so that we can
learn a lesson from it. That way, when God reverts the world to non-violence in the
future, we humans, the crown of his creation, can better appreciate his creation.
This explanation is arguably plausible within the creationists' time frame of 6,000
years. However, it falls apart if the earth is much older. Animals have been ripping each
other's necks out and eating each other for 540 million years.
What was God's purpose in allowing this to continue for so long? Certainly it was
not to teach us a lesson about the devil's rebellion, for we did not even exist back
then. Was the lesson meant for trilobites and dimwitted reptiles?
Moreover, the universe is even older – standing at 13.7 billion years. If the human
species is the crown of creation, the only organism "made in the image of God," as
Genesis 1 says, 10 then why did it take God 13.7 billion years to create us? What could
delay God for 13.7 billion years? Was it really necessary to create failed experiments
like dinosaurs and dodo birds before us? Was it really necessary to waste 3 billion years
creating different types of bacteria and sea scum before creating the first true plant or
animal? Once the Almighty Creator God is separated from the creationist time frame of a
6,000 year-old earth, it leaves that God in a theological no-mans' land, because there is
little cover from the many arguments that can be launched against that God's very
existence.
Also, if we are in the image of God, then why do our bodies have the marks of
evolution on them? We have a worthless organ called an appendix, which is good for
nothing but exploding and killing us. We have a large amount of DNA that is repressed
and doesn't even code for anything. It just takes up space and nutrients inside our
cells. Why would the Almighty Creator God include irrelevant DNA in his code for
life? Moreover, if we are the crown of God's creation, and if he created color and beauty
for our enjoyment, then why can't we see colors in their full glory? We are more
dependent upon our eyes than upon any other sensory organ, yet the color capacity of our
eyes is inferior to that of goldfish and chickens!
Far from being highly evolved, our color vision is no better than that of the most
primitive of living reptiles, the crocodiles! We have only three cones for color, but many
inglamorous species have more. These include flies, jumping spiders, and a certain
shrimp-like praying mantis that lives at the bottom of the ocean. 11
If we are to assume a divine hand was involved in the making of our evolutionary
history, it would have to be more along the lines of Intelligent Interference than
Intelligent Design, for God did not perfectly create us, nor did God perfectly morph us
from the lower apes. The human body was not designed by an all-knowing God. Rather,
it came about haphazardly and imperfectly.
1 McDougall, Ian; Brown, Frank; Fleagle, John. Stratigraphic Placement and Age of
Modern Humans from Kibish, Ethiopia. 2005, Nature 433, p 733-736
2 Stringer, Chris. Human Evolution: Out of Ethiopia. 2003, Nature 423, p 692-694
3 Cann, Rebecca L; Stoneking, Mark; Wilson, Allan C. Mitochondrial DNA and
Human Evolution. 1987, Nature 325, p 31-36
4 Genesis 2:14
5 Jobling, Mark A; Hurles, Matthew E; Tyler-Smith, Chris. Human Evolutionary
Genetics: Origins, Peoples & Disease. 2004, Garland Publishing, New York, NY, p
260-261
6 Gould, Stephen Jay; Andrews, Peter; Barber, John; Benton, Michael; Collins,
Marianne; Janis, Christine; Kish, Ely; Morishima, Akio; Sepkoski, J John Jr; Stringer,
Christopher; Tibbles, Jean-Paul; Cox, Steve. The Book of Life: An Illustrated History of
the Evolution of Life on Earth. 2001, W W Norton & Co, New York, NY, p 208
7 Jouzel, J; Lorius, C; Petit, J R; Genthon, C; Barkov, N I; Kotlyakov, V M; Petrov,
V M. Vostok Ice Core: A Continuous Isotope Temperature Record Over the Last
Climatic Cycle (160,000 years). 1987, Nature 329, p 403-408
8 EPICA Community Members. Eight Glacial Cycles from an Antarctic Ice
Core. Jun 10, 2004, Nature 429, p 623-628
9 Isaiah 11:6-9
10 Genesis 1:27
11 Kelber, Almut; Vorobyev, Misha; Osorio, Daniel. Animal Color Vision –
Behavioral Tests and Physiological Concepts. 2003, Cambridge Philosophical Society,
Biological Reviews, Vol 78, Issue 1, p 83-85
Graphic of Bison Skull, Utah Museum of Natural History
Graphic of Crocodile, University of Wyoming Geological Museum
Ice Age Migration? New DNA study suggests people moved between continents before
recorded history
Rob Waugh
The Daily Mail
Tue, 27 Mar 2012 08:22 CD
© Unknown
Early man: A third of people in modern Europe show genetic traces of populations
from sub-Saharan Africa, leading researchers to conclude that people migrated
between the continents as early as 11,000 years ago.
People moved between Africa and Europe long before recorded history - and the
migrations might have been driven by Europeans moving south to 'weather' ice ages.
The genetic traces of long-forgotten migrations from Africa to Europe live on in
Europeans today.
A third of the genetic traces of sub-Saharan lineages in today's Europe come from
prehistory.
Researchers think that Europeans 'pushed south' by glaciers might have met with
populations from sub-Saharan Africa.
People moved between the continents as early as 11,000 years ago.
Geneticists used mitochondrial DNA to look for the traces of ancient migrations.
Mitochondrial DNA is passed directly from mother to child with no DNA from the father
- and tiny changes in the sequence come to 'characterise' different populations, which can
be used to trace movements and migrations of groups of humans in the past.
Large numbers of people moved between Africa and Europe during recent and welldocumented time periods such as the Roman Empire, the Arab conquest, and the slave
trade - but the researchers found that a third of sub-Saharan lineages came from before
these movements.
'It was very surprising to find that more than 35 percent of the sub-Saharan lineages in
Europe arrived during a period that ranged from more than 11,000 years ago to the
Roman Empire times,' said Dr. Antonio Salas of the University of Santiago de
Compostela and senior author of the study.
'The other 65% of European haplogroup L lineages arrived in more recent times.'
The authors explain that these contacts likely connected sub-Saharan Africa to Europe
not only via North Africa, but also directly by coastal routes.
Salas said that it still remains unknown why there was genetic flow between the Africa
and Europe in prehistoric times.
One possible scenario is that some bidirectional flow was promoted when the last
glaciation pushed some Europeans southward, until the glacier receded and populations
returned north.
In addition to tracing the genetic links of Africa and Europe back to prehistoric times,
Salas expects that their work will also help those individuals who want to learn more
about their own ancestry.