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Professor Paul Turnbull
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Life has now been evolving for probably near four billion years on earth. We tend to think
of the evolution of life as a process resulting in the appearance on this planet of ever more
complex organisms. However, while it is true that there are indeed many species of complex
organisms on earth, we would do well to see that the emergence of biological complexity is
due solely to it having given particular species of plants and animals’ advantages favouring
their ability to survive and reproduce within the environment they inhabit.
When we look at the natural world around us, we find that there are many simple
organisms that have remained unchanged for many millions of years. The archaebacteria
discussed in our previous lecture are a good example of lack of complexity being
evolutionarily advantageous. These very simple organisms are well adapted to living and
reproducing deep in the earth much as they did 3.5 billion years ago. Individual
archaebacteria exhibiting variations from their simple form would probably not survive to
reproduce, unless the variation they exhibited gave them a clear advantage – by enabling
them, for example, to extract more energy, for example, from elements and chemical
compounds in their surrounding environment.
The archaebacteria are among the simplest of what scientists calculate are around 70,000
species of bacteria on earth, that at this moment number some five million trillion trillion
individual organisms on this planet. These organisms are both very simple and very well
adapted to their environments. Indeed, as we have discovered in the case of streptococci and
other bacteria harmful to humans, their simplicity allows them quickly to evolve so that they
can survive our best attempts to prevent their reproduction in the human body by
antibiotics.
Besides bacteria, there are many others species within the animal kingdom that are both
relatively simple in form and highly successful in evolutionary terms. For example, copepods
are tiny crustaceans found in all the earth’s seas and virtually every freshwater environment
on earth. They exist in greater numbers than any other class of animals.
The vast number of simple organisms that have lived successfully on earth over the past
3.8 billion years underscores an important point made by the late Stephen Jay Gould. This is
“that there is no progress in evolution.” As Gould explained in interview, a transcription of
which was published in 1995:
The fact of evolutionary change through time doesn't represent progress, as
we know it. Progress is not inevitable. Much of evolution is downward in terms
of morphological complexity, rather than upward. We're not marching toward
some greater thing. The actual history of life is awfully damn curious in the
light of our usual expectation that there's some predictable drive toward a
generally increasing complexity in time. If that's so, life certainly took its time
about it: five-sixths of the history of life is the story of single-celled creatures
only.1
1
John Brockman, The Third Culture : Beyond the Scientific Revolution (London: Simon & Schuster, 1996), p.
52.
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Gould went on the argue in this essay – which I have included among your recommended
further readings for this week - that we humans have an ingrained tendency to focus on the
very small part of the history of life over the past 3.8 billion or so years that has involved
some measure of progress. What is more, in doing so, we have more or less implicitly
assuming that this is what evolution is heading towards. We will return to discuss this
tendency of ours to see evolution in progressive terms, and how it has shaped our
understanding of the course of human history especially over the last 250 or so years,
towards the end of this course. For the moment, however, I want to talk about briefly about
the course of evolution on earth, and what we can safely say about the emergence of
biologically complex species such as our own.
As David Christian observes when introducing his chapter on Maps of Time on the
evolution of life and our biosphere, we need to keep in mind Gould’s point that organisms
that are more complex than those which existed 3.8 billion or so years ago have evolved
because it has proved to give these species clear advantages within the environmental
conditions in which they live and reproduce. Progress is not inevitable, and there are many
examples of living beings becoming less complex because it gave them evolutionary
advantages. 2
The first forms of what we generally regard as living organisms to appear on earth were
most likely types of archaebacteria, or what many biologists call eubacteria.
Eubacteria are very simple organisms that like archaebacteria have no cellular nucleus.
The eubacteria we most commonly encounter today are blue green algae and streptococci
bacteria. Blue green algae flourished in earth’s early history, but are comparatively rare
today. When we now encounter them it is commonly in warm, still water where there are
high levels of phosphorus and nitrogen. Often we have provided the perfect environment
for these organisms by our polluting of coastal seas and inland waterways.
Figure 1: "A large bloom of cyanobacteria, more commonly known as blue-green algae, spread across a lake in
Guatemala, with their green filaments and strands that are clearly visible in this simulated-natural-colour image."
NASA image. Public domain,
http://commons.wikimedia.org/wiki/File:Harmful_Bloom_in_Lake_Atitl%C3%A1n,_Guatemala.jpg
2
David Christian, Maps of Time: An Introduction to Big History (Berkeley: California University Press, 2005),
pp. 107-8.
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The eubacteria called streptococci can prove dangerous to humans by causing infections if
they enter the body through cuts in the skin or though contaminated food. However, other
species of eubacteria have variously contributed to our biological and cultural evolution.
Much of the oxygen in our atmosphere originated from ancient eubacteria using sunlight to
convert carbon dioxide into energy rich compounds - the process known as photosynthesis.
In the course of creating sugars and other compounds from carbon dioxide, eubacteria
gradually released enough oxygen into the atmosphere to allow the evolution of oxygen
breathing organisms.
Humans have evolved to benefit from close symbiotic relations with simple bacteria.
Their presence in the digestive system has enabled some populations over successive
generations to become better adapted to digesting sources of food that were plentiful in
their environment. Humans have also exploited eubacteria over the past 12,000 years.
These organisms are essential to the making of cheese, wine and beer - rich sources of
energy that have enhanced the capabilities of many human groups to survive and reproduce
in climates marked by lengthy cold seasons.
Without the emergence of bacteria capable of photosynthesis we would not have
developed the forms of social organization that have greatly shaped the course of our
history over the past 12,000 years. For these forms of social organization have only been
possible because we have been able to secure much of the energy our bodies require from
eating cultivated plants that provide us with the biochemical source of the energy we
require by photosynthesis.
But more to the point, without Eubacteria we would not exist. Because of their
capability to create and store sources of energy through by photosynthesis, these ancient
simple organisms were to become the ancestors of the majority of the forms that life on
earth has assumed over the past 3.8 billion years.
Scientists believe that until around 2 billion years ago, eubacteria were what biologists
term prokaryotic cells – that is, they were cells with no cellular nucleus and possessed DNA
in the form of a single circular chromosome called a plasmid.
Figure 2: basic diagram on the elements
conforming a prokaryote cell by Mariana Ruiz.
Public
domain,
http://commons.wikimedia.org/wiki/File:Prok
aryote_cell_diagram.svg
These very simple organisms
reproduced by a process in which
their single chromosome split into
two and moved to opposite ends of
the cell. The middle of the cell
membrane then contracted to create
two compartments, each with a copy
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of the cell’s genetic information. The cell then formed two cells, a process biologists call
“binary fission.”
Figure 3: "Three types of cell reproduction are compared: the relatively simple Binary fission and two more
complicated types that either involve mitosis or meiosis." By J.W. Schmidt, GDFL license,
http://commons.wikimedia.org/wiki/File:Three_cell_growth_types.pn
However, it appears that around 1.7 billion years ago, some types of prokaryotic eubacteria
joined together mixing their genetic material so that they came to possess a cell structure
with a nucleus in which DNA was now located in a number of chromosomes. Organisms
with this cell structure, called a eukaryotic cell structure, have a much-enhanced ability to
reproduce their essential form (by mitosis or meiosis- as illustrated above). They could
reproduce through a process in which each cell took in nutrients and duplicated its
chromosomes, after which the cell’s nucleus divided and two separate cells were formed. It
was process that also allowed cells that exhibited slight variations better fitting them to
their environment to proliferate. As David Christian observes, this “first step towards sexual
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reproduction, had a profound impact on the pace of evolutionary change, for it gave natural
selection a greater variety of bodies to chose from in each generation.” 3
We gain a clear idea of how successful this process of cellular replication was when we
consider that all of the plants, animals and fungi on earth have eukaryotic cell structures.
Some prokaryotic-celled organisms – blue green algae for example - gained clear
evolutionary advantages from being organisms that clustered together in large numbers.
However, the interaction between individual prokaryotic organisms that clustered together
was limited to enhancing the survival of individual organisms.
By contrast, early eukaryotic-celled organisms were to evolve to develop closer,
symbiotic ties that maximized the survival of their common genetic information.
By about 600 million years ago multi-cellular organisms had evolved that possessed
different kinds of cells performing differing functions that could all be replicated by
creating a single cell containing the genetic information necessary for all these different cells
to be reproduced.
The appearance of multi-cellular organisms further accelerated the pace of evolution.
Such is the abundance of fossils of a huge number of different kinds of multi-cellular
organisms in rock strata dating from the beginning of what scientists call the Cambrian era
(around 543 million years ago) that many scientists believe that what occurred at this time
was an explosion of diversity which created the ancestral forms of most of the animals now
living on earth.
Prominent amongst fossils of the early Cambrian era are various kinds of animals with
protective calcium carbonate shells such as Trilobites and primitive forms of starfish.
Some of the most remarkable fossils of the Cambrian era, which have been dated to
around 550 million years ago, are to be found in a region of the Rocky Mountains in British
Columbia known as the Burgess Shale Formation.4 In a fascinating book he published in
1989, called Wonderful Life, the evolutionary biologist Stephen Jay Gould argued that the
diversity of fossils in the Burgess Shale suggest the Cambrian era was a period of
“evolutionary experiment” in which many forms of organism emerged only to become
extinct, but in the process gave rise to the essentials forms of modern animals. Other
scientists have disagreed with Gould on the issue of how diverse the forms of these
Cambrian organisms were, believing that virtually all of the Burgess Shale fossils are related
to modern animals.
3
Ibid., 117.
See
The
Burgess Shale, Smithsonian
http://paleobiology.si.edu/burgess/index.html
4
National
Museum
of
Natural
History,
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Figure 4: Picture of a Paradoxides davidis
trilobite. © Sam Gon III. Reproduced
under Creative Commons license,
http://commons.wikimedia.org/wiki/File
:Paradoxides_davidis.jpg
The organisms of the Cambrian
lived in water. It was not until
around 510 million years ago that
the first multi-cellular organisms
appeared on the earth’s surface.
These first multi-cellular
organisms were plants that were
quite similar to modern mosses
and ferns.
Within another 100 million years,
these plants had given rise to the
first seed bearing trees that were
types of cycads and conifers.
Figure
5:
Land
distribution
during
late
Cambrian.
http://commons.wikimedia.org/wiki/File:Late_Cambrian_%28514_Ma%29.jpg
Image
GFDL
licensed,
Much of the coal we have exploited for energy over the past 250 years was originally the
remains of these trees, which grew in vast forests.
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The first land animals so far discovered in the fossil record have been dated to between
396 and 405 million years ago. The earliest of these are primitive arthropods.
One of the oldest fossils so far discovered are the remains form of insect probably related
to the modern silverfish, found in ancient red sandstone at Rhynie in Scotland.
Animals possessing internal skeletons made of bones and cartilages radiating off a
backbone, rather than exoskeletons like insects and other arthropods, are known as
vertebrates.
The first vertebrates to appear on earth were probably creatures rather like worms;
although the earliest vertebrate fossils discovered to date are primitive kinds of shark and
fish. Probably the earliest of these fossils is that of an animal called Haikouichthys, which is
thought to have lived around 350 million years ago. The fossil remains we have of this
creature suggest that it had eyes, a brain and rudimentary vertebrae.
Figure 6: Reconstruction of Haikouichthys ercaicunensis. Based on actual fossil evidence. Public domain image,
http://commons.wikimedia.org/wiki/File:Haikouichthys4.png
By around 400 to 390 million years ago - towards the end of what is called the Silurian
period - earth’s waters contained a rich variety of fish, including a group of bony fishes
called the Choanichthyes, which are thought to be the ancestors of those vertebrates that
came to live on land.
The Choanichthyes are thought to be the ancestors of land living vertebrates on the basis
of studying eight species of these fish still living today. Of these eight species of fish, six are
what are commonly known as lungfish, because with the exception of one of the six species,
the Australian lungfish, they possess and breath through two lungs.
Unlike the species of lungfish found in South America and Africa, the Australian
lungfish has only one lung and can breath through either that lung or its gills. The other
species can only breath through their lungs.
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Figure 7: Australian Lungfish (Neoceratodus forsteri), Creative Commons licensed
http://commons.wikimedia.org/wiki/File:Queensland_Lungfish_%28Neoceratodus_forsteri%29.jpg
)
image,
These lungfish are highly significant in evolutionary terms because studying their genes and
embryology promises to reveal how the bodily features of these fish evolved so that their
ancestors became adapted to life out of water. The Australian lungfish, for example, can not
only breathe through its lung, but also travel over ten metres on dry land.
These remarkable creatures, however, are in danger of extinction. The African lungfish
has long been hunted for food, but is now caught in numbers beyond the species rate of
reproduction. The Australian lungfish is now found mostly here in Queensland in the
Burnett and Mary rivers, and is at grave risk of its spawning grounds being destroyed. There
are probably no more than 10,000 of these fish left, even though they have been protected
for over a century.
The lungfish’s ability to travel across land and encase itself in mud and mucus on the
bottom of the water in which it lives during times of drought highlight another important
evolutionary feature occurring with the development of vertebrae: nervous systems.
In Maps of Time, David Christian draws to the work of the biological anthropologist
Terence Deacon. Deacon has suggested that what distinguishes organisms with nervous
systems from more primitive forms of life is that they have the capability to sense the
external world in ways that enable them to react in more complex ways to their external
environment.5 Lungfish, for example, can sense that the water in which they live is drying up
and survive by creating protective mud chambers. Of course, primitive vertebrates do not
possess consciousness, as we understand it; but what we can see in these animals is the early
part of an evolutionary path resulting in the appearance of animal species with successively
larger and more complex brains. These larger and more complex brains that eventually
5
See Christian, Maps of Time: An Introduction to Big History, pp. 123-4.
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reached the point of giving these species forms of consciousness that enabled cooperative
behaviour.
Terence Deacon argues that what distinguishes the human species is our development of
symbolic communication.6 By our ability to convey complex information about the world
and ourselves by language we have radically changed the environmental conditions in which
the human brain has evolved. Even so, the human brain, at its most basic level of reacting to
environmental stimuli, still has much in common with that of the lungfish.
Fish like the modern lungfish were the ancestors of amphibians, which in turn were the
ancestors of reptiles, which first appeared around 320 million years ago, marking a further
important stage in the evolution of land animals. For unlike the amphibians, reptiles could
reproduce by produce offspring protected within eggs that did not have to be laid in water.
Figure 8: Fossil of Pelorocephalus tunuyaensis, a fossil amphibian, public domain image,
http://commons.wikimedia.org/wiki/File:Pelorocephalus_tunuyaensis.JPG
When we think of prehistoric reptiles, we invariably think of the dinosaurs. Dinosaurs,
incidentally, were given that name by Richard Owen, the great nineteenth century British
comparative anatomist. Owen derived the word dinosaur from combining the ancient
Greek words for terror or power and lizard. Some dinosaurs were among the largest
carnivores ever to live on earth. However, as we now know from studying the fossil record of
these reptiles, many dinosaurs were quite small, and the largest were docile plant eaters.
Indeed, what is perhaps most interesting about the dinosaurs is that they were remarkably
diverse kinds of animals that proved capable of adapting to a wide variety of environments.
6
See especially Terrence W. Deacon, The Symbolic Species : The Co-Evolution of Language and the Brain (New
York: W.W. Norton, 1997).
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Figure 9: model of an Iguanodon modelled after reconstruction by Richard Owen. The model was used for a
dinner in Owen's honour at London's Great Exhibition in 1854. public domain image,
http://commons.wikimedia.org/wiki/File:Crystal_palace_iguanodon.jpg
The first dinosaurs shared the earth with the early ancestors of mammals. These protomammals were small, nocturnal animals, which looked quite like modern shrews. These
creatures were to be the ancestors of modern marsupials and mammals. They evolved in
various ways largely as a consequence of the disappearance of the dinosaurs between 64 and
66 million years ago.
Figure 10: illustration of Gobiconodon by Pavel Riha, Creative Commons licensed
image, http://commons.wikimedia.org/wiki/File:Gobiconodon.jpg. Gobiconodon
live between 140 and 100 million years BP.
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Why did the dinosaurs become extinct? In 1980, a team of scientists at the University of
California at Berkeley theorised that they may have been the victims of catastrophic event,
having discovered that rock strata in various parts of the world formed between 64 and 66
millions years ago contained large amounts of the metal iridium. They further noted that
the concentration of iridium in these rock strata was similar to that found in the remains of
meteorites hitting the surface of the earth. This led them to conclude that a meteor as large
as 10 kilometres in diameter may have hit the earth. Now if this were so, it would have
crashed on our earth with such force as to produce a crater at least 100 kilometres in
diameter. The force of the impact would have explosively sent millions of tonnes of
liquidized rock and other debris into the atmosphere, only to rain down upon the surface of
the earth. Smaller particles from the explosion would have choked the earth’s atmosphere,
first causing the temperature of the earth to drop by blocking the rays of the sun. As the dust
settled on the earth it would have absorbed the sun’s heat, killing much of the vegetation
that survived the initial lowering of earth’s surface temperature.
Dinosaurs and many other animal species would have been unable to survive this global
catastrophe.
What gave substantial eight to this theory was investigation of an ancient impact crater
discovered in the late 1970s near the town of Chicxulub in the Yucatán peninsula of Mexico
(Figure 18.3). Drilling down into the Chicxulub crater, geologists found igneous rock at 380
meters containing high levels of iridium, which could be dated to around 65 million years
ago.
Figure 11: Painting by Donald E. Davis depicting an asteroid slamming into tropical,
shallow seas of the sulfur-rich Yucatan Peninsula in what is today southeast Mexico.
NASA
image
in
public
domain,
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Even so, not all scientists are convinced. Some have questioned the dating of the rock
samples obtained from the Chicxulub Crater. Other scientists have questioned whether any
meteoric impact was the cause of the dinosaurs extinction, arguing that other kinds of
reptiles, notably frogs, were more likely to have been completely wiped out by the
environmental conditions prevailing after a meteor 10 or more kilometres in diameter
crashed into the surface of the earth. Yet the fossil record shows that frogs survived.
Supporters of meteorically caused extinction have suggested that the species of frogs
that survived may have lived in underground habitats, like early mammals – which not only
survived the dinosaurs, but evolved so as to occupy many of the ecological niches they
occupied.
We human beings are members of the biological order known as primates, a word derived
from the Latin that means “of the first, or prime rank.”
Today most primates are to found living in the tropic and sub-tropical regions of Africa,
Asia and South America. Though we humans have been remarkably successful in colonizing
every continent on earth.
There are two distinct groups of primates, called prosimians and simians. As the names
imply, the prosimians are the oldest group in evolutionary terms. The prosimians include
lemurs, lorises and tarsiers. Fossil evidence suggests that the first primates appeared around
60 millions years ago.
Figure 12: engraving of prosimians from Book 8 of the 4th edition of Meyers
Konversationslexikon
(1885-90).
Public
domain
image,
http://commons.wikimedia.org/wiki/File:Meyers b8 s0008a.jpg
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Ancient prosimians were rather like squirrels, possessing hands and feet well adapted to
climbing trees and manipulating objects. Importantly, they probably also had the beginnings
of stereoscopic vision, which gave them clear advantages in securing food and escaping
predators. By around 33 million years ago, there were many kinds of animals with the
characteristics of modern lemurs, lorises and tarsiers. Where they differed from later forms
of these creatures was that their brains were smaller, they had more pronounced snouts and
did not move with their bodies erect as many as modern prosimians do.
We are simians. So too are monkeys and apes. In fact our evolutionary ancestors were
monkeys that evolved from prosimians at some time between 35 and 23 million years ago.
One of the earliest forms of monkey for which we have fossil evidence is a creature
known as Aegyptopithecus. It was a small, fruit-eating animal that lived in trees.
Anatomically this creature resembled a lemur, but interestingly it had the same number of
teeth as adult gorillas and humans.
Figure 13: drawing of Aegyptopithecus by mateus zica in October 2005 , image used GFDL
license, http://commons.wikimedia.org/wiki/File:Aegyptopithecus_ZICA.png
Between 35 and 23 million years ago, the movement of the earth’s tectonic plates was well
underway. Europe and North America had become distinct continents, and India had
reached the Asian continent, causing it to rise up forming the Himalayas.
These major geological changes had a profound effect on the earth’s climate. Northern
regions became colder, causing primate species to become concentrated in tropical regions.
By around 15 millions years ago, the movement of the earth’s plates had created various new
mountain ranges that caused further climate changes.
In the tropical regions where most primates now lived, open woodlands and dry
savannahs had gradually replaced dense forests.
This was greatly to shape the course of primate evolution. By around 20 million years
ago the first apes had appeared in Africa. Within another six million years, competition for
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scarcer food resources had led to one group, given the name Dryopithecines, to colonize
woods fringing grasslands across the southern regions of the Eurasian landmass.
By around 8 to 9 million years ago, the earth’s northern hemisphere had cooled, causing
the extinction of the Dryopithecines and various other species of apes apart from those that
had either remained in Africa, or were able to migrate back into Africa or into more
hospitable southern regions of Asia. The Dryopithecines surviving in Africa evolved into
two groups. One included the ancestors of modern gorillas; the other that of chimpanzees,
bonobos and the early hominids from whom we have evolved.
Geneticists have found, incidentally, that chimpanzees and humans are only one percent
different in their genetic makeup. However, that one percent difference may explain much
about the evolutionary divergence between us. Among their similarities, both chimpanzees
and humans have genes equipping them with sophisticated senses of smell that they use to
find food and select mates. They also share genes giving them the metabolic ability to digest
protein from meat easily, thus greatly increasing their range of potential food sources. But it
would appear that humans possess a gene that provides them with a richer frequency of
hearing that may have played an important part in humans’ development of language and
symbolic communication. Of course we have no way of proving whether this genetic
difference was influential in shaping the course of human evolution away from that of our
closest relatives in the order of primates; but is a small indication of the profound
differences that can exist between species that differ only slightly in their genetic makeup.
More will be said about the course of human evolution in our next lecture. Here, let me
conclude by saying something about the biologist James Lovelock’s theory that all earth’s
myriad organisms are causally interconnected in such rich and complex ways that we would
do well to regard them as one superorganic entity that ensures earth’s environment can
sustain the diversity and successful reproduction of life.
Figure 14: 2005 photograph of James Lovelock, scientist and author best
known for the Gaia hypothesis. Photograph by Bruno Comby. Creative
Commons
licensed,
http://commons.wikimedia.org/wiki/File:James_Lovelock_in_2005.jpghttp://
commons.wikimedia.org/wiki/File:James_Lovelock_in_2005.jpg
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At the core of Lovelock’s theory, which he first put forward in the 1960s, and described in
detail in a widely read book published in 1975, is the idea that evolution is actually a process
of co-evolution, in which those organisms that live and successfully reproduce do so
because together they contribute to making the environment favourable to all. Lovelock
illustrated his reasoning by inviting his readers to imagine a hypothetical world in which
there were only two forms of life: black and white coloured daisies.
On this daisy world, Lovelock argued, the black daisies would absorb sunlight and warm the
planet, while the white daises would check the rise in temperature by reflecting light. Too
many black daises would cause the temperature of the planet to rise to the point it would
threaten their survival, but
favour the survival of white
daises. Two many white
daises would cool the world
so that it favoured the black
daises. Lovelock took care
to point out that earth’s
biosphere was clearly much
more complex than this
hypothetical daisy world;
but he argued that close
examination would confirm
that the billions of different
life-forms on earth likewise
kept
the
planet’s
environment in a state of
equilibrium, rendering it the
best possible world for all its
living organisms.
Figure 15: "Outputs of a Daisyworld climate simulation. The used generator is from the freely useable
online, java based environment simulator "swingdaisyball" from Ginger Booth (http://gingerbooth.com), a
teacher at the math department at Yale.
Lovelock further argued that humanity had long been the greatest risk to this universally
beneficial equilibrium because of our capacity to exploit other organisms for what we have
perceived as being in our best interests. Indeed, Lovelock warned that since the emergence
of industrial society, we have caused such disequilibrium within earth’s biosphere that it
may already so damaged as the threaten the continued existence of many thousands of
species, including our own.
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Lovelock’s theory has gained many adherents worldwide since the 1970s; but there have
also been many critics. Some critics have made much of Lovelock calling his theory the
“Gaia hypothesis”, Gaia being the name of the ancient Greek’s primordial earth goddess.
They have dismissed Lovelock ideas and arguments as a form of secular religion, not
rigorous science. Other critics, including the evolutionary biologist Stephen Jay Gould,
have found the analogy between the fictitious daisy world and earth’s biosphere
unconvincing. They have argued that Lovelock did not offer any convincing empirical
proof of the processes by which the earth’s biosphere is a self-regulating equilibrium.
Lovelock’s response has been to argue that such processes cannot be easily empirically
proved or disproved. Rather, he has maintained that what he has offered is a hypothesis
similar to those formulated by cosmologists in trying to understand the complex
interconnections between different phenomena in the development of the universe. And
like hypotheses in fields such as astronomical and particle physics, his hypothesis concerning
earth’s biosphere is provisional, while nonetheless making powerful sense of observable
regularities in nature.
Regardless of what strengths and weaknesses Lovelock’s “Gaia hypothesis” has, it had
been very influential in focusing attention on the causal relations between geological
change, climate and organic evolution on earth. Lovelock has done much to stimulate
interest in and debate on how humans have affected our planet’s biosphere, and also in how
changes in ecological conditions have been important determinants of the nature and pace
of the evolution of life on earth.
One may disagree with Lovelock; but since the 1970s his work has proved an important
catalyst for our coming to see a pattern within the history of our species. As David Christian
points out towards the close of the chapter of Maps of Time set as required reading for this
week’s classes, our history can be helpfully summarized “in a formula: migration,
innovation, growth, over-exploitation, decline, and stabilization (MIGODS).” We will
consider this formulae and its applicability to human history at various points in coming
weeks, as we trace the biological and social evolution of our species. 7
Bibliography
Brockman, John. The Third Culture : Beyond the Scientific Revolution. London: Simon &
Schuster, 1996.
Christian, David. Maps of Time: An Introduction to Big History. Berkeley: California
University Press, 2005.
Deacon, Terrence W. The Symbolic Species : The Co-Evolution of Language and the Brain.
New York: W.W. Norton, 1997.
7
Christian, Maps of Time: An Introduction to Big History, p. 133.