Download Environmental change and human evolution

3. Environmental Change, Human
Evolution and Our Movement Around
the World.
Rates of plate movement (cm/yr)
Gamma Ray bursts;
 Gamma ray bursts are very short wave
(high temperature) emissions usually
associated with major cosmic events –
supernovae and the formation of black
holes. They may last from milliseconds to
hours.
Jet Streams and the Circumpolar
Vortex
 The circumpolar vortex is a
permanent feature of the Earth’s
general circulation, but its size and
behaviour vary seasonally.
 It expands during the winter and
contracts in summer.
 Its edge is wave-like (Rossby
Waves). The number of waves may
vary as may their amplitudes.
 The edge is also marked by a very
strong temperature and pressure
gradient aloft, and because of this, a
core of very strong winds – the polar
front jet stream.
 The location of the jet stream has
huge consequences for our weather.
It controls air mass dominance, and
directs weather systems including
tornadoes.
There is also a subtropical jet stream which
tends to be less persistent. Jet streams are
also found in the southern hemisphere,
although less is known about them.
Cross-section of the lower atmosphere
(troposphere)
The situation during much of the winter
Environmental Change and
Human Evolution
 Last week we looked at the second
major control of Earth environment –
solar radiation. Whereas climate
appears on the scale of millions of
years to be determined by plate
tectonics, the finer variations , only
discernible in the relatively recent
past, seem to be related to both the
supply and processing of solar
energy.
 Whatever the cause, one reflection of
environmental change is the fossil record.
It reflects evolution and the sometime
drastic response to climatic shifts. The
record is really only good for the last
600,000 years, since the Cambrian. The
apparently rapid diversification of life from
that time is called the Cambrian
Exlplosion.
 If we take a look at the record of
biodiversity through the last 600 million
years, we see that there is a logarithmic
increase in species through time. However,
the increase is not a smooth one. There
are five episodes at which biodiversity is
drastically reduced. These are called mass
extinctions. The largest occurred at the
end of the Permian when perhaps 90% of
species went extinction. The latest one, at
the Cretaceous-Tertiary (K-T) boundary,
had about a 70% loss.
 All except the K-T extinction appear to
have been caused by a complicated set of
mechanisms determined by plate
tectonics. At the K-T boundary we can
make a good case for asteroid impact as
the cause.
 That boundary is very significant for us.
From that time we see modernization of
the biosphere – the rise of mammals,
insects, birds, flowering plants, etc.
Evolution of animals v= birds, BB= placental
mammals
Evolution of plants and animals after the K-T
event . angiosperms=flowering plants,
ungulates=hoofed animals.
 One order that evolved was the Primate Order.
We evolved from these as the family
Hominideae, in the genus Homo. Our species?
Homo sapiens.
 This evolutionary process appears to have been
largely driven by the environmental changes
from the mid-Tertiary.
 As climate cooled, tropical rainforests contracted
and were replaced by grasslands and deserts in
the tropical zone. Until the mid-Tertiary, much of
the Arctic archipelago had forest cover.
Metasequoia (redwood) on Axel Heiberg
Island)
Evolution of primates.
 Our ancestors, the primates, were once widely
distributed, but they have always been largely
tropical fauna and largely arboreal.
 Thus, climatic cooling caused their contraction
into the current tropical zone and the loss of
forest cover stimulated some to ground dwelling.
This required the exploitation of different
resources so there were morphological and
behavioural changes. The most obvious of the
latter was the development of bipedalism.
Our Lineage
Early primate
Current distribution of primates
Changes in the distribution of primates in
the lemur family through the Tertiary. Red
indicates fossil locations.
Ring-tailed lemurs
We share about 96% of our genes with
gorillas
And 98% with chimpanzees
Our African Origins;
 Although climatic changes through the
Tertiary were global, the evolution of
hominids was not . It was confined to
Africa. Was this because Africa had
a suite of environmental conditions
that only allowed our evolution to
occur there or was it fortuitous?
 Before we get into the African question, let’s
consider how speciation is thought to work.
 Speciation seems to occur when subpopulations get isolated. How does
reproductive isolation occur?
 (a) geographic (allopatric)isolation –
rivers, seas, mountains, deserts, etc.
 (b) ecological or temporal isolation– subpopulations develop different habitat
preferences ( ecological isolation), or have
different mating schedules (temporal
isolation) etc.
 Why Africa?
 (a) increasing aridity and seasonality
leads to major contraction of once
extensive rainforest
 (b) expansion of tropical savanna
 (c) creation of isolated forest
fragments in a ‘sea’ of savanna
 (d) isolation facilitated by uplift and
volcanism in the Rift Valley system
 (e) high nutrient status of soils means high
primary productivity
 (f) strong coevolutionary associations
between herbivores, carnivores and
scavengers.
 Massive environmental changes then
triggered life-style and dietary changes in
hominids. Obvious consequences included
bipedalism and an increase in body (and
brain) size.
 These responses are evident with both
proto humans and all species of Homo.
The proto humans are usually considered
to be the Australopithecenes. A.
afarensis , Lucy, is the best known of
these. Recently, a new genus,
Ardipithecus, has been recognized. It
predates the Australopithecines.
 About 2 MYBP the first species of Homo
(H. habilis) appeared, then H. erectus.
Bipedalism;
 Bipedalism may be seen as a
response the change from being
arboreal to a life on the ground.
 Benefits – increasing mobility,
increasing travel range, arms no
longer required for locomotion,
height gain, tool preparation,
thermoregulation, etc.
Changes from knuckle-walking to full
bipedalism
Increasing Size;
 Increasing size was associated with
broadened dietary niche, increased
home range, increased mobility,
change in scavenging activities,
increase in thermoregulatory
efficiency, increased longevity,
slower reproductive rates,
increasing brain size, increasing
sociality.
Changes in Brain Size;
Out of Africa;
 Humans remained African until about
1.5 MYBP. Since then there have
been two pulses of emigration – the
first by Homo erectus after 1.5 MYBP
and the second by H. sapiens after
about 100,000 YBP.
 Both of these exoduses involved
movement northward into areas that
were climatically more severe. How
and why could this happen?
 Did our two diasporas have any
obvious environmental consequences?
 There’s no evidence that Homo erectus
did, but the coincidence of the arrival of H.
sapiens and a mass extinction event
suggests that the first impact of modern
humans was both selective and massive –
the extinction of late-Pleistocene
megafauna.
 Was it technological? Was it a
population issue?
Late Pleistocene Megafaunal Extinction;
 50,000 years ago all of the continents had
a rich megafauna (animals larger than 44
kg), but by 10,000 BP , most large
mammal genera had disappeared except
in Africa.
 The pattern of extinction is peculiar;
 The continents were not impacted equally.
N and S America, and Australia lost almost
all of their megafauna. In Eurasia losses
were less. In Africa there was little loss.
 The timing of extinction varies between
continents.
 In N and S America, extinctions took place
mostly between 12,000 and 10,000 YBP.
 In Eurasia, the extinctions started earlier,
but finished at about the same time as in
the Americas.
 In Australia, extinctions occurred early,
about 50,000 to 45,000 YBP.
North American Megafauna
Australian Megafauna
 So explanations for these peculiarities need to
accommodate the following questions;
 A. Why were extinctions confined to
large animals?
 B. Why were extinctions in Australia
large and early?
 C. Why were extinctions in N and S
America large and late?
 D. Why were extinctions in Eurasia
small and protracted?
 E. Why was there little extinction in
Africa
 Most explanation revolve around (a)
environmental change, and (b) human
impact.
 Environmental Change – most
hypotheses involve the large and massive
climate changes that occurred at the
Pleistocene/Holocene transition (ice age to
interglacial). This may work for some
places, but not for all. A strong case can be
made only if there is a clear synchronicity
between the time of climate change and
the extinction event.
Extinction of Irish Elk and Mammoth in
Eurasia
 Human Impact – Were stone age people around
in large enough numbers and with a sufficiently
sophisticated technology to eliminate huge
numbers of animals over a short time ? Is there
some coincidence between the time of arrival of
people and the extinction phase?
 Paleolithic hunters had a fairly sophisticated
stone tool technology and strong social
organization. There is evidence from kill sites of
their effectiveness as hunters.
Pattern of Extinction in North America
Clovis Points
Cave Art – Altamira, Spain
Cave Art – Lascaux, France.
Mammoth and mastodon remains are often
found in wet sites – planning or preservation
bias?
Mastodon bone house – Mizhirich,
Ukraine
 In N and S America and in Australia extinctions
coincide with the arrival and spread of humans,
but this is not true of Eurasia and Africa.
 In N and S America, the overkill or blitzkrieg
hypothesis has been proposed.
 The lack of pronounced extinction in Eurasia
and Africa and the massive extinction in N and S
America and Australia has been attributed to
prey naivete. The long association of people
with large animals in Europe and Africa meant
that those animals were wary of contact. In the
other continents prey was naïve and easily
eliminated.
 So, the first major environmental impact of
humans may have been the mass
extinction event in the Late
Pleistocene/Early Holocene.
 It’s also from this time that we have the
first evidence of plant and animal
domestication.
 Domestication brought food surpluses,
allowed population increase,
urbanization, stratification of society –
the emergence of civilization.
We’ll look at plant and animal
domestication and its
consequences next week.