Climate Change and Human Evolution

The evolutionary surge that led to Homo habilis began during the transition between the Pliocene and Pleistocene Epochs around 2.5 million years ago when climates were becoming cooler and drier.  All later species of Homo evolved during the Pleistocene (2,600,000-11,700 years ago).  This was generally a time of more extreme world cooling and recurrent glaciations (ice ages).  During the coldest periods,  global temperatures dropped by about 9º F. (5º C.) and long-lasting ice sheets spread out from the poles and high mountains.  Between the four or more major glaciations of the Pleistocene, there were interglacial warming periods with temperatures similar to now.  Both the glacials and the interglacials lasted tens of thousands of years.  Very likely, we are now in an interglacial that began 10,000-12,000 years ago. 

Changing Climate Patterns of the Pleistocene Epoch
Epoch Climate Years Ago *
Regional Name for Climate Stage
European Alps North America
Holocene interglacial 10,000 to the present    
Pleistocene glacial 110-70,000 to 12-10,000    Würm    Wisconsin
interglacial 130-125,000 to 110,000    Riss-Würm   Sangamon
glacial 200,000 to 130-125,000    Riss    Illinoian
interglacial 380-300,000 to 200,000    Mindel-Riss    Yarmouth **
glacial 455-410,000 to 380-300,000    Mindel    Kansan **
interglacial 620,000 to 455-410,000    Günz-Mindel    Aftonian **
glacial 680,000 to 620,000    Günz    Nebraskan **
Evidence of glacial and interglacial episodes during the early Pleistocene
is less clear.  However, it is believed that there were 2 or more glacials with
intervening interglacials.  Between the Pleistocene and the preceding
Pliocene Epoch was a long period of gradual cooling.
Pliocene 5.3-2.6 million years ago--mostly warmer conditions than the Pleistocene
*   These time ranges are approximations and do not reflect the fact that temperature changes between glacials and interglacials usually occurred over thousands of
(principal sources: P. Gibbard and T. Van Kolfschoten (2004) "The Pleistocene and Holocene Epochs", ch. 22, in F. M. Gradstein, A Geologic Time
(2004), Cambridge University Press)
**  The use of pre-Illinoan climate stage designations for North America is controversial because the terms have been used inconsistently to describe different climatic
     events.  Many geologists prefer for now to lump the Yarmouth, Kansan, Aftonian, and Nebraskan together into a single pre-Illinoian stage.

The continents of the northern hemisphere were more affected by glaciations than the southern ones, which generally remained mostly tropical and subtropical, though more humid during ice ages.  The coldest regions of the world became arctic deserts.  However, the great hot deserts of North Africa and Western North America today were mostly vast grasslands with large permanent lakes and abundant large game animals during the Pleistocene ice ages.  Sea levels were up to 450 feet (137 m.) lower than today during the coldest periods as a result of a substantial volume of the world's water being locked on the continents in 1-2 mile (ca. 1.5-3 km.) thick glacial sheets covering thousands of square miles.  As a consequence, vast areas that are now shallow sea and ocean bottoms were exposed for thousands of years.  Twice during the last ice age, lowered sea levels resulted in Siberia being connected to Alaska by a 1200-1300 mile (1900-2100 km.) wide corridor.  Asian hunters used this route to migrate into the western hemisphere to become the first Native Americans.

         Extent of major glaciers at the height of the last ice age (20,000 b.p.)
         (27% of the earth's land surfaces were covered by ice at that time)

  photo of Ice age conditions in the northern hemisphere during the Pleistocene Epoch summers

  Ice age conditions in the northern  
  hemisphere during the Pleistocene
  (persistent glaciers with tundra and
  cold forests at lower elevations
  farther south)

Human evolution was very likely affected strongly by the dramatic climate swings of the Pleistocene.  These changes no doubt presented powerful new natural selection pressures.  Many animal species were driven to extinction by the advancing and retreating ice ages.  Humanity survived primarily by becoming more intelligent and adaptable.  This allowed us to develop new cultural technology to deal with cold environments and changing food sources, especially during the last 1/4 million years.  One of the greatest problems in the cold regions would have been the relative scarcity of plant foods that humans could eat during the winters.  In response to this, our ancestors became more proficient at hunting animals, especially large ones that provided more calories.  This required inventing more sophisticated hunting skills as well as better weapons and butchering tools.  These changes in subsistence pattern were essential for our survival.

During ice ages, those species that were not driven to extinction by the cold commonly evolved larger, more massive bodies as a means of producing and retaining more heat.  This was especially true of mammals in the northern hemisphere.  This is to be expected, given the predictions of Bergmann's rule.  Humans evolved larger bodies during the Pleistocene as well.

  Likely effects of extreme cold conditions on human evolution
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NOTE:  Global cooling events that result in ice ages have occurred a number of times during the earth's history.  Some of these cold periods have lasted for 10's of millions of years.  The ice ages of the Pleistocene were just the recent part of a longer progressively cooling trend that began about 60 million years ago.  By 35-30 million years ago, it was finally cold enough for the most recent polar ice sheets to form.

Major Long-term Cold Periods Over the Last 1/2 Billion Years
(shown in blue)

graph of the 4 major cold periods over the Last 1/2 billion years
(These time ranges are approximations and do not reflect the fact that temperature changes between major cold
and warm phases of the earth's history usually occurred over long periods of time and that the cold periods varied
in temperature and were not consistently cool.}

Research by Eelco Rohling of the University of Southampton in England suggests that we are now 2,000-2,500 years overdue for another ice age and that the reason it has not arrived yet has been the impact of humans on the global climate.  Specifically, it is thought that deforestation, the burning of fossil fuels, and other human activities have resulted in an atmospheric "greenhouse effect" which is responsible for prolonging relatively warm interglacial conditions.

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