The idea that man evolved from apes is accepted by most people today. There is also evidence that the entire world was populated by a relatively small group of people who emigrated from Africa. But what caused these developments? Scientists now believe that climate change and global warming had a great effect both on human evolution and our ability to colonise the world.

The classical story of how we evolved

It is well established through fossil evidence that man-like creatures (hominins) started to evolve in East Africa about 5 million years ago. Bipedalism (walking on two legs instead of four) preceded a gradual increase in brain size.

What stimulated this evolution? The classical explanation is that the East African climate gradually became dryer. The humid forest, where our ancestors climbed trees, gave way to grassland. Some of the apes, which were already partially bipedal, evolved to walk fully on two legs since this is an efficient way of traveling through tall grass. Then, somehow, they became clever and evolved into Man.  However, there is a problem with this simple, slowly changing picture. Grasslands existed well before there was much hominin evolution. Modern theories describe something more complex.

A 21st Century perspective

Almost all the hominin fossils found in Africa are in the Rift Valley. Therefore something very unusual must have happened in this place. The Rift Valley is a massive stretch mark which runs 6000 km North /South, having a width of 600 km. It was created by movements of the earth's crust pulling the continent apart between 10 and 5 million years ago. It changed the landscape from being boring and flat, to exciting and varied, with mountains, deep lakes, and plateaus. It also affected the local climate. The East Rift mountains prevented moist air from passing over Africa creating cloud forest on one side of the valley, and desert scrub on the other.

There is geological evidence that the East African climate did not become dryer gradually. Rather, this long term trend was punctuated by short episodes of alternating periods of drought and wetness made more extreme by the Rift Valley landscape. This forced  the hominins to adapt to a constantly changing environment, or become extinct. The fossil record reveals two ways they adapted: some species developed massive jaws to eat anything, whatever the conditions. Others, including Man, evolved bigger brains to think with and figure out how to deal with tough times.  Many hominin species came into existence and went extinct, hence the variety of fossils found there. Big game animals underwent a similar burst of evolution.

Famine and population crashes following abrupt climate change may have helped to develop cooperation and communication as well as innovation in early man. Times must have been very hard. There is genetic evidence that at one time there may have been as few as 2000 people left to carry humanity forward. 

Genetic studies reveal that almost all of us alive today are descended from a few thousand people who lived in East Africa 60,000 years ago.  Some of these decided to leave their homeland. Their descendants  gradually spread throughout the world as the periods of glaciation in northern latitides became less severe. Humans arrived in Spain only 30,000 years ago.

It was not an easy journey. Ice cores from Greenland show that there have been more than 20 abrupt climate changes over the last 60,000 years. Things would suddenly get warmer (over a period of decades), and then cool down slowly. One really spectacular example happened 11,500 years ago when temperatures in Greenland rose 10 degrees in 10 years. Changes to the environment, forests, glaciers and food resources must have been enormous. Mankind learned to deal with them, survived and reproduced to become the 6.7 million people we are today.

Climate Change and The Future

The world climate has been relatively stable over the past 10,000 years and is not pre-programmed to do anything dramatic soon. However, over the last 25 years it has been increasingly variable. Scientists believe that man-made greenhouse gases may be pushing the climate quickly towards a tipping point. Global warming has caused rapid melting of the Greenland ice sheets which is reducing salinity in the North Atlantic, and, by changing ocean currents, could soon create a sudden cooling called a "cold reversal". This could have severe world-wide knock-on effects.

Let us hope that our qualities of cooperation, communication and innovation, which evolved in response to climatic upheavals in the past, can help us through abrupt climate change in the future.

by Dr Christine Betterton-Jones BSc. (Zoology), PhD (Parasitology)

Bibliography

Dennis O'Neil - Evolution of modern humans - Behavioral Sciences Department, Palomar College, San Marcos, California. Updated January 2009.  http://anthro.palomar.edu/homo2/default.htm

Beth Christensen and Mark Maslin, 2008. Rocking the Cradle of Humanity:New thoughts on climate, tectonics and human evolution. Geotimes - January 2008. http://www.geotimes.org/jan08/article.html?id=feature_humanity.html

William H. Calvin 2002 . A Brain for all seasons: Human evolution and abrupt Climate Change . University of Chicago Press.

Doron Behar cited in Before the Exodus - The Economist April 2008

Gary Stix, 2008. Traces of a Distant Past. Scientific American July 2008

N. Wade, 2003. Why Humans and Their hair parted ways. New York Times, http://query.nytimes.com/gst/fullpage.html?res=9C03E0DE1030F93AA2575BC0A9659C8B63

A Paleo Perspective on abrupt Climate Change NOAA (National Oceanic and Atmospheric administration.USA) Updated June 2008 http://www.ncdc.noaa.gov/paleo/abrupt/index.html