Like most other scientists, Rick Potts believed that early humans evolved in a dry eastern African climate. Yet after studying some pushed-up ground layers in this earthquake-prone region, Potts began to question the conventional wisdom. Taking an old idea and giving it new currency, Potts may be poised to topple a theory that has roots dating back to Charles Darwin, the "father of evolution."

Based on Potts's interpretation of ground layers in Kenya's Rift Valley, this region experienced rapid changes in climate over the past two million-or-so years. This is roughly the period over which several adaptations that gave human ancestors survival advantages emerged. These adaptations include bipedalism (walking on two legs), the ability to make tools, and the emergence of a larger brain. When he factored in further evidence that suggested past volcanic activity, Potts wondered whether climate variability—and the environmental instability it produced—had been a stronger force than habitat alone (Darwin's idea) in shaping human evolution.

Looking at the fossil record, Potts's idea may make sense. For example, after a period of about four million years, over which no significant change in cranium (skull) size is visible, larger craniums appear to have evolved about two million years ago. Larger brains would have helped human ancestors make tools and clothing in order to survive fluctuations in temperature. What scientists have concluded about the climate during this phase of human evolution is consistent with the hypothesis. Overall, the climate record shows that there were wider fluctuations between warmer, wetter periods and cooler, drier ones than in the preceding several million years. What's more, these fluctuations occurred in short cycles lasting only tens of thousands of years.

According to Potts, being able to adapt to changing climate conditions can mean the difference between a species surviving or going extinct. Over the history of life, almost all species have proven themselves adaptable, but only to a certain degree. The ability to create a variety of tools, for example, and to eat many different types of food suggests a much higher degree of adaptability in the face of instability. This hallmark of modern humans may serve us well in an uncertain future.

How do scientists know how quickly and how much climate changed in the past? Because written climate records were not kept in ancient times, scientists rely on substitute records, called proxies, to determine ancient climates. Proxy climate data sources include tree rings, ice cores, and sediment cores. Even the fossilized remains of ancient life can serve as a useful climate proxy. To study Africa's past climate, scientists looked at sediment cores taken from lake bottoms and the ocean floor off Africa's eastern coast.

In these deep-sea core samples, layers of marine sediments are stacked. Each layer corresponds to a set of environmental conditions, with lower layers offering snapshots of conditions deeper in the past. A sediment layer contains most everything that was carried in the air or in the water at the time it was deposited. From deep-sea cores, scientists can analyze samples of dust, pollen, and ancient life forms. Pollen grains, for example, have distinctive shapes that can be used to identify the type of plant that produced them. Since pollen grains are well preserved in sediment layers, an analysis of the pollen grains in each layer tells us which kinds of plants were growing at the time the sediment was deposited. This, in turn, can inform us of the climate conditions that would have supported these plants.

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