August 4, 2004
Even though today Olorgesailie is a comfortable place, with good water, easy access to towns, and relative safety from animals, in the past it would not have been so. What I find so amazing is that, even during the tumultuous climate changes of the past when populations of other mammals apparently left the area, early humans were able to stick it out. I described earlier how animal species changed through time, and how sometimes dramatic events, like volcanic eruptions or the lake drying up, caused many animals to leave. Throughout all of these big changes, hominins were able to stay in the region (we can tell because their handaxes and other tools are found in almost every stratum) and seemed to thrive, no matter if it was wet or dry, cool or warm.
Take a look at the picture, which shows you a column of sediment for the Olorgesailie Formation. This is a kind of drawing that geologists make from their field observations of the sediment layers. In fact, Kay and I drew this one to reflect the stratigraphy of Olorgesailie from about 1.2 million to 493,000 years ago. The white areas reflect the times of lake sediment (sometimes called "lacustrine" deposits); the cross-hatched layers represent periods when soils formed; and the argon-argon dates (left side of the picture) come from ash and pumice indicated by layers with dots.
Now look at the arrows on the right side of the column - these blue arrows reflect times when there were really significant environmental changes in the Olorgesailie region, such as those I mentioned above. We have several major collections of fossil animal bones that occur in the layers immediately after these shifts. And in each collection, the animal species differ. I mentioned in an earlier dispatch that sometimes zebras were dominant, at other times baboons were the most plentiful, while pigs and hippos were the top species in still other times.
Now look at the red dots on the left side of the column - these are the layers in which we find the stone artifacts of the hominins who made handaxes. So far in our studies, it looks like the hominins just kept going and going, even in the face of large shifts in their surroundings. Maybe they had to leave the area for a short while, but they were amazing in their ability to keep coming back whether the lake was large, small, or absent, and during times of vastly altered landscape.
This pattern led me to think that perhaps, out of all animals, hominins had developed ways of adapting to a wide variety of surroundings very early on - hundreds of thousands of years before the origin of our own species. In fact, the evolution of human beings may have depended on the enormous changes in environment that occurred over the past several million years. For a long time, researchers thought that early humans were best adapted to the African savanna, or to the ice-age surroundings of Europe. I think that idea isn't correct. Instead, a hallmark of human evolution is the ability to adapt to the dramatic manner in which environments can vary. According to this idea, our ancestors weren't specialized to survive in just one or two types of environment. Rather, they seem to have had the ability to manage just fine when things changed. Eventually, humans became extremely competent at exploiting their surroundings when it stayed stable for a time. Ours is a very versatile species, and my explanation of this is that human ancestors became more and more adept at adjusting to very changeable environments.
Like all scientific hypotheses, it's an idea worth questioning and testing: Exactly when did early humans develop better and better ways of adapting to environmental changes that were more and more severe? This question gives you an idea of why I'm so interested in finding out about environmental change at Olorgesailie, and in finding the stone tools and fossil bones that attest to the presence of early humans and other animals. I hope that our work will inspire other scientists to investigate this question.