July 12, 2011
Today we picked up where we left off in our geology walk from Sunday, July 10. Do you remember the members? The members are the major geological units of the Olorgesailie Formation, and they occur in a sequence through time.
Member 8, where we started today, was the beginning of a time of harsher fluctuations between moist and dry, marked by vivid red colors in the sediment. The red indicates where the diatomite was burned to such a high temperature that the silica skeletons of the diatoms actually melted and fused together. We see evidence in the reddened diatomite of carbonized leaves and stems of marshy vegetation, which accumulated during a moist period. This period of accumulation was followed by intense drying and then burning after the vegetation had been buried and completely dried out. Wet and then astonishingly arid. We think it’s likely that the burning took place as lightning strikes ignited the underground heaps of dried vegetation, and the ground itself acted as a kind of kiln allowing the heat to be trapped and immense temperatures to be reached.
The reason why we’re very interested in these reddened beds is that the oldest ones at Olorgesailie are about 780,000 years old, right at the bottom of the Member 8 sediments. And that date roughly corresponds to when the strongest fluctuations between glacial and warm times began to occur at higher latitudes. Could it be that those cold-warm shifts in the temperate zone magnified the dry-wet fluctuations in tropical East Africa?
The visiting geologists from China were intrigued by this part of our discussions as we walked along the outcrop. Many scientists today have a keen interest in ancient climate and how environments changed and posed vital challenges to the ways in which organisms evolved and adapted to their surroundings. Over the past two decades, an important part of our team’s studies at Olorgesailie has focused on how past environments shifted over time and the effect those variations had on how human ancestors and other species evolved.
Through this work I proposed an idea, 15 years ago now, that environmental variability itself was a critical factor in how populations of early human ancestors evolved better ways of adjusting to change in their surroundings. During times when environmental change was ramped up, you might imagine the benefits of having improved mental abilities, social behaviors that encouraged cooperation, new aspects of cultural learning, and eventually the ability to plan, invent innovative technologies, to imagine and think abstractly, and to create complex ways of offsetting the effects of the environment, such as by building shelters and fire, making clothing, and growing food in surplus whenever possible. The basic idea is called variability selection – times of strong shifts in the environment favored ways that improved how well human ancestors could adjust to their surroundings. These ways included, of course, the ability to modify the immediate surroundings to a greater and greater extent – a hallmark of our species today, for better or for worse.
In short, many different shifts in the climate and landscape, rather than one single environment, have been the most important stimulus to evolution. As our little group discussed and debated these ideas, we ended our walk through time. We got in a car and drove off to an area south of Mt. Olorgesailie, where we hope to drill a long hole in the ground, about 250 meters deep, that will give our scientific team a detailed look at climate change over the past 500,000 years, as recorded in the lengthy core. We plan to do this work next year, and the Chinese researchers wanted to get a look at the proposed climate core site. As we drove over the flat grassland, my guests saw a real-life portrait of the East African savanna – one of the most famous habitats in the world. We wondered what lay underground, the archive of the past lake, its surrounding habitats of grassland and woodland, and the ancestors of the savanna wildlife. And we wondered: What will this environment become in the future?