When looking for clues about the origin of humanity, the Great Rift Valley in East Africa is the place to go. Human fossils found there span the last several million years of evolution, so understanding the conditions in this region over this time period is important to understand who we are and how we got here. Some scientists have suggested that humans evolved as this region was becoming drier and grassier, and that standing on two feet helped early humans to see over the grass and look for predators. Another view is that the important evolutionary force shaping humanity was change itself.
Lake Malawi in the Great Rift Valley is a deep lake going down to 2,300 feet, but this has not always been the case; researchers studying Lake Malawi have found that its depth has varied dramatically in the past. Dr. Andy Cohen of the University of Arizona in Tucson has studied Lake Malawi and other lakes in the area, drilling into the mud in the bottom of the lake to explore its history. When researchers very carefully pull up the mud, they can sort through the layers to examine microscopic remnants of pollen and fossils of marine creatures that were alive at the time. In dry times, less pollen would blow into the lake and get deposited.
The Lake Malawi region today is forested and receives abundant rainfall, but from 135,000 to 90,000 years ago, the lake sediments reveal that the lake’s water level dropped over two thousand feet, leaving a much shallower and murkier lake. The pollen and signs of vegetation in the area dwindled as a megadrought gripped the region before rainfall increased again and the lake filled back to its present level. The impact on humans at the time was certainly profound. Fewer human fossils are found in this time period, no doubt because food and water were hard to find.
Looking deeper in the lake sediments and at the ocean-floor sediments in the area, researchers have pieced together a picture of climate change extending even farther back in time, covering the last few million years that were crucial in the course of human evolution.
By looking at these, Dr. Matt Grove from the University of Liverpool has found that periods of rapid climate change over the last few million years coincided with periods of rapid human evolution and tool development. One period of climate change about 2.7 million years ago was followed rapidly (relatively) by the appearance of the first stone tools 2.6 million years ago. Homo erectus produced only very simple stone tools, a so-called hand-ax, and continued to use this simple tool for almost a million years with little change. Homo erectus lasted through this period while other species of early humans did not, perhaps due to its adaptability, living in a broad range and over a period of 1.5 million years. To put that into perspective, our own species, Homo sapiens, has been around only for about 160,000 years, a mere blink of the earth’s eye.
Looking at the course of human evolution and comparing this with the record of a changing climate in the area, a surprising chain of connections starts to emerge. For long periods, humans would reach a stable state in their biology and culture, and the climate in which they lived would be stable as well. When a shift in climate occurred, or the climate became highly variable, new species of early humans and new innovations in tools would emerge and spread.
This pattern repeated itself more than once. While the discussion was once whether humans evolved in grassland or forest, the true driving force in human evolution was probably change itself. The rapidly changing world early humans lived in forced humans to develop the versatility that allowed us to survive a changing world and eventually dominate the changing world we live in today. It’s ironic that climate change shaped us and that we, in turn, shaped our world, creating a new era of climate change. How we deal with this new challenge of modern climate change is another story that is still playing out.
Glenn Croston is the author of “The Real Story of Risk”, exploring our twisted sense of risk and how it plays out in the world we live in today. He is also the author of “75 Green Businesses You Can Start to Make Money and Make a Difference”, and “Gifts from the Train Station”, with stories of people living through great challenges and finding new lives filled with purpose.
