In 1996, Smithsonian anthropologist Rick Potts, director of the Human Origins Program at the Natural History Museum, turned heads in scientific circles when he proposed that environmental fluctuations were the driving force in human evolution. He spoke with the magazine’s Beth Py-Lieberman.
Conventional wisdom has long posited that African forests became grasslands, prompting our ancestors to climb down from trees, walk upright and use tools. That theory doesn’t work for you?
As I delved into the scientific data concerning ancient climate, I began to see fluctuations—between wet and dry, warm and cool—as a very important signal that other researchers seemed to be missing. While everyone else looked at the overall trend to drier grasslands, where we all assumed early humans lived, I started to notice the instability of environments. The fluctuation was perhaps a more important driving force. And then a new idea all started to come together.
Other researchers were focusing on excavating bones and tools, you had a much larger ambition to understand the ancient landscape and the environment.
The idea of digging a hole in the ground here or there, and taking everything back to the museum to study wasn’t quite broad enough. So I developed an approach and a research team that began to excavate landscapes. Rather than a single place in the ground to dig, we saw individual layers that we could follow over many square miles. By focusing on these targets of excavation, we could figure out: Where did early humans hang out in that particular environment? Where did they live, and what were they doing? What places did they avoid? The stone tools that they left behind, of course, were the big indicator of their presence on the ancient landscape. That was the first time this approach to excavating had been done.
So, our early ancestors didn’t live only in arid grasslands?
For the first four million years of human evolution our ancestors walked upright but also retained very long powerful arms and relatively short legs—which indicates that they were very comfortable climbing in the trees. This suggests that the origin of our ability to walk on two legs was not just an adaptation to grasslands, but rather an adaptation to the tendency of the environment to switch back and forth, between landscapes with many trees and very few. This means that our ability to walk upright was originally part of a very flexible adaptation to environmental fluctuation or instability, not just a single type of habitat.
Is there other evidence for this adaptation?
Lots of evidence. For instance, we can now look at stone tools in an entirely different light. A simple sliver of rock is a knife sharper than a carnivore’s canine tooth, while a hammer stone can pound and crush as effectively as an elephant’s molar. The aptitude to create such a diversity of tools suggests an ability to adapt to changing surroundings and eat many different kinds of food. In this way of viewing evolution, stone tool making was not an adaptation to the grassland, but an adaptation to change in the surroundings. This is adaptability – a hallmark of human evolution.
Did you have a Eureka! moment at Olorgesailie, when it suddenly made sense to you?
Yeah in a way I did, even though it took many years to have that moment. It came from studying the fossils of other animals besides early humans. The fossil mammals that dominated at Olorgesailie, one layer after another, were forms of zebra, baboons, pigs, and elephants, that were very well adapted to grassland environments. At first, this seemed to indicate that, yes my goodness, the grassland idea of human evolution might well be correct. However, they are the ones that are now extinct, I then realized. And as I traced the fossil record, I wondered which were the zebras and baboons and other animals that survived, and which were the ones that went extinct. It was always the specialized grass-eating forms that went extinct during the last one million years. I sort of hit myself on the head, and said ‘Rick, you’re an anthropologist; maybe this has relevance for humans.’ Our team was figuring out the record of environmental change, and we found that when the grassland species—what I call the large lawnmowers of the Pleistocene—became extinct, they were survived by relatives that were quite able to change from grasslands to tree-dominated environments, and they were the ones that survived until the present.
Describe the kinds of wild climatic fluctuations that were occurring between 700,000 and 150,000 years ago. What was life like for our ancestors?
Take this one site, Olorgesailie in Southern Kenya, as an example: What you see is that the lake was sometimes very, very broad. And then sometimes right in the middle of the lake sediments you see evidence of a hard calcium carbonate line, which indicates a drought. And then the lake comes back and is really large again. And then the lake dries up; it contracts. It dries up, and it’s replaced by grassland. And then there are times when it was wetter—more of a woodland and bushy type of environment. So it really fluctuated back and forth, which affected the kinds of food that our ancestors could find. It affected the amount of water that they could secure. It affected the kinds of diseases and the kinds of predators in their environment. And so all of the challenges that would have affected the life and death of these early ancestors of ours, would have been altered, would have been periodically revamped, by these changes in climate, and also by volcanic eruptions and earthquake activity. This led me to think that our basic adaptations, the adaptations that define the emergence of our species and our lineage going way back in time, may have been the result of adaptation to change itself, rather than any single environment.
How does your theory fit with the “survival of the fittest” idea? “Survival of the fittest” means that there is a particular kind of challenge that is consistent over long periods of time. Traditionally, scientists judge an organism as “fit” based upon its ability to survive and reproduce better than other organisms in a specific environment. But you can’t have survival of the fittest when the definition of ‘fittest’ keeps changing, based upon fluctuations in the climate. I prefer to think of human evolution as “survival of the versatile.”
So other hominid species that couldn’t adapt fast enough to new or changing environments died out, until just one, Homo sapiens, remained. Are you saying that climate change is what drove us to become human?
That’s a very interesting question. It’s important to realize that even those species that came before us were also adaptable. But what we see in almost all species over the course of earth’s history is that, they’re adaptable only to a certain degree. For example, a close fossil cousin of ours, Paranthropus boisei, had very powerful jaws, and big muscles for chewing food, and walked upright on two legs, had small canine or eye teeth, like we do. It had some of the defining characteristics of a human being. In fact, that particular species lasted for about a million years—five times longer than our species has been around. I think of this species as the eating machine in our family history. With such powerful jaws, it could eat almost anything it wanted to. But there were limits to how long that approach could succeed. In fact, during a time when climate change was ramped up, in East Africa and world environments overall, Paranthropus went extinct. It seems that climate change is not only a driver of evolutionary innovation, of evolutionary adaptations. When it increases beyond a certain degree, climate uncertainty can also be the death knell of species that have been successful for quite a long time. It’s a matter of how you trim the branches of our evolutionary tree so that earlier species that were close cousins of ours ultimately became extinct after a successful run. When you look at the record of earth’s climate, instability is the big driver of adaptability and extinction.
How will versatility benefit humanity as it confronts global climate change?
The rate of global climate change poses a serious challenge today. I’m especially concerned about the element of instability, which causes uncertainty about precisely how we should respond.