The Turkana Basin in Kenya, where the average daily temperature has been around 100 degrees for the past 4 million years, may have been the place where humans first began to walk upright. Johns Hopkins University earth scientist Benjamin Passey says that the need to stay cool may explain why pre-humans learned to walk upright, lost the fur that covered the bodies of their predecessors and became able to sweat more. Passey’s work in measuring ancient temperatures lends support to the so-called “thermal hypothesis” of human evolution.
According to the thermal hypothesis, our pre-human ancestors gained an evolutionary advantage in walking upright because doing so was cooler (when it is sunny, the near-surface air is warmer than air a few feet above the ground) and exposed their body mass to less sunlight than did crawling on all fours. The loss of body hair (fur) and the ability to regulate body temperature through perspiration would have been other adaptations helpful for living in a warm climate.
“The message of our study,” said Passey, “is that this region, which is one of the key places where fossils have been found documenting human evolution, has been a really hot place for a really long time, even during the period between 3 million years ago and now when the ice ages began and the global climate became cooler. In order to figure out if (the thermal hypothesis) is possibly true or not, we have to know whether it was actually hot when and where these beings were evolving. If it was hot, then that hypothesis is credible. If it was not, then we can throw out the hypothesis.”
Evaluating whether the ancient Turkana Basin climate was, in fact, the same scorching place it is today has been difficult up until now because there are very few direct ways of determining ancient temperature. Efforts to get a handle on temperatures 4 million years ago through analysis of fossil pollen, wood and mammals were only somewhat successful.
In the case of soil carbonates common in the Turkana Basin, the amount of rare carbon-13 bonded directly to rare oxygen-18 provides a record of the temperature during the initial formation of the mineral. It told the team that soil carbonates there formed at average soil temperatures between 86 and 95 degrees Fahrenheit, leading to the conclusion that average daytime air temperatures were even higher.
“We already have evidence that habitats in ancient East Africa were becoming more open, which is also hypothetically part of the scenario for the development of bipedalism and other human evolution, but now we have evidence that it was hot,” Passey said. “Thus, we can say that the ‘thermal hypothesis’ is credible.”