JOHNS HOPKINS (US)—In the Turkana Basin of Kenya the average daily temperature has reached the mid-90s or higher, year-round, for the past 4 million years, which may explain in part why pre-humans learned to walk upright, lost the fur that covered the bodies of their predecessors, and became able to sweat more.

“The ‘take home’ message of our study,” says Benjamin Passey, assistant professor of earth science at Johns Hopkins University, “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.”

Passey says the study lends support to the so-called “thermal hypothesis” of human evolution that suggests that pre-humans gained an evolutionary advantage in walking upright because, in sunny areas, the air is cooler a few feet up than it is at ground level.

Further, standing erect would have exposed their body mass to less sunlight than did crawling on all fours; the loss of fur and the ability to regulate body temperature through perspiration would also have been helpful adaptations, the hypothesis states.

Details are published in the online early edition of Proceedings of the National Academy of Sciences.

“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,” he says. “If it was hot, then that hypothesis is credible. If it was not, then we can throw out the hypothesis.”

The Turkana Basin is a rough, remote area around Lake Turkana, a desert lake in the Great Rift Valley in Kenya and Ethiopia in east Africa.

Evaluating whether the prehistoric basin also had today’s scorching climate has been difficult 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, as they reveal more about plants and rainfall and less about temperature, Passey says.

Passey, however, previously was part of a team at the California Institute of Technology (Caltech) that developed a geochemical approach to the “temperature problem.” The method involves analysis of carbonate minerals in soil and “clumps” of rare isotopes they contain.  (Isotopes are atoms of the same element that have different masses due to differences in the number of neutrons they contain.)

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 minerals.

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. In other words, it was hot way back then in what is now northeastern Kenya.

“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 says. “Thus, we can say that the ‘thermal hypothesis’ is credible.”

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