Heat might explain why we lost our fur and now strike an upright and slim (in theory, anyway) pose.
If our ancestors lived somewhere really hot, the theory goes, it would have made sense for us to lose body hair, start sweating more, become slender and even walk upright — to create distance between our bodies and the hot ground.
A new study supports the theory that heat helped drive human evolution, by showing that a key cradle of human evolution in East Africa has indeed been really hot for at least 4 million years.
"That's something that's been hard to get at," said Ben Passey, a geochemist at Johns Hopkins University in Baltimore. "It's nice to say that these things would be advantageous to living in hot, open environments. But was it actually hot and open?"
To find out, Passey and colleagues analyzed dated soil samples from the Turkana Basin, a well-studied region in Kenya and Ethiopia that contains lots of fossils from our human and pre-human ancestors. In particular, they looked at weighted carbon and oxygen atoms, called isotopes.
As temperatures drop, a rare isotope of carbon called carbon-13 tends to clump together with a rare isotope of oxygen called oxygen-18 within soil. Through a fairly simple relationship, the more clumping the scientists see between these isotopes, the colder they are able to say a sample is.
Their results, published in the Proceedings of the National Academy of Sciences, showed that dirt in the Turkana Basin has remained above about 85 degrees Fahrenheit with spikes above 95 degrees F over the past 4 million years. Since soil absorbs heat from the air, that means that the region has been really hot for a really long time.
Today, air temperatures in the Basin regularly exceed 100 degrees Fahrenheit, Passey said, with nighttime temperatures in the 70s. Temperatures average in the mid-80s all year-round. The landscape is sparse with grasses, shrubs and bushes.
"The heat is sort of unrelenting," he said. "You think, God, it could not have been as hot when humans were evolving here. It must have been much a nicer, lush place. Our results say no, it was still hot."
Because the researchers could look only at soil temperature, Passey added, it's possible that the air was even warmer millions of years ago than it is today, but with more vegetation and more shade that could have cooled the soil a little bit.
"There is no question that the results are fascinating," said Harvard anthropologist Daniel Lieberman, who studies how and why the human body looks the way it does.
For one thing, at some point we developed a unique ability to regulate our body temperatures while running, which might have helped people catch prey in hot and dry conditions. "The hotter it is, the more humans have an advantage over other mammals, especially when running."
"No one knows for sure when we became proficient at sweating and when we lost our fur," he added. "But this paper provides strong evidence that the climatic conditions that would have favored such adaptations intensely were present for a long time."