"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."