Humans hot, sweaty, natural-born runners

Hairless, clawless, and largely weaponless, ancient humans used the unlikely combination of sweatiness and relentlessness to gain the upper hand over their faster, stronger, generally more dangerous animal prey, Harvard Anthropology Professor Daniel Lieberman said.

The evidence belies the long and firmly held belief that humans are the animal world's biggest wimps and, if not for our big brains and advanced weapons, we'd be forced to subsist on fruits and vegetables, always in danger of being gobbled up by fiercer predators.

The problem with that theory, Lieberman said, is that we began adding meat to our diets around 2.6 million years ago, long before we developed advanced weapons like the bow and arrow, which was developed as recently as 50,000 years ago.

While some of our ancestors' meat-eating may have been due to scavenging, Lieberman said the appearance about 2 million years ago of physical adaptations that have no impact on walking but that make humans better endurance runners provide evidence that early scavengers became running hunters.

Specifically, we developed long, springy tendons in our legs and feet that function like large elastics, storing energy and releasing it with each running stride, reducing the amount of energy it takes to take another step. There are also several adaptations to help keep our bodies stable as we run, such as the way we counterbalance each step with an arm swing, our large butt muscles that hold our upper bodies upright, and an elastic ligament in our neck to help keep our head steady.

Humans, he said, have several adaptations that help us dump the enormous amounts of heat generated by running. These adaptations include our hairlessness, our ability to sweat, and the fact that we breathe through our mouths when we run, which not only allows us to take bigger breaths, but also helps dump heat.

"We can run in conditions that no other animal can run in," Lieberman said.

While animals get rid of excess heat by panting, they can't pant when they gallop, Lieberman said. That means that to run a prey animal into the ground, ancient humans didn't have to run further than the animal could trot and didn't have to run faster than the animal could gallop. All they had to do is to run faster, for longer periods of time, than the slowest speed at which the animal started to gallop. Most animals would develop hyperthermia -- heat stroke in humans -- after about 10 to 15 kilometers, he said.