People are astonishingly successful endurance runners, “and I don’t think it’s just a fluke,” Lieberman says. He and Bramble argue that not only can humans outlast horses, but over long distances and under the right conditions, they can also outrun just about any other animal on the planet—including dogs, wolves, hyenas, and antelope, the other great endurance runners. From our abundant sweat glands to our Achilles tendons, from our big knee joints to our muscular glutei maximi, human bodies are beautifully tuned running machines. “We’re loaded top to bottom with all these features, many of which don’t have any role in walking,” Lieberman says. Our anatomy suggests that running down prey was once a way of life that ensured hominid survival millions of years ago on the African savanna.
Although Bramble has studied locomotion in animals ranging from tortoises to jackrabbits for 40 years, he was first tipped off to the hypothesis that humans were born to run by one of his students, David Carrier. In the 1970s, Carrier was assisting with Bramble’s studies of how dogs, horses, and people regulate breathing while running. A marathoner himself, Carrier began to wonder about the role of endurance running in human evolution. People, he noted, can shed heat quickly—not by panting, like most animals, but by perspiring through millions of sweat glands. A lack of fur also helps dissipate heat more quickly.
Still, Bramble eventually came to realize that people turn in remarkable performances. He once filmed a horse cantering, with Carrier running alongside at the same pace. The movie showed that Carrier’s legs were churning more slowly than the horse’s, which meant that the student’s strides had to be spanning more distance per step than the horse’s.
Although Carrier moved on to other research, Bramble grew convinced that his student had discovered something. During a visit to Harvard in 1991, Bramble encountered Daniel Lieberman, then an anthropology Ph.D. student, making a pig trot on a treadmill. To glean insights into how bones grow—and thus to better interpret fossilized human jaws and skulls—the student wanted to see whether the repeated impact of running would spur a thickening of the pig’s skull. “You know,” Bramble said, “that pig’s not holding its head still.” He went on to explain that adept runners like horses, dogs, and rabbits keep their noggins remarkably steady as they lope, thanks to an obscure bit of anatomy called the nuchal ligament. It’s a tendonlike band that links the head to the spine. People, he said, have a version of this band.
Rummaging through a collection of replicas of fossilized primate bones in a nearby lab, Bramble pointed out that the nuchal ligament leaves a trace—a delicate ridge—where it attaches at the base of the human skull. Then the scientists noticed the ridge in a pitted, yellowed skull of our 2-million-year-old relative Homo erectus—but not in older hominids known as australopithecines, who walked the earth as far back as 4.4 million years ago. “Holy moley!” Lieberman thought. “There’s something going on here, and what’s more, we might be able to study it in the fossil record.”