Last week's New York City Marathon may have been a demonstration of athletic excellence, but according to a report this week in Nature, it was also a display of a key innovation in human evolution. New research suggests that the ability to run long distances emerged 2 million years ago, possibly enabling our ancestors to become better scavengers. The findings may also help explain why our bodies are so different from those of other primates.
Humans may not be known for their speed, but they excel over most mammals in sheer endurance. Our species can run for hours while using relatively little energy, making us unique among the primates. But where this skill comes from has been an open question.
To identify human adaptations for running, University of Utah biomechanics expert Dennis Bramble and Harvard physical anthropologist Daniel Lieberman put people and animals on treadmills and measured the activity of various muscles and ligaments, along with the forces a running body generates. The nuchal ligament, for example, which connects the base of the human skull to the base of the neck, stands out. "It's an elastic band that has repeatedly evolved in animals that run. Apes don't have it," says Bramble. He and Lieberman hypothesize that the nuchal ligament helps keep an endurance runner's head from bobbing violently. Humans also have a special arrangement of tendons in their legs (including long Achilles tendons) that can act like springs. These tendons store about half of the energy of each stride and release it in the following one.
In addition, Bramble and Lieberman have zeroed in on the importance of having a large rear end. By attaching electrodes to the gluteus maximus muscles of very cooperative volunteers, they have found that these muscles contract during each running stride but not during walking, probably to stabilize the trunk. Chimps, by contrast, "have tiny rear ends," says Lieberman.
The fossil record suggests that these adaptations for endurance running emerged together about 2 million years ago, in the early species of our own genus, Homo. Endurance "running is the only known behavior that would account for the different body plans in Homo as opposed to apes or australopithecines," says Bramble.
John Fleagle, an anatomist at Stony Brook University in New York, is impressed by Bramble and Lieberman's argument and wonders why no one thought of it before. "It's a real head-slapper," he says. A number of their predictions remain to be tested, he points out, because the fossil record of early Homo is still incomplete. But he expects Bramble and Lieberman's paper to generate a lot of new research.