Bramble adds that walking cannot explain the changes in body form that distinguish humans from Australopithecus.
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The study details various adaptations found in early humansincluding fossils of Homo erectus and Homo habilis,which are required only for running.
These adaptations include long, springlike tendons, such as the Achilles tendon, which store energy and reduce the metabolic costs of running by half. Fossil records suggest the Achilles tendon was absent in Australopithecus.
Likewise, the longitudinal arch of the footanother well-developed set of springs important to runningappears to have evolved with Homo habilis.
Long legs are also vital for endurance running, because speed is gained by increasing the length, not rate, of strides. The researchers say long legs, relative to body mass, first appeared with Homo erectus some 1.8 million years ago.
And because running exposes the body to much higher stresses than walking, the study suggests humans evolved relatively large joint surfaces in the lower body to act as shock absorbers.
Bramble even points to human facial features as evidence that our appearance was shaped by selection for running. Compared to our apish ancestors, which could run only short distances, we have a more balanced head, flatter face, and smaller teeth and nose.
"This shifts the center of mass back, so it's easier to balance your head when you are bobbing up and down," Bramble explained.
Similarly, broader shoulders, a narrower waist, and shorter forearmsall characteristic of humans among primateshelp the upper body to counterbalance the lower body while running.
Even our large buttocksconspicuous by their absence in our closest living relativesare considered critical to stabilization while running. "Have you ever looked at an ape?" Bramble asked. "They have no buns!"
If natural selection had not favored running, Bramble said we would still look a lot like apes.
So why did early humans run when they could walk?
Because they sought protein provided by meat, marrow, and brain, said study co-author Daniel Lieberman, anthropology professor at Harvard University.
He said, "What these features and fossil facts appear to be telling us is that running evolved in order for our direct ancestors to compete with other carnivores for access to the protein needed to grow the big brains that we enjoy today."
Some scientists have suggested that endurance running was used for pursuing animals before spears, arrows, nets, and other hunting tools were invented. Later it may have been employed by hunters to exhaust their prey, allowing them to get close enough to use projectiles.
This strategy is still used by the San Bushmen of southern Africa to hunt herd animals like the kudu antelope.
However, Lieberman adds, "There is very little observational study of this behavior, largely because researchers can't keep up with the hunters!"
Another possibility is that humans used their swift feet to reach dead mammals before other scavengers. Lieberman said they would have followed clues such as vultures circling in the distance.
He added, "We know from the earliest sites that early hominids smashed open limb bones of animals killed by predators and extracted marrow."
Chris Stringer is the head of human origins in the department of paleontology at the Natural History Museum in London. He thinks the new study provides an important new look at human anatomy.
"I find [Bramble and Lieberman's] reexamination and sometimes reinterpretation of features in fossil and recent humans to be generally plausible, although it will require much more complete evidence for the evolution of the skeletons of early humans below the neck to test their ideas properly," Stringer said.
He added, "From my own research focus, it will be interesting to see how the Neanderthals fit into this scenario, given their distinctive anatomy, body shape, and the temperate-cool environments in which they generally lived. Could they have been more suited to close-range ambush hunting as a strategy, compared with their ancestors and with modern humans?"
Today running is regarded as a form of exercise. But perhaps a jog in the park goes much deeper than that: It could be affirmation of the very reason that we are human.
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