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Faster Than A Hyena?
Depending on your point of view, last week's New York City marathon was a demonstration of athletic excellence or of unparalleled masochism. But according to a report in this week's issue of Nature, it was also a display of a key innovation in human evolution. University of Utah biomechanics expert Dennis Bramble and Harvard physical anthropologist Daniel Lieberman argue that the human body is exquisitely adapted for endurance running. They marshal evidence that the ability to run long distances emerged 2 million years ago, possibly enabling our ancestors to become better scavengers. If the researchers are right, running goes a long way toward explaining why our bodies are so different from those of other apes.

It may come as a surprise to hear that humans excel in running. Obviously, a leopard can leave us in the dust in a short sprint. But over longer distances leopards and most other mammals flag. €œMost mammals can't sustain a gallop over 10 to 15 minutes,€ says Lieberman. Humans, on the other hand, can continue running for hours while using relatively little energy. €œHumans are phenomenal endurance runners, in terms of speed, cost, and distance,€ says Lieberman. €œYou can actually outrun a pony easily.€ And yet, he points out, €œno other primates out there endurance run.€

Bramble and Lieberman believe that much of this skill comes from a large inventory of special adaptations in our muscles, tendons, and bones. They emphasize that these adaptations are not all-purpose traits that also help us walk upright. €œRunning is not fast walking,€ says Lieberman. €œYou do not use the same mechanics.€

To identify adaptations for running, the researchers have 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 stretches from the base of the human skull to the base of the neck, stands out. €œIt'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. €œEvery time your heel hits the ground, your head wants to topple forward,€ says Lieberman.

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. €œChimps don't have these springs,€ Lieberman says, also noting that recovering energy is important for endurance running but not for sprinting.

Bramble and Lieberman have also zeroed in on the importance of 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, €œhave 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. Paleoanthropologists have long noted that some early Homo were markedly different from earlier hominids. Australopithecus afarensis, which lived from about 4 million to 3 million years ago, stood 0.9 to 1.2 meters tall and had long arms and a wide pelvis. Homo ergaster, which lived in Africa between 1.9 million and 1.6 million years ago, was about as tall as modern humans and had long legs and relatively short arms. Most researchers ascribed these changes to adaptations for efficient walking. €œThat's the standard story you'll get in most textbooks,€ says Lieberman. But Bramble and Lieberman have a different theory. Endurance €œrunning 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. €œIt'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.

The €œsketchiest part€ of their hypothesis, admits Bramble, is why hominids ran long distances. Paleoanthropologists generally agree that early Homo were primarily scavengers, using stone tools to cut meat off carcasses and crack open bones. €œIf you get [to the carcass] before the hyenas and the other hominids, you would have a lot of protein and fat at your disposal,€ says Lieberman.

By allowing hominids to get to more protein and fat, Lieberman suggests, running might have fueled the evolution of big hominid brains. €œLarge brains occur after the evolution of this modern, humanlike body form, and it may have been that the ability to do endurance running released a constraint on human evolution,€ he says. He speculates that the importance of endurance running only faded once humans invented hunting weapons.

€œThe importance of running to the tens of thousands of people who run marathons every year is not just a fluke,€ says Lieberman. €œI think it's a result of some important evolutionary history. It may have been lost with the invention of the bow and arrow, but the traces are still there in our bodies.€

Copyright 2004 Carl Zimmer

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