Array of seemingly inconsequential traits arose to separate lumbering chimps from spry humans
CAMBRIDGE, Mass. -– A collection of seemingly random physiological traits that arose millions of years ago, at the evolutionary split of chimpanzees and Homo erectus, conspired to make humans unusually strong endurance runners, permitting our ancient ancestors to compete for food with speedy four-legged carnivores and greatly shaping the distinctive human body form that we know today.
So say anthropologists at Harvard University and the University of Utah, who report on the evolution of traits such as a small ridge at the base of our skulls, shoulders decoupled from our heads, an extensive series of springy tendons along the back of our legs and feet, and well-defined buttocks in the Nov. 18 issue of the journal Nature.
"These esoteric anatomical features make humans surprisingly good runners. Over long distances, we can outrun our dogs and give many horses a good race," says Daniel E. Lieberman, professor of anthropology in Harvard's Faculty of Arts and Sciences. "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."
Protein and fat derived from prey are excellent food for growing the bodies and brains of predators; Lieberman suggests running may have fueled growth of the human brain by relaxing constraints on human acquisition of protein and fat.
"We are very confident that strong selection for running -– which came at the expense of the historical ability to live in trees –- was instrumental in the origin of the modern human body form," says Dennis M. Bramble, a professor of biology at Utah. "Running has substantially shaped human evolution. Running made us human -– at least in an anatomical sense. We think running is one of the most transforming events in human history. We are arguing the emergence of humans is tied to the evolution of running."
This work started 13 years ago when Lieberman and Bramble began questioning why pigs are such poor runners. Bramble noted that unlike horses, dogs, cheetahs and other good runners, pigs lack a so-called nuchal ridge at the base of their skulls which attaches to a broad band of tissue that keeps an animal's head steady when it runs.
Traces of this ridge can be found in human skulls dating back several million years, but the fossils of early humans at Harvard's Peabody Museum of Archaeology and Ethnology showed Lieberman and Bramble that neither the earliest prehumans nor the chimps that are their nearest relatives have a nuchal ridge. The scientists were struck by ancient humans' acquisition of this feature, since the stooped predecessors of Homo erectus, known as australopithecines, spent much of their time in trees and seldom ran.
"As we started to think more about the nuchal ridge, we became more excited about other features of bones and muscles that might be specialized for running, rather than just walking upright," Lieberman says. "One that comes immediately to mind is our shoulders. The burly shoulders of chimps and australopithecines are connected to their skulls, the better to climb trees and swing from branches. The shoulders of modern humans are disconnected from our skulls, allowing us to run more efficiently."
Another of humankind's most distinctive features, the gluteus maximus muscle that comprises the buttocks, also helps establish humans as far more skilled runners than their ancestors: A quick look at fossil australopithecines reveals that their pelvises, like those of chimps, support only a modest gluteus maximus.
"Your gluteus maximus stabilizes your trunk as you lean forward in a run," Lieberman says. "A run is like a controlled fall, and the buttocks help to control it."
Runners also get a lot of help from their Achilles tendons, tough bands of tissue that anchor the calf muscles to the heel bone, and related tendons along the back of the leg and foot. This extensive system of springs in the leg and foot effectively store and release significant elastic energy during running, but they're not needed for walking.
"There were 2.5 million to 3 million years of bipedal walking [by australopithecines] without ever looking like a human, so is walking going to be what suddenly transforms the hominid body?" Bramble asks. "We're saying, no, walking won't do that, but running will."
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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