Humans appear to have evolved puny muscles even faster than they grew big brains, according to a new metabolic study that pitted people against chimps and monkeys in contests of strength. The upshot, says biologist Roland Roberts, is that "weak muscles may be the price we pay for the metabolic demands of our amazing cognitive powers." Scientists have long noted that the major difference between modern humans and other apes, like chimps, is our possession of an oversize, energy-hungry brain. It was the development of that brain that drove the evolution of our early human ancestors away from an apelike ancestor, starting roughly six million years ago.
But the question of just why and how we evolved such big brains, which consume 20 percent of our energy, has long bedeviled science. "A major difference in muscular strength between humans and nonhuman primates provide one possible explanation," suggests thenew study, led by Katarzyna Bozek of Germany's Max Planck Institute for Evolutionary Biology. The study, published Tuesday in the journal PLoS Biology, looked at how rapidly the metabolic needs of various organs, ranging from our brains to our kidneys, have evolved. Some scientists have suggested that the rapidly evolving metabolism of the human gut, for example, drove the brain's evolution. Instead, the new study suggests that muscles and brains have essentially traded off their energy use.
The researchers found that in the last six million years, people have evolved weaker muscles much more rapidly—eight times faster—than the rest of our body changed.
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