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Scientists have long noted that the major difference between modem humans and other apes, like chimps, is our (26) of an oversize, energy-hungry brain. It was the development of that brain that drove the evolution of our early human (27) away from an apelike ancestor, starting roughly six million years ago. But the question of just why and how we (28) 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 (29) ," suggests a new study. The study, (30) Tuesday in the journal PLoS Biology, looked at how rapidly the metabolic needs of various organs, (31) 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 (32) 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 (33) our body changed.
Our early ancestors likely possessed apelike strength, at least for the skeletal muscles analyzed in the new study. Today our brawn is much (34) , while other body tissues, like kidneys, have remained (35) unchanged over millions of years. Over the same time period, the brain evolved four times faster than the rest of the body.