No matter how many weights a man may lift, a chimp is likely to be stronger.Corbis

In elementary school, we learned that our brains make humans a unique species. But a new study shows that human muscle may have evolved even more than the brain.

Researchers found that the metabolome (products of metabolism such as sugars, vitamins, amino acids and neurotransmitters) of the human brain has evolved four times faster than the chimpanzee brain. Intending to use muscle as a control, the researchers were taken aback when they realized that human muscle changed over eight times that of chimpanzee muscle.

"It's a rather drastic change in both brain and muscle," said Philipp Khaitovich, one of the study's authors and a researcher at the Shanghai Institutes for Biological Sciences. "Of course, muscle was the most surprising. It was the control tissue; [we thought] muscle should be the same. But it turned out to be even more dramatic."

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The international team of researchers analyzed over 10,000 different metabolites in tissues of four species: humans, chimpanzees, macaques, and mice. While most of the tissue followed the same pattern as genomic change, muscle tissue did not.

The researchers quickly ruled out lifestyle as a factor in the changes by putting macaques on a "couch potato" diet of limited exercise and fatty foods. That accounted for just about 3 percent of the changes.

Could there be a link, then, between brain and brawn, the researchers wondered? Did human muscle weaken, perhaps, as the brain grew stronger? (While the analysis of metabolites confirms a change, it says nothing about what type of a change is happening in the muscle.)

Although there could be hundreds of hypotheses, the researchers decided to take a stab at their idea that humans "sacrificed" strength of body for strength of mind. They recruited college basketball players and professional mountain climbers to compete in a strength test against chimps and macaques. Despite their training, the athletes could raise only half of the weight that untrained, captive chimps and macaques did.

Still, it would take more research to parse out exactly why the animals display greater strength. It could be, for example, that human muscle adapted to favor endurance over strength, suggested Roland Roberts, associate editor of PLOS Biology, the journal the study is published in today.

"The authors focused on strength because there are anecdotal stories that nonhuman primates are unbelievably strong, more than you'd expect from something that's human-sized," said Roberts, who wrote an accompanying synopsis to the study. "And the common sense thought is that since there's no obvious immediate advantage to become weak, the brain could be the possible payoff because we live by our brains."

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No matter the outcome of the brain vs. brawn question, Khaitovich says this is just the tip of the iceberg in the study of evolutionary metabolome.

The genome has long been the focus of evolutionary distinctions in humans, but only a few genetic changes are responsible for what makes humans unique. The study of metabolites could uncover other pathways by which humans have changed. And finding those human metabolome distinctions could eventually shed light on metabolic disorders, such as diabetes," said Patrick Giavalisco, who led the metabolome measurement in Germany, in a press release.

"Even after so many years studying evolution, here's something that's still completely new, something that people didn't know about and something that's very fundamental," Khaitovich said.