You don't need be a scientist to see that it's the brain that sets humans apart from the rest of the animal kingdom. Humans may not be the fastest or biggest, but they're the smartest, clever enough to populate the globe.

But just as there's no such thing as a free lunch, bigger brains don't come on the house. According to a new study, human intelligence came an evolutionary price. Modern humans sacrificed brawn for brains.

In analyzing more than 10,000 different metabolites -- intermediates and products of metabolism -- in tissue samples from humans, chimpanzees, macaques and mice, researchers found that the human brain evolved up to four times faster than that of chimps. Muscle evolved eight times more quickly.

But whereas the human brain became bigger and more capable, human muscle became less efficient.

"It's a rather drastic change in both brain and muscle," said Philipp Khaitovich, 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."

"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.

The study helps explain why humans, even in peak physical shape, aren't nearly as strong as chimps and other monkeys of similar size.

"Amazingly, untrained chimps and macaques outperformed university-level basketball players and professional mountain climbers," Rolad Roberts, a scientist with the Public Library of Science, told National Geographic. "Weak muscles may be the price we pay for the metabolic demands of our amazing cognitive powers."

The study was published Tuesday in the journal PLOS Biology.