Our human intelligence may have come at an evolutionary cost, researchers say, a trade-off that saw our muscles grow ever weaker so that our cognitive powers could get stronger.

Letting our muscles go weak is how we began to develop the brainpower that let us evolve away from an ape-like early ancestor, they suggest.

In comparison to modern apes like chimpanzees, humans posses an oversize brain that makes huge demands on our energy, requiring as much as 20 percent of our body's total energy output.

Why our brains are so much bigger, and how they got that way, is an evolutionary question that has long puzzled scientists.

A new study of human and ape metabolism that matched people against monkeys and chimps in tests of strength may have provided some clues to the way in which we evolved our powerful, complex brains, researchers say.

"A major difference in muscular strength between humans and nonhuman primates provide one possible explanation," study leader Katarzyna Bozek at the Max Planck Institute for Evolutionary Biology in Germany says.

The study looked to determine the speed with which the metabolic energy requirement of our various organs evolved, and its findings suggest that in humans the brain and muscles have essentially swapped their energy demand levels as we evolved.

Over the last 6 millions years, the researchers say, our muscles changed -- getting weaker -- much faster than any other parts or our body.

Our strength has been much reduced from the muscular brawn our apelike ancestors possessed while other tissues in our bodies, such as kidneys, have seen little change over those millions of years, the researchers found.

In the study, the comparative strength of humans and modern apes like rhesus monkeys and chimps was tested by having them pull weights upward.

The human study participants -- including a number of professional athletes -- fared poorly against the apes, which proved capable of handling nearly twice the weights in the testing.

"According to our results, an average adult chimpanzee is approximately two to three times stronger than an average adult human," researcher Phillip Khaitovich of the Key Laboratory of Computational Biology in Shanghai said.

The findings tend to support an evolutionary trade-off where our human ancestors swapped brawn for brain, the researches say.

 "It is a very simple explanation, and it could be completely wrong," Khaitovich said. "In evolution, however, simple explanations often work well."

The study has been published in the journal PLOS Biology.