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Humans evolved puny muscles even faster than they grew brains, according to a new metabolic study that compared people with chimps and monkeys in contests of strength.

It has been long known that it was the development of the brain that distinguished our early human ancestors from an apelike ancestor. However, the question how and why we grew such big brains continued to bedeviled science and scientists. 

"A major difference in muscular strength between humans and nonhuman primates provide one possible explanation," suggested the new study, led by Katarzyna Bozek of Germany's Max Planck Institute for Evolutionary Biology, according to National Geographic. 

The study looked at how rapidly the metabolic needs of various organs ranging from brains to kidneys, have evolved. The study then suggested that muscles and brains have essentially traded off their energy use.

The upshot, according to biologist Roland Roberts is that "weak muscles may be the price we pay for the metabolic demands of our amazing cognitive powers," as quoted by National Geographic.

According to the study, in the last six million years, people have evolved weaker muscles eight times faster than the rest of our body. 

"Human muscle has changed more in the last six million years than mouse muscle has since we parted company from mice back in the early Cretaceous," noted Roberts.  

The study has been published in the journal PLoS Biology.