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.