We humans may be weaklings by nature.

Humans appear to have evolved puny muscles even faster than they grew big brains, according to a new metabolic study that pitted people against chimps and monkeys in contests of strength.

The upshot, says biologist Roland Roberts, is that "weak muscles may be the price we pay for the metabolic demands of our amazing cognitive powers."

Scientists have long noted that the major difference between modern humans and other apes, like chimps, is our possession of an oversize, energy-hungry brain. (Related: "Human Origins Project.") It was the development of that brain that drove the evolution of our early human ancestors away from an apelike ancestor, starting roughly six million years ago.

But the question of just why and how we evolved such big brains, which consume 20 percent of our energy, has long bedeviled science.

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

The study, published Tuesday in the journal PLoS Biology, looked at how rapidly the metabolic needs of various organs, ranging from our brains to our kidneys, have evolved. Some scientists have suggested that the rapidly evolving metabolism of the human gut, for example, drove the brain's evolution.

Instead, the new study suggests that muscles and brains have essentially traded off their energy use.

The researchers found that in the last six million years, people have evolved weaker muscles much more rapidly—eight times faster—than the rest of our body changed.

Our early ancestors likely possessed apelike strength, at least for the skeletal muscles analyzed in the new study. Today our brawn is much reduced, while other body tissues, like kidneys, have remained relatively unchanged over millions of years.

Over the same time period, the brain evolved four times faster than the rest of the body.

Roberts, a scientist with the Public Library of Science who wasn't involved in the study, called it a "tantalizing preliminary enquiry" in a commentary accompanying the new paper.

He notes that "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." That was about 130 million years ago.

Geeks Rule

To confirm their findings, which were based on analysis of 10,000 metabolic molecules, the researchers pitted people, chimps, and macaques—another kind of monkey—against each other in a contest of strength. (Related video: "Genius Chimp Outsmarts Tube.")

All participants had to lift weights by pulling a handle.

"Amazingly, untrained chimps and macaques outperformed university-level basketball players and professional mountain climbers," Roberts says. People were indeed only about half as strong as the other species.

Looking for an explanation, the team also subjected the macaques to two months of a "couch potato" lifestyle: little exercise, high stress, crummy food.

At the end of the two months, a strength contest with the couch potato macaques found that the animals' strength hadn't declined much. In fact, the scientists deduced from those macaques that humanity's "soft" lifestyle accounts for 3 percent of the strength difference between people and monkeys.

That appears to confirm the idea that weak muscles, along with a weakness for the couch—so conducive to brain-intensive exercises like watching movies and reading—could be our evolutionary inheritance.