Humans are sort of the runts of the animal kingdom, combatively speaking—we've got soft underbellies, frail nails, dull teeth and lackluster night vision. We're slow, we're weak, we're not particularly good climbers, swimmers, or gliders, and we've got these big round heads perched atop easily snappable necks. So what do we have? A good throwing arm, fancy thumbs, strong communication and, yes, those big round heads.

Related Content New Fossil Discovery May Change What We Know About Human Evolution

Our brainy skulls do give us an advantage—more than enough, obviously, to compensate for all those other shortfalls. That we have an abundance of brain and paltry brawn isn't just a coincidence, though. Instead, it was a direct evolutionary trade off. Our big brains take lots of energy, and evolution deemed fit to fuel that growth by rapidly shedding muscle mass, says new research, described by Dan Vergano for National Geographic:

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.

There was a specific divergence in evolution compared to other close relatives like Chimpanzees, says Charles Choi for Live Science. Though still quite clever, chimps are much more burly than squishy little humans, said Philipp Khaitovich, one of the scientists involved, to Choi: