Human brawn may be the key to why human brains are so big, according to a new hypothesis linking exercise to the evolution of our oversize noggins.
The idea has yet to be thoroughly tested, but researchers writing in the journal Proceedings of the Royal Society B say that it's worth a deeper look. In fact, some evidence in mice and rats suggests that simply breeding the most athletic animals could create offspring with sharper minds.
"Across mammals there seemed to be this broader relationship between exercise and activity and the brain," study researcher David Raichlen, a University of Arizona anthropologist, told LiveScience. Raichlen and his colleagues suspect humans are no exception.
Humans have big brains relative to body size, with well-developed frontal lobes that outstrip those of humanity's closest primate relatives. There are many theories about what triggered the evolution of these big brains, with ideas ranging from the need to navigate complicated social relationships to the need to navigate complex environments as human ancestors migrated away from the equator.
Now, Raichlen and his colleagues have a new idea to add to the mix: Perhaps human brains didn't just grow because our species was facing more mental challenges. Maybe, instead, the shift to an increasingly aerobic hunter-gatherer lifestyle about 1.8 million years ago boosted our species' athletic prowess. Because of links between exercise and the brain, this natural selection for faster, more agile humans might have resulted in smarter, bigger brains. [10 Ways to Keep Your Mind Sharp]
The bigger brains may have been a crucial piece of this puzzle, given that more cognitive ability would allow one to hunt and gather farther afield than those who came before, Raichlen said. Or, humanity's cognitive capacity may be a simple side effect, a neurobiological change that sort of got "dragged along" with aerobic capacity.
Evidence for fit brains
The mice and rat studies suggest the side-effect explanation may hold up, given that those animals were bred purely for brawn, not brain, yet their offspring still ended up smarter. Either way, multiple lines of research link the body's fitness with the brain's, Raichlen said.
For example, exercise in old age keeps the brain from losing volume. In fact, if you're elderly, a 2012 study in the journal Neurology suggests that your brain can get a big boost from a brisk walk, while mental exercises such as brain teasers do little.
Studies in children also show increased brain volume in fit kids versus couch potatoes. Exercise seems to boost the body's production of neurotrophins and brain-related growth factors, substances crucial for building and maintaining brain cells.
Thus, natural selection for fitness in human ancestors could have triggered an increase in actual brain juice, prompting growth and development.
"It's a really intriguing possibility, right?" Raichlen said. "Because you can get changes in the brain that maybe have very little to do with selection acting on cognition."
Unfortunately, Raichlen and his colleagues wrote, little is known about the aerobic fitness of humans' closest ancestors, given that they're not around to jump on a treadmill today. However, scientists can estimate fitness based on hind-limb length — longer legs mean a more efficient, faster stride — and inner ear development. Because the inner ear keeps humans balanced, this organ's size is linked to speed and agility.
Raichlen and his colleagues compared those features on humans with those on the extinct human ancestor Homo erectus, modern great apes and the extinct Australopithecus. They found the more aerobically fit the primate, the greater its brain size is in relation to its body.
None of this evidence proves the hypothesis, Raichlen warned. More work, particularly selective-breeding studies on animals, is needed. Nor do the researchers think exercise explains the entirety of Homo sapiens' evolution in brain growth.
"The evolution of the human brain is probably the result of a lot of complex selection pressures interacting with each other," Raichlen said. "I don't think we're going to find just one pressure that drove all of human brain evolution."