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Exercise and the Ever-

Smarter Human BrainBy GRETCHEN REYNOLDS

Michael Poehlman/Getty images

Anyone whose resolve to exercise in 2013 is a bit shaky might want to consider an emerging scientific

view of human evolution. It suggests that we are clever today in part because a million years ago, we

could outrun and outwalk most other mammals over long distances. Our brains were shaped and

sharpened by movement, the idea goes, and we continue to require regular physical activity in order

for our brains to function optimally.

The role of physical endurance in shaping humankind has intrigued anthropologists and gripped thepopular imagination for some time. In 2004, the evolutionary biologists Daniel E. Lieberman of

Harvard and Dennis M. Bramble of the University of Utah published a seminal article in the journal

Nature titledEndurance Running and the Evolution of Homo, in which they posited that our

bipedal ancestors survived by becoming endurance athletes, able to bring down swifter prey through

sheer doggedness, jogging and plodding along behind them until the animals dropped.

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Endurance produced meals, which provided energy for mating, which meant that adept early joggers

passed along their genes. In this way, natural selection drove early humans to become even more

athletic, Dr. Lieberman and other scientists have written, their bodies developing longer legs, shorter

toes, less hair and complicated inner-ear mechanisms to maintain balance and stability during

upright ambulation. Movement shaped the human body.

But simultaneously, in a development that until recently many scientists viewed as unrelated,

humans were becoming smarter. Their brains were increasing rapidly in size.

Today, humans have a brain that is about three times larger than would be expected, anthropologists

say, given our species body size in comparison with that of other mammals.

To explain those outsized brains, evolutionary scientists have pointed to such occurrences as meat

eating and, perhaps most determinatively, our early ancestors need for social interaction. Earlyhumans had to plan and execute hunts as a group, which required complicated thinking patterns

and, its been thought, rewarded the social and brainy with evolutionary success. According to that

hypothesis, the evolution of the brain was driven by the need to think.

But now some scientists are suggesting that physical activity also played a critical role in making our

brains larger.

To reach that conclusion, anthropologists began bylooking at existing dataabout brain size and

endurance capacity in a variety of mammals, including dogs, guinea pigs, foxes, mice, wolves, rats,

civet cats, antelope, mongeese, goats, sheep and elands. They found a notable pattern. Species like

dogs and rats that had a high innate endurance capacity, which presumably had evolved over

millenniums, also had large brain volumes relative to their body size.

The researchers also looked at recent experiments in which mice and rats were systematically bred to

be marathon runners. Lab animals that willingly put in the most miles on running wheels were

interbred, resulting in the creation of a line of lab animals that excelled at running.

Interestingly, after multiple generations, these animals began to develop innately high levels of

substances that promote tissue growth and health, including a protein called brain-derived

neurotrophic factor, or BDNF. These substances are important for endurance performance. They

also are known to drive brain growth.

What all of this means, says David A. Raichlen, an anthropologist at the University of Arizona and an

author of a new article about the evolution of human brains appearing in the January issue of

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Proceedings of the Royal Society Biology, is that physical activity may have helped to make early

humans smarter.

We think that what happened in our early hunter-gatherer ancestors, he says, is that the more

athletic and active survived and, as with the lab mice, passed along physiological characteristics thatimproved their endurance, including elevated levels of BDNF. Eventually, these early athletes had

enough BDNF coursing through their bodies that some could migrate from the muscles to the brain,

where it nudged the growth of brain tissue.

Those particular early humans then applied their growing ability to think and reason toward better

tracking prey, becoming the best-fed and most successful from an evolutionary standpoint. Being in

motion made them smarter, and being smarter now allowed them to move more efficiently.

And out of all of this came, eventually, an ability to understand higher math and invent iPads. Butthat was some time later.

The broad point of this new notion is that if physical activity helped to mold the structure of our

brains, then it most likely remains essential to brain health today, says John D. Polk, an associate

professor of anthropology at the University of Illinois at Urbana-Champaign, and co-author, with Dr.

Raichlen, of the new article.

And there is scientific support for that idea. Recent studies have shown, he says, that regular

exercise, even walking, leads to more robust mental abilities, beginning in childhood and

continuing into old age.

Of course, the hypothesis that jogging after prey helped to drive human brain evolution is just a

hypothesis, Dr. Raichlen says, and almost unprovable.

But it is compelling, says Harvards Dr. Lieberman, who has worked with the authors of the new

article. I fundamentally agree that there is a deep evolutionary basis for the relationship between a

healthy body and a healthy mind, he says, a relationship that makes the term jogging your

memory more literal than most of us might have expected and provides a powerful incentive to be

active in 2013.