Anyone whose resolve to exercise this year is a bit shaky might want to consider an emerging scientific view of human evolution.
This suggests we are clever today partly 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 physical activity for our brains to function optimally.
The role of physical endurance in shaping humankind has intrigued anthropologists for some time. In 2004, evolutionary biologists Daniel E. Lieberman of Harvard and Dennis M. Bramble of the University of Utah published a seminal article in the journal Nature titled ''Endurance Running and the Evolution of Homo'', positing 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 finally dropped.
Human endurance produced meals, which provided energy for mating, which meant that the adept early joggers passed along their genes. In this way, Lieberman and other scientists have written, natural selection drove early humans to become even more athletic, 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 mind and body. But simultaneously, in a development that until recently many scientists viewed as unrelated, humans were becoming smarter. Their brains were rapidly increasing in size.
Today, humans have a brain that is about three times larger than would be expected, anthropologists say, given the body size of our species compared with that of other mammals.
To explain those outsize brains, evolutionary scientists have pointed to developments such as meat-eating and our early ancestors' need for social interaction. Early humans had to plan and execute their hunts as a group, which required complicated thinking patterns, and, it has been thought, rewarded the social and brainy with evolutionary success. According to this hypothesis, the evolution of the brain was driven by the need to think.
But some scientists are now suggesting that physical activity played a critical role in making our brains larger. To reach that conclusion, anthropologists began by looking at the existing data about brain size and endurance capacity in a variety of mammals, including dogs, guinea pigs, foxes and wolves.
They found a notable pattern. Species such as dogs and rats that have a high innate endurance capacity, which presumably evolved over millennia, have large brain volumes relative to their body size.
The researchers looked at recent experiments in which mice and rats were bred to be marathon runners. Those laboratory animals that willingly put in the most kilometres on running wheels were interbred, resulting in the creation of a line of lab animals that excelled at running.
After multiple generations, these animals began to develop innately high levels of the substances that promote tissue growth and health, including a protein called brain-derived neurotrophic factor, or BDNF. These substances are important for endurance, and they are known to drive brain growth.
What all of this means, says David Raichlen, an anthropologist at the University of Arizona and an author of a new article in Proceedings of the Royal Society about the evolution of human brains, is that physical activity may have helped to make the early humans smarter.
In our early hunter-gatherer ancestors, he said, the more athletic and active survived, and as with the lab mice, passed along the physiological characteristics that improved their endurance, including elevated levels of BDNF. Eventually, these early athletes had enough BDNF that some of it 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 ways of tracking prey, becoming the best-fed and most successful from an evolutionary standpoint. Being in motion made them smarter, and being smarter allowed them to move more efficiently.
And out of all of this eventually came an ability to understand higher maths and invent iPads.
The point of this new notion is that if physical activity helped to mould the structure of our brains, it probably 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 Raichlen, of the new article.
And there is scientific support for the idea, Polk says. Recent studies have shown that regular exercise, even walking, leads to more robust mental abilities, ''beginning in childhood and continuing into old age''.
However, the hypothesis that jogging after prey helped to drive the evolution of the human brain is just that - a hypothesis, Raichlen says, and almost unprovable.
But it is a compelling explanation, says Harvard's Lieberman, who has worked with the authors of the new article.
''I fundamentally agree there is a deep evolutionary basis for the relationship between a healthy body and a healthy mind,'' he said - 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 this year.
The New York Times