But whether a similar trade-off occurred with our muscles has remained in doubt. Muscles potentially provided another route to survival during our species’ early days. With sufficient brawn, animals, including people, could physically overpower prey and sprint from danger.
But muscles are also very calorically needy and, like brain tissue, use blood sugar as their primary fuel. So scientists have wondered whether and how early humans’ bodies balanced the fueling needs of their brains and their brawn. Did one take precedence over the other? If so, that choice could tell us something about the underpinnings of human development and also how best, even now, to manage thinking and moving.
Since experiments on cavepeople are, however, not practicable, researchers at Cambridge University in England decided instead to focus on the bodily machinations of 62 elite, collegiate rowers for their new study, which was published this month in Scientific Reports.
The scientists hoped to suss out what happens when both muscles and minds are stressed, and if one of those operations gets preferential treatment from the body.
To find out, they asked the rowers, who were all young, male and fit, to visit a university lab on three separate occasions.
During one visit, the men sat quietly while dozens of words were displayed on a large screen in front of them. The men had three minutes to memorize the words and then, immediately afterward (when the screen went dark) write down as many as they could remember. This was their mental task.
On another day, they rowed on a rowing machine as intensely as they could for three minutes while the researchers tracked their power output, testing muscular prowess.
Finally, on the last visit, they rowed for three minutes while simultaneously viewing a list of new words and then, immediately afterward, writing down as many as they could recall.
Then the researchers simply compared their performance on each task. Almost uniformly, the men had been able to produce fewer watts and recall fewer words when they performed the muscular and mental tasks together.
But the falloff in physical functioning was much steeper than the mental slump. The rowers lost almost 13 percent of their power output, a decline that was about 30 percent greater than their loss in word recall after the combined session.
“Our proposed explanation for this finding is that they were both competing for the same resource,” which in this case was blood sugar for fuel, says Danny Longman, a postdoctoral research fellow at Cambridge who led the study.
And the brain won. The implication of this victory is that thinking probably provided more advantage for us during evolution than brawniness, Dr. Longman says, and on those occasions that both systems needed to be fed, the brain got its portion first.
Of course, this study was very short-term and viewed the tug-of-war between brains and muscles only indirectly. The researchers did not track actual changes in blood sugar uptake by any tissues. They also looked only at quite-intense exercise and used memory recall as their sole marker for thinking.
But even with these limitations, the study to some extent advances our understanding of how we became the species that we are, Dr. Longman says.
“For me, the main message of this study is a bit philosophical,” he says. “An enlarged and highly functioning brain is one of the key factors that make us human. This study demonstrated, in a very simple way, this defining characteristic of our species.”
More humbly, the results also indicate that intense workouts may not be the optimal time to compose your next epic poem or calculate tax withholding.
