THAT swots are weedy and jocks are stupid is a high-school cliché. But a paper just published in PLOS Biology by researchers at the Chinese Academy of Sciences and the Max Planck Institutes in Germany suggests there may be an evolutionary grain of truth in it. When Katarzyna Bozek and her colleagues looked at how quickly human tissues have evolved, compared with those of other mammals, they found that as the human brain has got stronger, so the species’s muscles have got weaker. Intriguingly, in a demonstration of the importance of serendipity in science, this was not a hypothesis they had set out to prove.

Human brains are greedy. Though they constitute only 2% of an adult’s body weight, they consume a fifth of his or her metabolic energy. Indeed, according to a school of thought led by Richard Wrangham of Harvard University, keeping the brain running is so demanding that only the invention of cooking, which makes more nutrients available from a given amount of food than can be extracted from it in its raw state, permitted the neurological expansion which created Homo sapiens. This need to supply the brain with energy suggests that other human organs are under pressure to economise. And muscle, it seems, has economised a lot.

Dr Bozek and her colleagues began their study by looking at what is known as the metabolome. This is the mixture of small, water-soluble chemicals such as sugars, amino acids and vitamins that are the currency of biochemistry. The researchers examined almost 10,000 of these chemicals in five types of tissue taken from 14 individuals of each of four species of mammal: humans, chimpanzees, rhesus monkeys and mice. (The samples were removed from previously healthy, dead individuals. The humans had died mainly in accidents.) Three of the tissue types came from the brain, namely the prefrontal cortex, the visual cortex and the cerebellum. The other two were thigh muscle and kidney.

Metabolic enhancement

Roughly speaking, you would expect differences between the metabolomes of two species to be proportional to the amount of time they have been evolving separately. Larger differences in a particular type of tissue would suggest strong and specific evolutionary pressures on that tissue. For this reason, the researchers thought human prefrontal cortex (which has expanded enormously in the past few million years, and is the seat of complex cognitive behaviours such as abstract thought) would show markedly greater differences in metabolome from its equivalent in other species than was the case for the remaining tissues. These, they expected, would simply have chugged along.

Their guess about the prefrontal cortex was pretty accurate. Its metabolism has changed four times faster in humans than in chimpanzees since their lines split 6m years ago. The other brain regions, and the kidneys, showed no such acceleration—again, as the researchers had expected.

What they had not expected was that human thigh-muscle metabolism had changed even faster than frontal-lobe metabolism. It had altered eight times faster than in chimpanzee muscle since the lines split. Indeed, it had undergone more divergence in that period than mouse muscle had since the lines leading to mice and men separated from one another 130m years ago. That led Dr Bozek to wonder just what was going on.

Possibly, human muscle has strengthened. But anecdotal evidence suggests humans are weaker than other primates, so that seemed unlikely. To find out, the team did an experiment by organising trials of strength between people, chimps and monkeys. These involved subjects pulling suspended weights up against the force of gravity (in the case of chimps and monkeys this was done for a food reward). The tests showed that both chimps and rhesus monkeys are twice as strong as people, with untrained apes and monkeys outperforming university-level basketball players and professional mountaineers.

The team also checked the idea that human muscle is odd because modern people are, compared with their wild ancestors, lazy. They did this by enforcing a “couch-potato” lifestyle of high-sugar, high-fat diets and limited exercise on some monkeys for a month. It made little difference to their muscle metabolomes.

All this suggests that Homo sapiens has indeed sacrificed brawn for brains. So, although Dr Bozek and her colleagues did not actually compare individual people, when geeks refer to jocks in disparagingly simian terms they may have hit on an underlying reality.