We humans marvel at our big brains, which have made us the most advanced animals on the planet. But running them takes a lot of energy. A new study suggests that we paid a big price for being so smart. Over the course of our evolution, humans got weaker relative to other primates, trading brawns for brains.
At an average volume of 1400 cubic centimeters, our brains are three times as large as those of our closest living evolutionary cousins, chimpanzees. While researchers debate why our noggins got so big, one thing is for sure: The brain is a costly organ. Our brains use 20% of our energy expenditures when we are resting, more than twice as much as expended by chimps and other primates. Back in the 1990s, U.K.-based researchers Leslie Aiello and Peter Wheeler proposed what they called the expensive-tissue hypothesis, arguing that the human digestive system, which uses a great deal of energy to metabolize our food, had downsized considerably to help pay that price.
To see what other trade-offs might have occurred, a team led by Philipp Khaitovich, a biologist at the CAS-MPG Partner Institute for Computational Biology in Shanghai, China, looked at the energy use profiles of five different tissues in four animal species. Three of the tissues were in the brain: the prefrontal cortex (involved in advanced cognition), the primary visual cortex (which processes the sense of sight), and the cerebellar cortex (key to motor control). The other two tissues were the kidney and thigh muscle. The animal species in the study were humans, chimps, rhesus monkeys, and mice, whose tissues were sampled soon after their deaths.
Rather than measure energy use directly, the researchers used a proxy indicator called the metabolome—the ensemble of small molecules, or metabolites, that either fuel living tissues or make up their structures, including amino acids, fats, sugars, vitamins, and other compounds. The team detected about 10,000 different metabolites in each tissue type and compared the metabolic and genetic differences between these diverse animals, using a sample of 14 individuals from each of the four species. As the researchers report today in PLOS Biology, the differences in metabolome profiles between the mice, monkeys, and chimps were no greater than the relatively small genetic differences between them, meaning that evolution had probably not significantly altered any of their tissues. Nor was there evidence of significant evolutionary changes in the human kidney or the visual or cerebellar cortex.
On the other hand, the metabolome profile of the human prefrontal cortex was dramatically altered from that of other primates: Using the split between the human and mouse (130 million years ago) and between humans and monkeys (45 million years ago) as baselines, the team calculated that the metabolome had evolved four times faster than that of the chimpanzee over the roughly 6 million years since the human and chimp lines split. (The genetic differences between the two species, in contrast, are only about 2%.)
This result was not shocking, given the mountains of evidence for the greater cognitive prowess of the human brain compared with that of other primates. But what did surprise the team was the differences in the profiles of primate and human skeletal muscle: The human metabolome had evolved more than eight times faster than that of the chimps since the two species went their separate evolutionary ways.
To make sure this disparity wasn’t simply due to differences in environment and diet, the team exposed monkeys to something resembling the modern human lifestyle. The researchers took 12 macaque monkeys and divided them into two groups of six each. One group was put into individual, solitary cages to limit how much exercise they could get, and was fed a cooked diet high in fats and sugars; the second group was put into solitary cages but fed a normal diet of raw plant foods. When these 12 subjects were compared with a control group of 17 monkeys fed on normal diets and allowed to romp outside in family groups, the differences in their metabolomes were minimal, amounting to no more than 3% of the metabolic changes detected in humans. That rules out dietary or environmental explanations for the differences, the researchers conclude.
Finally, the team performed a key test: comparing the strength of macaques, chimps, and humans. Although very limited earlier studies had suggested that humans were the weaker species when body size is taken into account, no systematic comparisons had been done. So the researchers devised an experiment in which macaques, chimps, and humans had to pull an adjustable weight with all their strength, using the muscles of both their arms and their legs (see video). The monkeys and chimps were motivated by their desire to grab a food reward, whereas the humans—who included five university basketball players and four professional climbers—were motivated by the exhortations of the researchers to do their competitive best. The result: Humans were shown to be on average only half as strong as the other two primates.
The team concedes that it is not yet clear why the differences in metabolome between humans and other primates lead to weaker muscle strength; when the researchers looked at possible structural differences between chimp and human thigh muscle, they found none, leaving as-yet-unknown differences in energy use as the most likely explanation. And although the researchers caution that the differences between humans and other primates might have been due in part to different levels of motivation while pulling the weights, the consistency of the findings indicates that humans are indeed weaker overall. The scientists hypothesize that the parallel evolution of bigger brains and weaker muscles on the human lineage may not have been a coincidence, but rather due to a “reallocation” of energy resources between the two tissues. The idea of such a trade-off “is a very simple hypothesis,” Khaitovich says, “but in evolution simple explanations are often the best ones.”
Aiello, who is now president of the Wenner-Gren Foundation for Anthropological Research in New York City, says that recent research has suggested that “the energetic trade-offs relevant to [human] brain evolution are more complex” than she and Wheeler had originally suggested in their brain versus gut hypothesis, and that “this work demonstrates another possible trade-off between the metabolic requirements of the brain and skeletal muscle.”
However, Aiello and other researchers think that humans didn’t just get weaker, but started using their muscles in different ways that required less overall strength, for example for endurance running during hunting or other activities—an idea that has been championed by Daniel Lieberman, an anthropologist at Harvard University.
Lieberman says that the new paper “is very cool and interesting,” but he doesn’t buy its suggestion of a brain versus brawn trade-off during human evolution. “Humans are less strong than chimps, but I don’t believe we are less athletic,” Lieberman says. Thus, he argues that humans still used a great deal of muscular energy, but applied it to tasks that enhanced their survival over the long term rather than to feats of brute power. With our bigger and cleverer brains, Lieberman says, humans devised ways to be more energy efficient, by becoming more effective hunters, learning to cook our food, and sharing resources among larger groups. In other words, in the evolutionary sweepstakes, victory sometimes goes to the brainiest rather than the brawniest.
(Video credit: Kasia Bozek)