Humans Sacrificed Brawn for Brains As We Evolved

By Carl Engelking | May 27, 2014 4:39 pm

man and monkey boxingAmongst all the evolutionary changes since hominids split from apes, the one we associate most closely with our humanness is probably our brains. They’ve gotten larger and smarter as our cognition and society has gotten more complex. But new research shows that another aspect of our physiology has also had to rapidly adapt in the opposite direction: our musculature.

By comparing concentrations of human metabolites — fundamental molecules like sugars, vitamins, amino acids and neurotransmitters — between monkeys, mice and humans, researchers could pinpoint two areas where humans rapidly diverged from primates: brain tissue and skeletal muscle. Their findings, published today in the journal PLOS One, suggest that through evolution we traded in our brawn for the benefit of brains.

Metabolically Speaking

Analyzing the metabolome — the unique signature of metabolites in tissues — can reveal insights into physiological evolution that go deeper than studying the genome. To locate where humans diverged from primates, researchers compared the metabolomes of postmortem tissues taken from 14 humans and a larger number of chimpanzees, rhesus macaques and mice.

They discovered that humans’ prefrontal cortex, a part of the brain responsible for planning and cognition, evolved four times faster than that of chimpanzees since the two diverged — but that skeletal muscle changed a stunning seven times faster. It’s been known for quite some time that primates are much stronger than humans, but the reason why has remained a mystery. Since past studies showed that the human gut shrank in favor of a larger brain, researchers hypothesized that our muscles were also paying a price for a growing brain.

Testing Strength

Then, researcher pitted 42 humans (some athletes and some average folk) against five chimps and six macaques in a weightlifting test. Each species lifted weights with their legs and arms until they reached their maximum threshold. As expected, humans were far weaker than their primate cousins — on average primates could lift twice as much relative to their body weight as humans could.

Researchers thus believe that skeletal muscle and brain matter are linked in a competition for resources, which has been dominated by the brain. The more energy our ancestors devoted to developing our brains, the less energy went to building up our muscle strength.

Therefore, we may not be as strong as monkeys, but while they toil in the wild climes of Earth, humans have landed on the Moon and are setting their sights on Mars. Brains for the win!


Photo credit: Everett Collection/Shutterstock

CATEGORIZED UNDER: Living World, Mind & Brain
  • Ralph Haygood

    Today’s report is part of a growing body of work pointing in the same direction. For example, if you’ll pardon my tooting my own horn a little:

    O. Fedrigo et al., 2011, “A potential role for glucose transporters in the evolution of human brain size”, Brain, Behavior and Evolution 78:315-326.

    Abstract: Differences in cognitive abilities and the relatively large brain are among the most striking differences between humans and their closest primate relatives. The energy trade-off hypothesis predicts that a major shift in energy allocation among tissues occurred during human origins in order to support the remarkable expansion of a metabolically expensive brain. However, the molecular basis of this adaptive scenario is unknown. Two glucose transporters (SLC2A1 and SLC2A4) are promising candidates and present intriguing mutations in humans, resulting, respectively, in microcephaly and disruptions in whole-body glucose homeostasis. We compared SLC2A1 and SLC2A4 expression between humans, chimpanzees and macaques, and found compensatory and biologically significant expression changes on the human lineage within cerebral cortex and skeletal muscle, consistent with mediating an energy trade-off. We also show that these two genes are likely to have undergone adaptation and participated in the development and maintenance of a larger brain in the human lineage by modulating brain and skeletal muscle energy allocation. We found that these two genes show human-specific signatures of positive selection on known regulatory elements within their 5′-untranslated region, suggesting an adaptation of their regulation during human origins. This study represents the first case where adaptive, functional and genetic lines of evidence implicate specific genes in the evolution of human brain size.

  • KokoTheTalkingApe

    The article claims that our genes somehow shifted something from muscles to our brains. What was that something? Mitochondria? And why must must muscles lose if the brain gains? Wouldn’t it be a relatively small set of mutations to just increase mitochondria (or whatever it is) in the brain, while leaving the ones in the muscles unaffected?

  • Sue Thomason

    You wouldn’t think our brains have developed at all if you read some of the stuff on this site about ‘women’s attractiveness’.


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