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

      Fantastic! So I imagine a “glucose transporter” is a protein, not a cell or an organelle? And why would they be conserved? Are they extremely energy-intensive to make? Why couldn’t the body just make more of them to serve the growing brain, the way it grows more blood vessels or blood cells? Thank you!

      • Ralph Haygood

        Yes, glucose transporters are proteins. The proteins themselves tend to be well conserved, presumably because there’s little need for them to differ in different organisms; the chemistry of glucose transport varies little across large groups of organisms. However, regulatory DNA sequences near the sequences coding for the proteins aren’t so conserved, probably in part because different organisms need different allocations of glucose across tissues; the regulatory sequences affect how many copies of the proteins are present in various kinds of cell, which in turn affects how much glucose the cells extract from the bloodstream. The limiting resource isn’t the transporters but the glucose. Given a certain amount of glucose, which for an animal is ultimately determined by diet, more glucose for one tissue requires less glucose for others. One way to shift glucose from one tissue to another is to change how many copies of glucose transporter proteins are present in the cells forming those tissues. My colleagues and I suspect certain mutations in regulatory sequences of SLC2A1 and SLC2A4 were beneficial and hence fixed by natural selection in our ancestors because they contributed to redirecting glucose away from muscles and toward the brain, as our ancestors gained more in survival and reproduction through being increasingly brainy than they lost through being decreasingly brawny.

        • KokoTheTalkingApe

          Thanks! Maybe I used the wrong word. I meant “conserved” as “set at some amount within a single organism” the way energy or momentum are conserved in physics. So the article sounds as if “energy” within the body is “conserved”, meaning there is only a set amount that has to be allocated to one thing or another, and the total amount cannot be increased. But you say that transporters are conserved, but only because glucose is conserved (which makes sense; why make more transporters if there is only so much glucose available?) But then the question is, why is glucose limited that way? I would think that if humans evolved larger brains, they could simply eat more, and then keep their nice strong muscles AND have their lovely brains too. Why must there be an overall limit to available glucose?

          • Ralph Haygood

            The amount of glucose available to an animal per unit time isn’t exactly fixed, but it certainly isn’t unlimited. Ultimately, glucose comes from food, and there are limits on how much food is available in an animal’s habitat, how quickly the animal can harvest or capture, consume, and digest it, and how much energy – glucose by any other name – all this costs the animal. For most animals most of the time, these limits are significant. That’s true even for most humans living as nomadic foragers today. Of course, many adaptations are adaptive precisely because they raise the limits, for example, by enabling animals to eat previously inedible things (e.g., plants that synthesize nasty chemicals). However, raised limits are still limits.

          • KokoTheTalkingApe

            I know they are diverse, but I thought people in many hunter-gatherer cultures many of them spend only a few hours a day gathering or preparing food. In any case, a larger brain would presumably also make you a more efficient hunter or gatherer, so you would think there would be net benefit, as you say. In fact I would think that is what reduced the glucose consumption by muscles; smarter man-apes don’t need to be a strong to live long and prosper, instead of smartness causing man-apes to become weaker. Limits are limits, but only when you bump up against them. If larger brains increase the available glucose (by increasing hunting or gathering efficiency), then theoretically, glucose might never limit either brain size or muscle strength. Other limits, like the width of female hips or the length of childhood, might come into play.

  • 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’.

  • MechMan

    Humans have weaker muscles most likely for the following reasons:

    1) We stopped needing the physical strength of other primates

    2) Our big brains require a lot of energy to function. Maintaining a strong musculature also would require a lot of energy. In order to feed a human that had the physical strength of say a chimpanzee, you’d need a ton of food, and this would mean a reduction in population size, which would harm species survival of diseases and predators.

    3) Humans developed use of tools and projectile weapons, which meant we stopped using raw physical strength as much, and thus we gradually began losing it. Stone tools work just fine to process animals and vegetation and spears allowed humans to kill any animal.

    4) Differences in arm leverage. Humans are the only primates capable of an overhand throw. This is due to how the muscles and tendons in our arms are set. By contrast, for example, chimpanzee arms are very superior in terms of leverage via the muscles and tendons for pulling. Yet, chimps can’t throw worth anything. A twelve year-old human can throw with more force then a chimpanzee. So humans, being a plains ape, sacrificed the raw strength used for swinging through trees for the ability to throw projectiles in order to be able to kill large animals. This also makes sense considering that humans are endurance animals, designed to run distance.

    This use of distance running, projectile weaponry, and tool creation and use made humans the apex predator on the Earth. Because most predators use physical strength to bring down their prey, they are limited in what they can hunt. The weak little apes called humans, by contrast, can hunt anything, because they don’t rely on physical strength in the first place to bring the animal down. Instead, they’ll run it down over distance and/or then throw a spear into it. Humans also learned to use tools to make and set traps.


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