Our big brains likely came at a price. A comprehensive new study of metabolites suggests that as our brains got bigger, our muscles got weaker relative to other primates.
On average, human brains are about three times as large as those of chimpanzees. The organ consumes 20 percent of our total body energy when we’re resting -- that’s twice as much as that expended by chimps. So maybe we had to make some trade-offs in order to have such costly cerebral matter.
Metabolites are the small molecules that result from our breakdown of food -- like sugars, fats, amino acids, and vitamins. Some metabolites are flushed out of our bodies, while others are crucial for our various physiological functions. "Metabolites are more dynamic than the genome and they can give us more information about what makes us human,” says Philipp Khaitovich of Max Planck Institute for Evolutionary Anthropology. "It is also commonly known that the human brain consumes way more energy than the brains of other species; we were curious to see which metabolic processes this involves."
To examine the evolution of our metabolite compendiums (or “metabolome”), a large interdisciplinary team analyzed 10,000 metabolites from tissue taken from the kidney, thigh muscle, and three brain regions of humans, chimpanzees, macaque monkeys, and mice.
They found accelerated evolution of metabolites in the human brain as well as in human muscle (that second part was a surprise). Furthermore, the metabolome of the human brain -- particularly the prefrontal cortex involved in advanced cognition -- has evolved four times faster than that of chimps over the 6 million or so years since we split. That’s remarkable considering how genetically, chimps and humans are only different by about 2 percent. And as it turns out, human muscle accumulated an even higher amount of metabolic change: eight times that of chimps.
But what if that’s just because some of us are inactive couch potatoes? To rule that out, the team performed additional measurements on macaques who had been moved from a large, countryside facility to small indoor cages and served cooked, fatty, sugary food for weeks. These lifestyle changes only had a small effect on the macaque muscle metabolome. "For a long time we were confused by metabolic changes in human muscle,” Max Planck’s Kasia Bozek says, "until we realized that what other primates have in common, in contrast to humans, is their enormous muscle strength."
So, as a follow-up, the team tested the strength of humans and nonhuman primates. Several chimps, macaques, college basketball players, and even professional mountain climbers competed in a pulling strength challenge. Turns out, human strength is half that of primates. "This is common knowledge to all the zoo keepers, but it was never tested systematically,” Max Planck’s Josep Call says in a news release.
While it’s not clear why metabolome differences between humans and other primates led to our weaker muscle strength, the team does think that the metabolic roles of human brain and brawn are intertwined. Seems like we have some unique energy management system that allows us to allocate energy for “our extraordinary cognitive powers at a cost of weak muscle,” Bozek explains.
The findings were published in PLOS Biology this week.
Image: William Warby via Flickr
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