Humans have a uniquely high metabolism compared to other mammals, including our primate relatives. This allows us to fuel our large brains, extended lifespans, and high physical activity levels without sacrificing resting metabolic rates.
Credit: SciTechDaily.com
Humans have evolved an exceptionally high metabolic rate, unlike any other known animal. Unlike primates that conserve energy by being less active, humans can stay on the move thanks to sweating, which prevents overheating. This metabolic advantage may have been key to our evolutionary success.
Humans have significantly higher metabolic rates than other mammals, including our closest relatives—apes and chimpanzees—according to a new Harvard study. Researchers suggest that having both a high resting metabolism and an active one allowed our hunter-gatherer ancestors to gather enough food while also supporting larger brains, longer lifespans, and higher reproduction rates.
“Humans are off-the-charts different from any creature that we know of so far in terms of how we use energy,” said study co-author and paleoanthropologist Daniel Lieberman, the Edwin M. Lerner Professor of Biological Sciences in the Department of Human Evolutionary Biology.
Andrew Yegian and Daniel Lieberman.
Credit: Dylan Goodman
The Energy Puzzle: How We Burn Calories Differently
Published in the Proceedings of the National Academy of Sciences, the study challenges the long-held belief that human and non-human primates have similar or even lower metabolic rates than expected for their body size.
Using a refined comparison method that accounts for body size, environmental temperature, and body fat, the researchers found that humans, unlike most mammals—including other primates—have evolved in a way that avoids the typical tradeoff between resting and active metabolism.
Why Chimps Are Couch Potatoes and Humans Keep Moving
Animals take in calories through food and, like a bank account, spend them on expenses mostly divided between two broad metabolic categories: resting and physical activity. In other primates, there is a distinct tradeoff between resting and active metabolic rates, which helps explain why chimpanzees, with their large brains, costly reproductive strategies, and lifespans, and thus high resting metabolisms, are “couch potatoes” who spend much of their day eating, said Lieberman.
Generally, the energy animals spend on metabolism ends up as heat, which is hard to dissipate in warm environments. Because of this tradeoff, animals such as chimpanzees who spend a great deal of energy on their resting metabolism and also inhabit warm, tropical environments, have to have low activity levels.
Comparisons of resting, active, and total metabolic quotients among various species and human populations, as defined by the Harvard researchers’ new method.
Credit: Andrew Yegian
The Secret Weapon: Sweating for More Energy
“Humans have increased not only our resting metabolisms beyond what even chimpanzees and monkeys have, but — thanks to our unique ability to dump heat by sweating — we’ve also been able to increase our physical activity levels without lowering our resting metabolic rates,” said co-author Andrew Yegian, a senior researcher in Lieberman’s lab.
“The result is that we are an energetically unique species.”
How Our Bodies Became Metabolic Powerhouses
The team’s analysis shows that monkeys and apes evolved to invest about 30 to 50 percent more calories in their resting metabolic rates than other mammals of the same size, and that humans have taken this to a further extreme, investing 60 percent more calories than similar-sized mammals.
“We started off questioning if it was possible that humans and other primates could have comparatively low total metabolic rates, which other researchers had proposed,” Yegian said. “We tried to come up with a better way to analyze it using quotients. That’s when we hit the accelerator.”
What’s Next? Studying Metabolism in the Modern World
The research team — which includes collaborators at Louisiana’s Pennington Biomedical Research Center and the University of Kiel in Germany — plans to further investigate metabolic differences among human populations. For example, subsistence farmers who grow all the food they eat without the help of machines have significantly higher physical activity levels than both hunter-gatherers and people in industrial environments like Americans. However, all human populations, regardless of activity levels, spend similar amounts of energy for their body size on their resting metabolic rates.
“What we’re really interested in is variation among humans in metabolic rates, especially in today’s world of increasing technology and lower levels of physical activity,” said Yegian. “Since we evolved to be active, how does having a desk job change our metabolism in ways that affect health?”
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