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HOW HUMANS EVOLVED TO BE ‘ENERGETICALLY UNIQUE’  
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Anne J. Manning
November 18, 2024
The Harvard Gazette
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_ Human ancestors with higher metabolic rates outpaced ‘couch
potato’ primates thanks to sweat, says new study. “The result is
that we are an energetically unique species.” _

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

 

Humans, it turns out, possess much higher metabolic rates than other
mammals, including our close relatives, apes and chimpanzees, finds a
new Harvard study. Having both high resting and active metabolism,
researchers say, enabled our hunter-gatherer ancestors to get all the
food they needed while also growing bigger brains, living longer, and
increasing their rates of reproduction.

“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
[[link removed]], the Edwin M.
Lerner Professor of Biological Sciences in the Department of Human
Evolutionary Biology.

The paper, published Monday in Proceedings of the National Academy
of Sciences
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previous consensus that human and non-human primates’ metabolic
rates are either the same or lower than would be expected for their
body size. 

[ Comparisons of resting, active, and total metabolic quotients among
various species and human populations, as defined by the Harvard
researchers’ new method.]

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

Using a new comparison method that they say better corrects for body
size, environmental temperature, and body fat, the researchers found
that humans, unlike most mammals including other primates, have
evolved to escape a tradeoff between resting and active metabolic
rates. 

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.

“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
[[link removed]], a senior
researcher in Lieberman’s lab.

“The result is that we are an energetically unique species.”

“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.” – Andrew Yegian

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
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“We tried to come up with a better way to analyze it using
quotients. That’s when we hit the accelerator.” 

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?”

_Anne J. Manning is a Harvard staff writer_

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