Human metabolism has evolved to become 'energetically unique'

Some of us may rave about our fast metabolism, while others have witnessed how it slows down with age. But how does the human metabolic rate compare to that of other mammals? It turns out, we humans are unique metabolic hotshots.

According to a new study by Harvard researchers, humans have significantly higher metabolic rates than other mammals. That includes our close kin, the apes, and chimpanzees.

Human metabolism: A delicate balance

Metabolism refers to all the chemical reactions that occur in our bodies and that help us maintain life; this includes those reactions involved with converting food into energy.

Picture this as a bank account where you deposit calories via the food you eat and then spend them on various activities – such as just existing (resting metabolism), and, well, everything else (active metabolism).

This balance between resting and active metabolism is where we humans stand apart.

Human metabolic rate

The human metabolic rate is unusual and challenges previous scientific consensus. Humans, it turns out, are rather energy extravagant.

We defy the trade-off between resting and active metabolic rates commonly seen in other mammals, including apes and monkeys. But how did we manage to escape this trade-off?

The usual scenario looks like this: animals with high resting metabolic rates use up energy to maintain life at rest and consequently have to cut back on their physical activity.

It’s like spending a large chunk of your income on rent, which leaves you with less to spend on holidays.

The same happens with our primate cousins. Chimpanzees, with their large brains, long lifespan, and expensive reproductive strategy, tend to be “couch potatoes,” and spend a lot of time eating to fuel their high resting metabolism.

How is human metabolism different?

The research revealed that not only do we humans have higher resting metabolisms than our primate cousins, but we’re also great at being active without decreasing our resting rates.

Daniel Lieberman, a paleoanthropologist, the Edwin M. Lerner Professor of Biological Sciences at Harvard and co-author of the study, deems humans to be ‘off-the-charts’ in how they use 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,” explained co-author Andrew Yegian, a senior researcher in Lieberman’s lab.

Energetically unique humans

Yegian explains that, because of our high resting and active metabolic rates, humans can be crowned as an “energetically unique species.”

How unique are we? Consider this: monkeys and apes invest about 30 to 50% more calories in their resting metabolic rates than other mammals of the same size.

Humans aren’t content with just that. We go a step further, investing a whopping 60 percent more calories than other, similar-sized mammals.

Path to metabolic uniqueness

The evolutionary journey that led to this human metabolic anomaly likely involves several intertwined pathways.

Evolutionary pressures favored a high-energy lifestyle, driven by the caloric demands of our large brains and the physical requirements of our environments. Early humans likely benefited from a metabolism capable of supporting endurance activities like persistence hunting over vast distances.

This capability, alongside the development of complex social systems and tools, would have conferred an adaptive advantage. Moreover, our penchant for cooking and processing food allowed us to extract more energy from our diets, supporting both our brain growth and physical exertions.

These evolutionary adaptations highlight the intricate balance between energy intake and expenditure that underlies the human condition, and that sets us apart on the mammalian landscape.

Implications for modern life

Understanding our heightened metabolic rate has profound implications for health and lifestyle in the modern world.

On one hand, our energetic physiology underlines the importance of regular physical activity, as our bodies are well-equipped to handle and benefit from it.

However, modern lifestyles can diverge significantly from the active lives of our ancestors, potentially contributing to the rise of metabolic disorders such as obesity and diabetes. This disconnect suggests a need to harmonize our diet and activity patterns more closely with our evolutionary design.

Furthermore, the insights gained from studying human metabolism can guide public health policies and provide targeted strategies for disease prevention and management, making this field of study both fascinating and critically important.

Human evolutionary advantage

This unique metabolic feature has been instrumental in our evolution – it has helped our hunter-gatherer ancestors meet their food requirements, supported the growth of bigger brains, and increased our lifespans and rates of reproduction.

That’s not all. The researchers are now intrigued to explore metabolic variations within human populations. They are particularly interested to see how changes in lifestyle due to technology and decreased physical activity might be causing shifts in our metabolic profiles.

As we continue to evolve in a world that is increasingly dependent on technology, it will be interesting to see how our energetic uniqueness adapts.

The study is published in the journal Proceedings of the National Academy of Sciences.

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