Metabolism studies reveal surprising insights into how we burn calories—and how cooperative food production helped Homo sapiens flourish
It was my daughter Clara’s seventh birthday party, a scene at once familiar and bizarre. The celebration was an American take on a classic script: a shared meal of pizza and picnic food, a few close COVID-compliant friends and family, a beaming kid blowing out candles on a heavily iced cake.
These evolutionary shifts reverberate today. The cooperative foraging that pushed our hunting, gathering and farming ancestors to flout long-established ecological rules didn’t just change the foods we eat. It altered fundamental aspects of our biology, including our metabolism. The same unlikely series of events that gave us birthday cake has also shaped the way we eat it—and how we use the calories.
Humans are a striking example of this evolutionary book-keeping in action. The traits that distinguish us from the other apes, including our huge brains, big babies and long lives, all require a lot of energy. We pay for some of these costs by spending less on our digestive system, having evolved a shorter intestinal tract and smaller liver. But we have also increased our metabolic rate and the size of our energy budget.
A Metabolic Database My colleagues and I have begun to fill that gap in scientific understanding. In 2014 John Speakman, a researcher in metabolism with laboratories at the University of Aberdeen in Scotland and the Chinese Academy of Sciences in Shenzhen, organized an international effort to develop a large metabolic database.
We did find a decline in metabolism with age, but it doesn’t kick in until we hit 60. After 60, metabolism slows by around 7 percent per decade. By the time men and women are in their 90s, their daily expenditures are 20 to 25 percent lower, on average, than those of adults in their 50s. That’s after we account for body size and composition. Weight loss with old age, especially diminished muscle mass, compounds the decline in expenditure.
For the past decade I’ve been working with colleagues to understand the calorie economy in the Hadza community of northern Tanzania. The Hadza are a small population of 1,000 or so, and about half of them maintain a traditional hunting-and-gathering way of life, foraging on the savanna landscape they call home. No population alive today is a perfect model of the past, but groups like the Hadza, who continue these traditions, provide a living example of how these systems work.
It’s the positive feedback engine that propelled the human species to new heights. Hunting and gathering is so productive that it creates an energy surplus. Those extra calories are channeled to offspring, meaning they can take longer to develop, learning skills that make them effective foragers. Reaching adulthood, they’ll do just as their parents did, acquiring extra food and plowing those calories into the next generation.
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