Why do humans have an unexpected extra amount of energy and what is it for?

AdobeStock 261706134

Why humans have unexpected extra energy (and what it is for)

file 20230619 22241 oh4l8z.png?ixlib=rb 1.1
Shutterstock / Axel_Kock
Luis Ríos Frutos, Complutense University of Madrid

Metabolism is the set of transformations through which, from organic matter (food), water and oxygen, we build our body and generate energy for all vital processes. We expend energy when we exercise. But also when sleeping, maintaining and repairing the body. Or spending a pleasant afternoon reading: let's not forget that maintaining the brain "is difficult" for an adult 20 % of all your daily energy.

How humans obtain and expend energy has been the subject of discussion in the sciences and humanities. Even more so since it has been discovered that we have unexpected extra energy. Compared to other primates, we use more energy than might seem necessary given our physical characteristics.

Why do humans need that extra energy? And how do we get it?

One of the most exciting answers to both questions has to do with our biocultural nature.

The human energy paradox

During our evolution we have developed traits that differentiate us from our closest living relatives: we grow more slowly, we develop a larger brain, we reproduce more, we live longer and we are more resistant in physical exercise than chimpanzees, bonobos and gorillas.

These traits are energetically expensive, and making them possible throughout our lives is what has been called the human energetic paradox.

Until a few years ago, surprisingly no one had compared the total energy expenditure between humans and great apes. The first to do so was the experimental biologist Herman Pontzer, who proposed that Humans are metabolically “accelerated”. This means that the human lineage has experienced an acceleration in metabolic rate throughout evolution, providing energy for larger brains and faster reproduction without sacrificing organismal maintenance and longevity. Pontzer described it as the unexpected extra of human energy: we expend more energy per unit of mass than our closest relatives. And that is accompanied by a particularly accelerated metabolism.

The center is in the heart

If we follow the trail of energy expenditure, the path takes us directly to the heart, the organ that has embodied the essence of life throughout our history. It has always been the protagonist, since the prehistoric representation in the Pindal Cave in Asturias to today's emphasis on good heart health, to its weighing on the journey to the afterlife in ancient Egypt, or his transformation into a jaguar after death for the Amazonian Waorani.

The heart provides the entire body with the energy necessary for all physiological activity. We can quantify this supply with the cardiac output, which is the product of the amount of blood we pump in one heartbeat times the number of heartbeats per minute.

This flow has an approximate average value of five liters per minute. This way we maintain the circulation of raw materials (oxygen and nutrients) and waste (carbon dioxide, water) of our metabolism, which leads us to the human energy paradox. If compared to other great apes we are metabolically accelerated (we use more energy per unit of mass), we can expect that our cardiac output per unit of mass will also be greater.

One way to study it is by measuring the diameter of the “pipe” through which all the blood in a beat leaves the heart. From echocardiograms in living great apes we observe that our aorta is similar to that of gorillas, which can double our size. In fact, controlling for weight we have a larger aorta per unit of mass than gorillas. To feed our accelerated metabolism we have a higher cardiac output per unit of mass: a gorilla aorta in a human body.

Biocultural reproduction

And good? How do we sustain the system? How do we achieve that extra energy that allows us to develop a voracious brain or give birth more than any other primate?

An answer to these questions is found in our biocultural nature.

Let's think about an ingredient of the paradox: our big brain. It finishes growing in volume at approximately six years of age, in a process so expensive that it slows down body growth and uses no less than 40 % of children's total daily energy (and more than 65 % of resting energy). In fact, in childhood we have a cardiac output and we burn more energy per unit mass than adults.

Let's now think about another ingredient of the paradox: we reproduce more. Human mothers have the ability to have babies at a higher rate than other great apes. Humans wean the offspring to become pregnant again when it is still dependent and burns energy at a very high rate. This is only possible thanks to the provision of care and nutritious, processed food provided by the community, a uniquely human characteristic: we do not raise alone.

As human biologist Barry Bogin suggests, one key is that humans practice a biocultural reproduction. In our species, the care and provision of offspring is not just a matter for the parents. In this task it is essential the role of grandmothers, grandparents, older brothers and sisters, and other relatives. But also the role of other people in the group whose bond is defined culturally, beyond genetics (the “aunts” and “uncles” of childhood, actually close friends of our parents). Another uniquely human characteristic.

Following this biocultural energetic path, it has been proposed that, during our evolution, one of the solutions for the search for nutritious food was the development of hunting and gathering and horticulture. These social, group skills allowed us to improve our energy return rate.

Through a complex, intense and costly but profitable social process, we get more calories in less time than the other great apes. This is how we feed the machine.

survival of the friendliest

Biocultural reproduction and increased energy return cannot be explained without a continued increase in our sociability throughout evolution. An increase that surely occurred in a feedback that included other milestones such as fire, language and cooking food.

So striking is our sociability that it has been suggested that our evolution is a process of self-domestication, of "survival of the friendliest”.

Returning to the fundamental perspective of matter and energy cycles, hearts and brains are part of individual metabolisms woven into a social group. After all, they are part of an evolutionary biocultural matrix capable of paying for our energy paradox.

The essay of the recently deceased Nuccio Ordine The usefulness of the useless expresses the depth of these links. In it, Ordine writes: “The need to imagine, to create, is as fundamental as breathing.” Thus, according to Ordine: “This breath […] expresses the surplus of life with respect to life itself […], energy that circulates invisibly and that goes beyond life, even though it is immanent to it.”

An evocative reflection to respond to the unexpected extra of human energy.The Conversation

Luis Ríos Frutos, Professor of Anthropology, Complutense University of Madrid

This article was originally published in The Conversation. read the original.