Cracking Humanity’s ‘Energetic Paradox’
Researchers have now proved that humans have a faster metabolism and higher body fat percentage than our closest living evolutionary cousins. Dr Herman Pontzer believes his team’s new study could “aid efforts to promote and repair metabolic health for humans in industrialized populations and apes in captivity”.
An organism’s life history is the pace with which it grows, reproduces, and ages. Physiological demands of growth, maintenance and reproduction, compete for shares of an organism’s total available metabolic energy, which is mainly determined by body size. If one need, such as reproduction, receives a larger share of energy, then the other life processes’ share will be reduced.
Dr Herman Pontzer, Associate Professor of Anthropology at Hunter College, City University of New York, explains that variation in life history reflects evolved differences in energy expenditure. For example, if an organism’s energy is redirected towards reproductive output, and away from maintenance, the species would be expected to reproduce faster. However, the lack of repair would generally result in a shorter maximum lifespan.
Physiological demands compete for shares of an organism’s total
available metabolic energy, which is mainly determined by body size
Dr Pontzer’s study has expanded this trade-off framework to consider the energy needed to grow and maintain large brains among primates. Dr Pontzer previously led a team who discovered that orangutans, in proportion to their body mass, expend less energy than any other studied mammal in the world except the tree sloth. The 2010 study’s findings suggest orangutan’s energy efficiency is an adaption to the evolution of a metabolically costly, larger brain.
‘Energetic Paradox’
Humans are a unique anomaly in the hominoid family, also known as the Apes, that includes orangutans, gorillas, chimpanzees and bonobos. Humans have the largest brain size relative to whole-body energy needs. We also live longer than any of the great apes, and collectively produce more offspring per year. Our unusual life history results in what the researchers call an ‘energetic paradox’. It begs the question: how is it that we naturally produce more frequent and larger babies than any other living hominoid, yet also have the longest lifespans and the largest, most energy consuming brain (see below)?
Figure 1 | The human energetic paradox. Humans achieve greater reproductive output (black bar) and have larger brains (blue bar) relative to female metabolic mass than any of the great apes, yet also achieve longer lifespans (grey bar). Human data are from traditional hunter–gatherer and subsistence farming populations; ape data are from populations in the wild (black bar).
The collaborative project, including Dr Stephen Ross from Lincoln Park Zoo, Dr David Raichlen from the Arizona University, and Dr Amy Luke from Loyola University, aimed to test the hypothesis that the human lineage has experienced an acceleration in metabolic rate, providing energy for larger brains and faster reproduction without sacrificing maintenance and longevity. Using the doubly labelled water method, the scientists compared the total energy expenditure of mixed-sex humans to a range of primates over 7–10 days. The study’s subjects consisted of a motley crew of 56 captive primates (27 chimpanzees, 11 orangutans, 10 gorillas and 8 bonobos) and 141 humans, following their normal daily routine.
Harder Better Faster Stronger
Humans’ increased fat storage and faster metabolism gave humans stronger chance for survival in the past. The researchers found that body fat percentage was ‘markedly higher in humans that other hominoids’ in the study. Wild apes and humans in traditional foraging and farming populations will generally have lower body fat percentage than their urbanised and captive counterparts. However, even the fatter captive apes’ body fat percentages were still comparable to, or even below, average body fat percentages for humans in physically active, traditional hunter-gatherer populations. Humans were also the only species where significant differences in body fat percentages between sexes were observed, with the female average at 41.7%, while males came in at just 22.9%.
The team also discovered that total energy expenditure is remarkably consistent across populations within a species, regardless of activity level. For instance, humans living in cities and zoo-housed populations of primates, have similar size-corrected total energy expenditure to individuals from traditional hunter-gathering communities and counterparts living in the wild respectively.
The project proved that in comparison to apes, our closest living evolutionary cousins, we have a faster metabolism and a higher body fat percentage. Humans expend more energy, per day, than any other ape studied, except adult male gorillas. However, when considering body size and physical activity, humans expend more calories than chimpanzees, bonobos, gorillas and orangutans (see below).
Figure 2 | Predicted Total Energy Expenditure, Basal Metabolic Rate and fat mass for adult hominoids. Values are estimated for males (55 kg FFM) and females (45 kg FFM), using the same Free-Fat Mass across genera. For more information on the data see doi:10.1038/nature17654.
‘Untangling’ Human Metabolic Evolution to Combat Obesity
The team conclude that humans evolved an accelerated metabolic rate and larger energy budget to accommodate larger brains and increase the production of children, as well as to enable a longer lifespan — without the expected energetic trade-offs. The researchers hypothesise that, within humans, food sharing and increased body fat percentage coevolved with greater total energetic expenditure, in order to mitigate the inherent risk of increased energy demands.
By “untangling the evolutionary pressures and physiological mechanisms shaping the diversity of metabolic strategies amongst living hominoids,” Dr Pontzer offers an explanation for the current obesity epidemic that is sweeping our world. The research lifts the lid on why we are now living in a world of dangerously overweight industrialised populations. Our increased fat storage was advantageous for pre-historic Homo sapiens and their growing brain’s appetite. Nowadays, the abundance of high calorie, readily available food, as well as our sedentary lifestyles, means that this once useful evolutionary trait is resulting in widespread poor health due to excessive weight gain. The study provides an insightful resolution the human energetic paradox, guiding future research towards strategies
for combating obesity and metabolic disease.
• For more information on hunter gatherers visit hadzafund.org and for resources on human evolution go to australopithecus.org
Read more here.
