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
Did you get to spend a lot of time with primates during the project and what were the highlights of this experience?
All of the hands-on interactions with the subjects is done by the vet and caretaker staff at the zoos we partner with. But I do get to spend a lot of time observing them, which is always fun and eye opening. Observing apes and working with talented and energetic zoo personnel have been the highlight of the project.
Your research found that only humans have a significant difference in body fat between men and women. What do you think this means for the difference between the brains of men and women?
The sex difference in body fat in humans is most certainly a reflection of reproductive demands – women invest a lot more calories in report diction and their additional fat provides an important buffer. The fat difference isn’t related to brains, which are essentially the same in men and women.
Can you explain the connection between metabolic acceleration and the current human obesity epidemic?
We’re still putting all of the pieces together with this issue. My current thought is that metabolic acceleration led to selection for increased body fat, to act as a reserve fuel tank. In today’s environment, that tank is unnecessary, as there’s plenty of food, but our bodies are still eager to add fat. That’s a recipe for obesity if we don’t watch our calorie intake.
How will this study help future researchers to improve metabolic health for humans in industrialised nations and apes in captivity?
We found that our closest relatives, chimpanzees and bonobos, don’t put on fat like we do. Even in zoos they stay very lean. That suggests we might be able to identify the genes and mechanisms that differ between them and us, making them lean and us prone to obesity. If we can identify the mechanisms involved we might develop new strategies to combat obesity.
How do you plan to expand your research in the future?
This study provides a broad framework for metabolic differences among apes, but there are many questions that arise. What mechanisms are behind these metabolic differences? How are metabolic differences related to healthy aging? How do chimpanzees remain lean in captivity? These are some of the questions we want to tackle next.
Funding
National Science Foundation
Partners
Dr Stephen Ross, Lincoln Park Zoo, ape metabolism work
Dr David Raichlen and Dr Brian Wood, hunter-gatherer work
Dr Amy Luke, Loyola University, recent human work
Info
Further information can be found at doi: 10.1038/nature17676
BIO
Dr Herman Pontzer, Associate Professor of Anthropology at Hunter College and the City University of New York, investigates how evolution shaped the human body. A native of western Pennsylvania, he attended Penn State as an undergraduate and received his PhD from Harvard University. His work incorporates lab and field studies of humans and apes, living and extinct, and has taken him from studies of chimpanzee climbing in Uganda, to human fossil excavations in the Republic of Georgia, to measuring energy expenditure and daily activity among traditional hunter-gatherers in northern Tanzania. When he’s not in the field, the lab, or the classroom, Dr Pontzer enjoys playing outdoors with his family, rock climbing, and pondering life’s mysteries over pretzels and cold beer.
Contact
Department of Anthropology
Hunter College
City University of New York
695 Park Ave, New York, 10065
