Humans are remarkably fuel-efficient, or at least, our brains are. The lump of tissue inside our skulls is three times larger than that of a chimp, and it needs a lot more energy to run. But for our size, we burn about as much energy as a chimp. We’re no gas-guzzlers, so how did we compensate for the high energy demands of our brains? In 1995, Leslie Aiello and Peter Wheeler proposed an answer – we sacrificed guts for smarts.

The duo suggested that during our evolution, there was a trade-off between the sizes of two energetically expensive organs: our guts and our brain. We moved towards a more energy-rich diet of meat and tubers, and we took a lot of the digestive work away from our bowels by cooking our food before eating it. Our guts can afford to be much smaller than expected for a mammal of our size, and the energy freed up by these shrunken bowels can power our mighty brains.

This attractive and intuitive idea – the so-called “expensive tissue hypothesis” became a popular one. But Ana Navarrete from the University of Zurich thinks she has disproved it.

She wanted to see if the same brain-gut trade-off would apply to other mammals, so she measured the organs of 100 of them, including 23 primates (the group we belong to). She dissected hundreds of specimens, and weighed their kidneys, spleen, liver, stomach, intestines, heart and lungs. “Too often in this field, authors cull data from previous publications, sometimes cherry-picking,” says Robert Martin, who studies human evolution at the Field Museum.”Navarrete painstakingly collected an entirely new dataset.”

It wasn’t easy. The organs had to be fresh or frozen, and vets and pathologists usually remove the internal organs from dead animals they come across. “Getting the cadavers was really hard,” says Karin Isler, who led the study. “It was a huge effort. Ana was dissecting for more than two years.”

After all this work, she found no connection between the relative size of a mammal’s brain and its other organs. For the group as a whole, bigger brains don’t go hand in hand with smaller guts. “As I see it, we can now say goodbye to the expensive gut hypothesis,” says Martin.

Others are not entirely sold. Both Aiello and Wheeler say that doing a comparison across mammals is stretching their idea beyond its original boundaries. The expensive tissue hypothesis was focused on human brains and it was never intended as a one-size-fits-all explanation that applied across all mammals. The original paper even said that “the cost of the additional brain tissue could have been met by strategies other than a reduction in gut size.” Wheeler says, “The specific example of balancing the human energy budget is not a general principle that would necessarily be expected to apply across groups.”

Benjamin Campbell from the University of Wisconsin-Milwaukee agrees that the new study is “a point against” the expensive tissue hypothesis but he points out that the Navarrete’s general conclusions don’t hold up for primates, the group that would be most instructive for humans. When Navarrete considered her 23 primates separately, she found a positive connection between their brains and the size of their hearts and guts. That would imply, counter-intuitively, that the big brains go with big guts – a result that is never really discussed in the paper.

Campbell says, “This suggests that either primates are different or that the group needs more data.” Isler agrees. She says that they looked at the weight of each mammal’s organs relative to its overall body size, and they measured body size after first removing stores of fat. However, many smaller-brained primates such as lemurs can store fat in odd places like their tails. So Navarrete may have overestimated the size of some of her species, obscuring a relationship between their brains and their other organs.

Nonetheless, the primate figures still run contrary to the expensive tissue hypothesis, and that creates a problem: you need a new way of explaining how mammals compensate for larger and more energy-sapping brains. Navarrete has an answer: mammals pay for larger brains by auctioning off their body fat. In her hundred species, as brain size went up, fat stores became smaller.

Navarrete suggests that body fat also saps energy – it’s heavy and you need to carry it around, but it also helps to buffer you against starvation when food is scarce. A large brain can take over that role, allowing a species to behave more flexibly and cope with environmental changes. Simply put, a more intelligent mammal can afford to be leaner.

But again, this connection disappears among the primates. Indeed, humans have much larger fat deposit than our closest relatives. Fat makes up around 14 to 26 percent of a healthy adult’s body, but it accounts for just 3 to 10 percent of a chimp’s or bonobo’s.

Rather than trading off brains and fat, Navarrete suggests that our ancestors balanced their soaring energy budget in other ways. They ate and cooked richer food, worked together to hunt and forage, gave more food to females with children, and used their growing intellect to stabilise their supplies. They could also have saved energy in other ways, including growing more slowly than other primates, and walking on two legs – a more efficient style of movement than knuckle-walking or climbing. “This wasn’t restricted to a short time. It went on for two million years,” says Isler.

None of the concepts in this “expensive brain framework” are unique to humans, but Navarrete thinks that they all came together to drive the evolution of our extraordinary brain. Whether this is true remains to be seen. Likewise, it’s still unclear if the primates, and humans in particular, are a special case.

I have to ask…what population of humans were used to establish that 14-26% of body weight is made up of fat? As Carol notes, western populations (especially that of the U.S.) are obese. What needed to be measured was the percentage of body weight made up of fat in humans in their “natural habitat” – i.e., before we developed agriculture, animal husbandry, scientific farming, etc.

Unortunately, there is to our knowledge no published value of body fat content in a natural population of human (e.g. hunter-gatherers). BMI data are available, but the predictive power of BMR for body fat content is weak. It would be great to have such data, but they may be difficult to obtain (measuring in living individuals would require calibration with a few cadavers first, which is ethically problematic). From non-quantitative reports is seems, however, that temporal fat storage is abundant also in natural populations of humans.
The 14-26% are however not from obese people, but healthy, fit, younger adults. males is about 15%, females about 25%. Obese would mean much more, 30-50% or more.
Also, keep in mind that the non-human primate values are also not from natural populations, but usually from zoo animals, that may be rather slim from an illness, or rather fat from lack of training. This induces a high amount of error variation. We completely agree that to assess the case of primates, further studies with better samples are needed.

Why isn’t the positive relationship between brain and gut size in primates generally supportive of the original theory — assuming that humans do not follow this trend? Only humans cook/process food extensively with tools, so it would not obviously be a surprise that they were the only outlier.

@JMW regarding the ‘natural habitat’ of humans, if you were measuring body composition of an ant species that survived from the energy provided by domesticated aphids or the fungus it cultivates would you also go back and study only the ants that didn’t cultivate domesticates to be a more ‘natural’ study? While I agree it doesn’t make sense to use an extreme outlier population to define the norms of a species, I think we have to be careful what we call natural and unnatural.

Why isn’t the positive relationship between brain and gut size in primates generally supportive of the original theory — assuming that humans do not follow this trend? Only humans cook/process food extensively with tools, so it would not obviously be a surprise that they were the only outlier.

Could one postulate that the relationship held true in humans up to the moment cooking was invented (the beginning of brain enlargement in the human lineage predates the earliest evidence for use of fire and cooking), but then the advent of cooking caused the “rules to change”, decoupling the brain-gut relationship, leading to human brains getting even bigger while gut size stayed constant, thus pushing humans into being the outlier?

There is a difference not just in the overall mass of muscles in humans vs. other apes, but also in how they operate. In nonhuman apes there is a strong bias towards firing muscle groups all at once, in parallel, thus a power-heavy operation. In humans we go for serial firings, which is much ore finesse-heavy. My guess is that our cognitive modes have a similar split, so that we can handle all sorts of shades of meanings, nonliterality, etc., while the other apes are limited to the concrete, and absolute.

One question I would ask is just how much we trust the science on caloric metrics in the first place. If we do trust the metrics, then there remains the question of what is going on that makes it so that humans and chimps can operate under the same caloric budget. I would rather point to muscle than organ tissue or body fat, if we’re just guessing. But there is probably some really smart reason to discount that idea.

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Ed Yong is an award-winning British science writer. Not Exactly Rocket Science is his hub for talking about the awe-inspiring, beautiful and quirky world of science to as many people as possible.
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