The human lineage combines several unique traits. One of the most peculiar of these combinations is that we're both fat and smart. Amongst the primates, a species either tends towards a big brain or lots of body fat. Our closest living relatives - the apes - tend to only have 1 - 10% body fat. They combine this with a brain around the size of a coke can. It might not seem impressive, but for a primate that's huge.
Meanwhile, a healthy human's body is made up 12 - 24% body fat; depending on gender. And this isn't a product of modern culture, hunter-gatherers have similar percentages. So humans are unique in two ways. We began prioritising body fat, unlike our ape relatives. And we kept a big brain.
What's the deal with our blubber?
Fat and smart: alternate strategies
A species getting fat isn't that unusual. Despite the problems obesity now causes, a bit of extra belly is often a good thing. It serves as an energy store that helps individuals get through tough times. This could include future uncertainty, seasonal shifts, or even getting pregnant. All of these can be made less challenging with a bit of extra bulk. It helps buffer you from potentially harmful changes.
However, that isn't the only way you can buffer yourself against change. Being smart also helps. This is known as the cognitive buffer hypothesis, for what is hopefully obvious reasons. The gist of it is given away the name. By using intelligence you can deal with changing environments. Maybe learn how to exploit a new food source, or develop a new tool that helps counteract change.
This helps explain why other primates don't combine fat and brains. It's redundant. What's worse, it's also wasteful. Sustaining fat stores requires a fair bit of energy. Obviously, you have to eat all the extra food to grow them in the first place. But the tissue itself also requires maintenance, which isn't exactly inexpensive. And the brain has a similar problem. It requires a lot of energy to grow and then maintain. In fact, the brain contains the most expensive tissue in the body.
This makes it difficult to fuel both options, so most primates tend to pick one and stick with it.
The exception to the rule
Humans, of course, are the exception to this rule. That's the whole reason I'm pointing this out in the first place. It probably wouldn't be that interesting an article if I just went "and nothing weird was found". Science thrives on the weird.
The reasons for fat and smart being rarely combined are many. The big one, as I explained above, is that it's both redundant and very costly. But it's also rarely that helpful. A primate with more mass is going to have more trouble climbing trees. Which is what they tend to spend a lot of time doing. That added weight is going to make any movement - particularly vertical movement - more costly.
Which introduces the exception to the "don't be chubby and smart at the same time" rule: animals that live on the ground. Researchers thought that they would suffer from similar constraints as primates in the trees. Getting both attributes is still costly wherever you live.
However, a larger review of mammals found that ground-based species don't have that same constraint (or to be more accurate, the cost of having both is drastically reduced). For some reason, they can afford to be both fat and smart. The researchers speculate that it might be linked to the fact that big bellies aren't as big a problem when you don't have to climb, reducing the costs of taking both options. However, the authors caution they can't rule out other possible explanations for this relationship.
Conclusion
Humans famously made the transition from arboreal primates to living on the ground. Our bipedality is one of the most notable facts about this. And it turns out it's less costly for animals that live on the ground to use fat as a buffer. Perhaps it's no coincidence that our species started doing that too. Our chubby bellies could be the result of a shift towards life on the ground.
References
Heldstab, S.A., van Schaik, C.P. and Isler, K., 2016. Being fat and smart: A comparative analysis of the fat-brain trade-off in mammals. Journal of Human Evolution, 100, pp.25-34.
