Humans live an energy intensive life. We have big brains that need hundreds of calories a day to maintain. We produce big brained babies that take extra calories to mature. And after the baby has been born, women recover quickly, which - you guessed it - also requires a lot of energy. All of this puts a hefty strain on our metabolism.
For many years scientists have speculated that the energy to fuel these adaptations came from our ancestors eating better food. Perhaps we started cooking our food, breaking it down so we could extract extra calories from it. Or maybe switched to more calorie dense sources (like meat).
Yet eating enough food isn't the entire picture. We need to be able to digest it, break it down, and put that energy to work. Which raises the question: is an ape metabolism sufficient to power the human body (when given enough food) or did we evolve a faster digestion?
The first research into this question reveals it's the latter. Humans have evolved an extra fast metabolism to keep our body burning.
Food for thought
Humans have the highest daily energy usage of any ape - with the exception of male gorillas. This is pretty impressive considering we have a similar body weight to chimps, our closest living relatives. Yet we wind up using more than 25% extra energy compared to them. I don't even feel that bad about coming in second place to gorillas. They're massive, so you'd expect them to use a lot of energy. Besides, I get the feeling they'd crush my skull if I tried to steal their place. Unless I had some sort of laser gun.
There are numerous reasons why we have such a high energy expenditure. Perhaps the biggest is our big brains. Brain tissue is one of the most energetically costly in the human body (up there with guts, often the most costly organ in other species). Over the past few million years our brain has tripled in size and with it so has it's energy demand. We now spend almost 1/4 of our daily calorie budget feeding the damn thing. For context, that's about as many calories as are in a Big Mac.
Not that I'm saying you should eat McDonalds every day. In the long run that probably wouldn't help your brain.
The other big cost comes from reproduction. Normally, animals follow one of two strategies. They invest a lot in a small number of babies and reproduce slowly, or they invest only a little bit in lots of babies. Think rats versus elephants. However, humans try and have it both ways. The large brains of infants require us to invest a lot in our kids. Not just during gestation, but afterwards to. They're dependent on their parents for a long time - which puts an energetic strain on the adults. As if that wasn't tough enough humans also reproduce relatively quickly. In fact, we're one of the fastest reproducing of all the apes (second only to the bonobo), despite the fact we produce the kids with the biggest brains that take the longest to grow.
In fact, even adaptations to reduce energy wind up costing us energy in the long run. For example, walking upright is one of the more efficient ways of moving in the primate family. So we can travel further than your average monkey. In fact, we wind up travelling so far that it costs us more energy than if we'd just stayed closer to home with a rubbish monkey way of moving. This "efficiency adaptation" ultimately causes a net loss in energy.
All in all, humans have to eat between 2,000 - 2,500 calories a day (as all that nutrition information keeps reminding you). Most apes, on the other hand, need to eat <2,100 calories a day. Often closer to 1,500.
Metabolism 2.0
All of this presents a bit of a problem for our ancestors. Without any McDonalds to provide them the required Big Mac a day, how were they to meet this extraordinary calorie demand? There's a long list of possible answers.
We could have reduced the size of other bits of our body, freeing up energy for our brains, walking, and reproduction. Some claim this happened to the gut. However, the evidence for this is circumstantial to say the least. Alternatively, we could have started consuming more calories. This obvious answer has a lot more support ( although the exact manner in which these extra calories were acquired is still contested).
But this just creates another problem. How to get all that energy out of the food. Would an ape metabolism be up to the challenge? It might have been, but it's clear that there might be benefits to speeding the process up. And if there's a possible benefit, evolution can get to work.
It seems that's exactly what happened. The amount of energy we burn just sitting around is much higher than when other apes are lazy. This points to an increased metabolism providing the fuel for our high energy lifestyle. Further evidence comes from our stomachs. Which can get big. Apes, on the other hand, remain relatively fat free. In fact, to have as little fat as the average ape would be downright dangerous for us. This suggests our metabolism is able to blast through food at such a rate we can accumulate this body fat. Apes, on the other hand, can only sustain themselves. They don't produce a surplus to store.
In other words, changes to our internal chemistry were almost as important as changes to our behaviour when it comes to explaining how humans got big brains. A fascinating discovery that - given the amount of popcorn I eat - I'm glad happened.
Summary
Humans need a lot of energy to survive. So we eat a lot of calories. However, we also evolved a faster metabolism to process those extra calories, allowing us to fuel our energetically expensive bodies.
References
Aiello, L.C. and Wheeler, P., 1995. The expensive-tissue hypothesis: the brain and the digestive system in human and primate evolution. Current anthropology, 36(2), pp.199-221.
Isler, K. and van Schaik, C.P., 2012. How our ancestors broke through the gray ceiling. Current Anthropology, 53(S6), pp.S453-S465.
Pontzer, H., Raichlen, D.A. and Rodman, P.S., 2014. Bipedal and quadrupedal locomotion in chimpanzees. Journal of human evolution, 66, pp.64-82.
Pontzer, H., Brown, M.H., Raichlen, D.A., Dunsworth, H., Hare, B., Walker, K., Luke, A., Dugas, L.R., Durazo-Arvizu, R., Schoeller, D. and Plange-Rhule, J., 2016. Metabolic acceleration and the evolution of human brain size and life history. Nature.
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