However you want to look at it, our brain is kind of a big deal. From language to learning, it's probably our greatest attribute. Unfortunately, all of this awesomeness comes at a hefty price. Although it only makes up 2% of our body weight, it demands more than 20% of our body's energy supply. That's about 500 calories or one big mac. 
All of this sounds very impressive on paper. 1/5 of everything you eat is going to just keep the mush between your ears going. In fact, some have gone on to argue that this must have been a key factor in our evolution. Our ancestors discovered fire and started cooking food to feed our brain extra energy.
But is it really that unusual? How much extra appetite does our brain have because it's so big? Compared to other apes, we do seem to have to give extra energy to our brains. However, when you start looking at other primates the picture becomes a lot more muddled.
Expensive apes
When trying to track how our brain - and its energy demands - have changed over our evolution we're faced with a fairly major problem. Brains don't fossilise. At best, we get casts of the inside of the skull. Although they show cool stuff like the size, shape, and organisation of the brain, their inorganic nature limits what they can tell us about its energy demands. Fortunately, palaeoanthropologists are quite good at thinking outside the (brain) box.
See, our brain gets the energy it needs from the blood. The brain's blood supply comes from two main sources, the vertebral artery and the carotid artery. The exact names aren't important (yet). What is important is that they enter the skull through a series of holes. These are called foramen (foramina plural), which is just "hole" in Latin. Because everything sounds extra science when you put it in Latin.
Language lessons aside, the important thing is that these foramina do fossilise (along with the rest of the skull). And the size of them is linked to the size of the arteries which flow through them. The size of those arteries is, in turn, linked to how much blood (and thus energy) the brain demands. Long story short, a bigger foramen means a more energy-intensive brain.
So scientists have studied the size of these foramina in our family. As you might expect, they show that as our relatives evolved bigger and bigger brains, their energy demands rose accordingly. All of this culminates in modern humans having the most expensive brain in our family.
Energy-intensive monkeys
In fact, that last section undersold our evolutionary achievement. Not only is our brain the most energetically expensive of any hominin, it demands more energy than any other ape. Chimps, gorillas and orangutans have practically starved brains compared to ours. But whilst we may have won our local league, how do we fare in the world cup of brains? After all, there are many non-ape primate species out there.
To compete in the primate-wide tournament, things get a little bit more confusing. Remember how the brain is fed by two arteries (the vertebral artery and the carotid artery)? Well, the ape brain is mostly fed by the carotid artery, so you can get a good feel for its energy demands by only examining that foramen. However, this isn't always the case. Different primates priorities different arteries.
The general principle still holds true. Bigger foramina mean a more expensive brain. It's just now we have to look at more foramina to get the complete picture. And what is that picture? Do we still come out on top?
Not quite. Whilst we still have one of the most expensive brains of any primate, several branches match us (and a couple even overtake us).
Interestingly, if you look at that chart it doesn't seem to make much evolutionary sense. There's not a single branch of primates that seems to have evolved a costly brain. Rather, a few species in several different lineages seem to have evolved it. For example, the classic ring-tailed lemur spends 29% of their energy on their brain (compared to our measly 27%) but the sportive lemur clocks in with the relatively small 18%.
Reconciling the differences
The sporadic occurrence of an expensive brain suggests that there was no single evolutionary moment that led to it. Rather, the ancestral primate brain is relatively low cost. Then, several different branches evolved a more energetically expensive brain independently.
This raises the question: why does it keep happening?
Well, our brain became more expensive as it became bigger. Sure enough, this seems to hold true for the other species, with (relatively) bigger brained ones needing more blood. But this can't be the whole story. After all, we - relatively speaking - have the biggest brain yet it isn't the most expensive. This gap may come from our body size. Larger animals are more energetically efficient. There's a whole biological law about it. Perhaps the natural efficiency of our large body offsets the energetic costs of our big brain, pushing us down the league table.
Regardless, the fact remains that our brain is costly for an ape. That's still a problem evolution had to deal with in our ancestors. It just turns out we weren't the only one in the primate family with these issues.
References
Boyer, D.M. and Harrington, A.R., 2018. Scaling of bony canals for encephalic vessels in euarchontans: Implications for the role of the vertebral artery and brain metabolism. Journal of Human Evolution, 114, pp.85-101.
Seymour, R.S., Bosiocic, V. and Snelling, E.P., 2016. Fossil skulls reveal that blood flow rate to the brain increased faster than brain volume during human evolution. Royal Society open science, 3(8), p.160305.
Seymour, R.S., Bosiocic, V. and Snelling, E.P., 2017. Correction to 'Fossil skulls reveal that blood flow rate to the brain increased faster than brain volume during human evolution'. Royal Society Open Science, 4(8), p.170846.
Wang, Z., Ying, Z., Bosy-Westphal, A., Zhang, J., Schautz, B., Later, W., Heymsfield, S.B. and Müller, M.J., 2010. Specific metabolic rates of major organs and tissues across adulthood: evaluation by mechanistic model of resting energy expenditure. The American journal of clinical nutrition, 92(6), pp.1369-1377.
