What makes humans special? Whichever attribute you pick is probably made possible by our big brains. However, these brains aren't all benefits and sunshine. They're very energy intensive. So it was thought that cooking - which allows us to squeeze more energy out of food - made these massive brains possible.
Or maybe it didn't.
A review of a mathematical model of primates reveals that when faced with similar problems, they shouldn't cook their food ( yet). Rather, they should increase their foraging efficiency, squeezing more food out of their environment. So which strategy did out ancestors go for? Well, it looks like the latter. Fire appears in the archaeological record several hundreds of thousands of years after big brains appear in our family.
These two datapoints show that cooking probably wasn't key to the evolution of our big brains.
Cooking and our big brains
There is a relationship between brain size and body size. The bigger a body you have, the larger a brain is needed to control it. However, we dial this up to 11. Humans have a brain 7 times as big as it should be; given our body size.
Trying to explain why humans have such big brains has always been a question of two parts: why and how. Why did evolution drive us to get such large brains and how were they made possible. After all, our brains do have some significant downsides. Like making birth hard (because the big brains don't fit right), children stupid (because the big brain takes a long time to grow), and our bodies hungry (with our brains requiring 500 calories a day).
Explanations for the "why" are fascinating and have been the focus of a lot of attention.All of this research means we have a fairly good idea of the main factors driving the evolution of our big brains. Most other primates need a big brain to help them be social. It's pretty intellectually demanding living in a big group as you have to remember who everyone is, how they're all related, what their status is and so forth. Big brains are needed to do this. Humans seem to have taken this to the extreme, having some of the largest groups (needing the largest brains to boot).
Another key factor is environmental variability. Our ancestors lived through some pretty difficult times. Climate change (and some fairly major ice ages) made life hard. Big brains helped our ancestors to learn and adapt to these changes, "insulating" them from the changing conditions.
Of course, it should be noted that biology (and evolution in particular) and complex subjects. There was no single cause of our big brains. It was probably a mixture of those; along with a few other minor ones we've yet to identify.
But whilst there's a lot of explanations for why our big brains evolved, there's only a handful explaining how it was made possible. And most of these revolve around a single issue: our brains are energetically expensive. It takes about 1/4 of your daily calories to fuel them. That's the equivalent of a big mac each day (not that I'm saying you should eat that many; it would probably be bad for you and your brain).
So the real question is how could our ancestors have fuelled these brains? Perhaps they started exploiting a new, energy rich source of food. Like meat. Or maybe they modified their food, making it easier to digest. Grinding and cutting plants breaks them down, making it easier for us to extract nutrients from them.
Cooking could have been a great way to modify existing food sources. Cooked plants become 30 - 100% easier to digest. Some rather amusing research noting how raw-foodists, in abandoning this key human adaptation, suffer some widespread negative side effects. Cooked meat also gains some benefits as it becomes more protein dense (although the importance of this is debated).
Extinguishing ancient fires
For these reasons, cooking has been viewed as one of the important "hows" behind our big brains for about a decade now. However I - and many other researchers - have been skeptical of it as the timing just doesn't seem to line up.
The earliest confirmed evidence for fire comes from Wonderwerk Cave, South Africa, and dates to around 1 million years ago. Yet Homo erectus (one of the first species to develop a big brain) evolved closer to 2 million years ago. There has been some circumstantial discoveries in the intervening periods. But these are so scattered and rare it's difficult to tell if they were deliberate fires made by our ancestors.
A new review throws a bit more fuel onto this skeptical fire. As well as confirming there's no real correlation between archaeological evidence of fire and brain size increases; they create a model of how primates would adapt to increasing energy costs. It turns out the preferred strategy would be to more intensively exploit their territory to gain extra food, rather than modifying their normal food. I'm not so sure if this model is applicable to humans. After all, at a certain point there just isn't enough food in a territory to sustain you. If humans reached that point we would have had to switch to something like cooking.
But they thought of my criticism and present a few additional lines of data too. The most significant being a little experiment they ran, feeding two groups of mice raw and cooked meat respectively. The cooked population (or, the group that received cooked food, not the mice that were cooked) didn't experience any additional weight gain; so they clearly weren't getting any extra energy from them. However, cooking could have been used to cook plants, which is something they should have tested.
Ultimately the best evidence still comes from the disconnect between the origins of fire and the origins of our big brain. But this extra data certainly dumps an additional bucket of water on the situation.
tl;dr
Cooking with fire is thought to have given us the energy needed to evolve big brains. But the disconnect between when we invented fire and when our brains got big suggests this isn't the case.
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.
Carmody, R.N. and Wrangham, R.W., 2009. The energetic significance of cooking. Journal of Human Evolution, 57(4), pp.379-391.
Cornélio, A.M., de Bittencourt-Navarrete, R.E., de Bittencourt Brum, R., Queiroz, C.M. and Costa, M.R., 2016. Human brain expansion during evolution is independent of fire control and cooking. Frontiers in Neuroscience, 10, p.167.
Dunbar, R.I., 2003. The social brain: mind, language, and society in evolutionary perspective. Annual Review of Anthropology, pp.163-181.
Grove, M., 2011. Change and variability in Plio-Pleistocene climates: modelling the hominin response. Journal of Archaeological Science, 38(11), pp.3038-3047.
