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.
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.
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.
Cooking fueled brains?
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.
However, this model is imperfect.
Mice on Atkins
The model essentially calculates how many calories the body needs. Then it identifies how much surplus various feeding strategies would provide. The gist of the idea being that this surplus can be invested in big brains. Ultimately, they find that sufficient surplus to fuel our brains could be provided by increasing foraging efficiency. Fire, reducing energy spent on other organs, and other options are somewhat unnecessary.
However, all of this based on certain assumptions of how much energy the brain needs. Which was calculated by inferring how much energy it would take to fuel all our neurons. Yet recent research has found our ancestors' brains needed more energy than could be explained by their neurons alone. Fossils show extra blood flowing into the brain, indicating there was more energy needed than included in this model. So the conclusions are dubious, to say the least.
However, that's not the craziest part. This research also put some mice on the Atkins diet.
Of course, it wasn't intentional. These researchers investigated whether mice gained weight when fed cooked meat. Which involved cutting out other sources of food (from which mice get most of their carbs, i.e., putting them on Atkins). After 4 days of study, the cooked food mice weren't any fatter than the raw food mice. Might the benefits of cooked food been overstated?
This conclusion stands in stark contrast to previous research on mice. This showed that mice fed cooked food gained more weight. Why were these two results different? Well, this earlier research allowed the mice to eat as much raw or cooked food as they wanted. The more recent work gave them allotted portions. This distinction is key, and once again makes this part of the research essentially useless.
tl;dr
Ultimately the best evidence still comes from the disconnect between the origins of fire and the origins of our big brain. 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.
