Humans have stupidly big brains. Which are also stupidly important, making all of our fancy technology possible. As such scientists have spent an awful lot of time trying to figure out just why we have big brains. And they have an answer! Several, actually. It turns out the evolution of our big brains wasn't a simple thing; with multiple different factors driving it. Now researchers have identified another one: heat!
Or to be more precise: too much heat. Brains must be kept nice and warm. So our big brains require more heat than the average ape to keep toasty (since there's more brain to heat). In fact, compared to a chimp-sized brain our big brains require more than double the amount of energy to heat. In a warm environment this would put us at a disadvantage relative to those chimps since we're wasting all that extra energy on heat.
But what about in a cooler environment? Well, then animals will have to produce extra heat anyway in order to stay warm. Thus the extra heat we were already giving to our big brains isn't "wasted". Suddenly this disadvantage disappears and there's no reason for humans not to evolve large noggins.
And that's the gist of this new hypothesis. Over the course of human evolution the environment got colder. So animals had to spend more energy keeping warm. Thus spending more energy keeping a bigger brain warm didn't put our ancestors at a disadvantage; allowing us to evolve larger brains (for one of the other reasons referenced in the introduction).
Now, some of you with a decent understanding of surface area and heat might think that a larger brain would need even more energy to heat in colder climates. Wouldn't this nullify the thermal benefit? However, enlarging organs is how animals typically keep warm in colder environments. So the chimps would be keeping their brains warm by growing them bigger. So again, we'd all be on the same evolutionary page and our ancestors still wouldn't be at a disadvantage for having large brains.
(Fortunately for us chimps never encountered cold climates so never had the chance to get big brains. However, Neanderthals lived in even colder environments than us and they had even bigger brains).
And when you examine the fossil data this idea does hold water (or retain heat; if you're more pun inclined). As environmental temperature goes down larger brains become more viable and soon begin to evolve.
However, it doesn't seem that heat is the single most important factor in the evolution of large brains. Other factors have a stronger correlation with brain size in our ancestors. But then, this hypothesis doesn't claim heat is the single most important driving force. Rather, it made it viable for big brains to emerge and thus for other factors to make them evolve.
So, another variable in the evolution of big brains has been discovered. Our understanding of its evolution is now a bit better but - for people like me who have to learn all this stuff - sadly a bit more complicated.
Reference
Naya, D. E., Naya, H., & Lessa, E. P. (2015). Brain size and thermoregulation during the evolution of the genus Homo. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology.