Our development of larger brains had much to do with our evolution as a species. But the question is, what fueled the development of those larger brains?
The most popular theory among anthropologists is that we switched to an omnivorous diet. Our ancestors are believed to have evolved in a less forested, more open grassland environment than our cousins, the chimpanzees. This would have given them a clear view of large herbivores. Cut marks on animal bones suggest that around 2.5 million years ago our ancestors did indeed begin hunting. Because meat is rich in nutrients, including essential amino acids, it may have spurred the increase in brain size.
The trouble is that the big increase in brain size occurred with Homo erectus, which appeared just 1.8 million years ago. If meat was largely responsible for the change, then why are there cut marks on bones nearly a million years earlier?
Other anthropologists think it's because meat-eating isn't the key at all, that the real key is cooked tubers, like yams.
They say an increase in calories is what's important when it comes to the development of larger brains, and that cooked tubers would have provided just such a boost. And root vegetables are believed to have been plentiful in the environment where humans evolved
Cooking would have made tubers easier to digest, and therefore richer in calories. Cooking is the target of major criticism though. Most archaeologists believe humans didn't begin cooking until about 250,000 years ago.