Don: 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?

Ya?l: Isn't it that our ancestors switched from a herbivorous diet to an omnivorous diet?

D: That's the most popular theory among anthropologists. 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.

Y: Sounds reasonable.

D: 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?

Y: That is strange.

D: Other anthropologists think it's because meat-eating isn't the key at all, that the real      key is cooked tubers.

Y: As in baked yams?

D: Yep. 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 evolvedY: And I guess that cooking would have made the tubers easier to digest.

D: Right, and therefore richer in calories. Cooking's the target of major criticism though.      Most archaeologists believe humans didn't begin cooking until about 250,000 years      ago.

Y: Oh. That is a problem, but then again, perhaps we started cooking earlier than      scientists think.

D: Time may tell.