In his new book Catching Fire, primatologist and author Richard Wrangham uses that question to help answer an even deeper one. For decades archaologists have held that man came down out of the trees and became man because of a theory called 'Man the Hunter'. This theory states that the changes in Australopithecus (a very early hominid that eventually gave rise to the line that lead to humans) came about because they learned to hunt. The hunting gave them access to a better source of protiens (vegetarian diets are notoriously short on protiens) which gave them the ability to eventually grow larger brains. In the 'Man the Hunter' theory autralopithecus learned to hunt about 2 million years ago, however it wasn't until much later (after they gave rise to Homo hablis and they gave rise to Homo erectus) that mankind eventually discovered cooking.

Wrangham states that this is wrong and in fact, the changes in Australopithecus are best defined by them being the first pre-humans to discover cooking. So how does this tie into the question about how long you spent eating? He spends the first several chapters of the book talking about the physical and chemical changes that occur in the food because of the cooking. Regardless of the food type application of heat starts the process of breaking down the food; the protien chains, the cell walls, connective tissue, etc. This is all labor that creatures who don't cook have to perform either in the mouth (chewing) or in the gut (fermentation).

So get to the point already, Sumo. OK. In order for a primate of about human size to get enough calories from raw food to survive they have to spend no less than 6 hours a day... chewing. Just chewing, not hunting, not foraging, not interacting, just chewing. If Australopithecus were the first creatures to accidentally drop a leg-bone near the fire and pick it up an hour later or so then they suddenly and dramatically changed the balance of how many hours per day were spent doing what. This could have changed everything and given them the head start towards eventually becoming humans.

The book is a good read, very accessible and Wrangham makes some very persuasive points. If you have any interest in anthropology and happen to be a foodie then I strongly recommend this book. It's amazing how efficient our food intake behavior is when you look at it from his point of view. I can't think of any animal out there that can gather enough calories in one day that they can afford to eat just for the heck of it. That's an amazing change.