The austrolopithicines, the predecessors of our prehuman ancestors, lived in savannahs with dry uplands. They would often have encountered natural fires and food improved by those fires. Moreover, we know from cut marks on old bones that our distant ancestor Homo habilis ate meat. They certainly made hammers from stones, which they may have used to tenderize it. We know that sparks fly when you hammer stone. Its reasonable to imagine that our ancestors ate food warmed by the fires they ignited when they prepared their meat.
Now, once you had communal fires and cooking and a higher-calorie diet, the social world of our ancestors changed, too. Once individuals were drawn to a specific attractive location that had a fire, they spent a lot of time around it together. This was clearly a very different system from wandering around chimpanzee-style, sleeping wherever you wanted, always able to leave a group if there was any kind of social conflict.
Wrangham’s hypothesis falls into a long tradition in paleoanthropology – the “umbrella hypothesis”, a term coined by John Langdon (1997). In Wrangham’s version, cooking was the fundamental change from which most of the other changes in early Homo can be derived. Other well-known umbrella hypotheses include the “expensive tissue” hypothesis, the aquatic ape hypothesis, and the “killer ape” hypothesis.
An umbrella hypothesis isn’t necessarily false just because it relies on a single cause. Hey, maybe cooking really did cause all that other stuff. Many well-respected scientific theories started out as umbrella hypotheses, like continental drift, or the K-T impact hypothesis.
But an umbrella hypothesis can be difficult to test because its supporters may draw in many facts that are explained equally well by other causes, or worse may be irrelevant. Take for example the argument that a fire provides an attractive location for social interactions. That is certainly true in many recent human hunter-gatherers. But food-sharing hominids may have had home bases attractive for social interactions without fire. And ethnographic hunter-gatherers really do leave groups because of social conflicts. They are much freer to move than male chimpanzees are, and this freedom to move has nothing obvious to do with cooking.
Anyway, I’m looking forward to reading Wrangham’s book – not because I think I’ll agree with it, but because it can be so useful to line up the facts in different ways.
UPDATE (2009-04-21): A reader asks if I could add some more detail – what do I really think about cooking/diet change/brain evolution? That’s a tall order; it will take a while to write it up but I’m happy to do it.
Especially since I’ve come to think something completely counter-intuitive. The brain of early Homo erectus didn’t grow relative to body size. If anything, it shrank.
Langdon JH. 1997. Umbrella hypotheses and parsimony in human evolution: a critique of the Aquatic Ape Hypothesis. J Hum Evol 33:479-494. doi:10.1006/jhev.1997.0146