You may be familiar with Harvard anthropologist Richard Wrangham’s book Catching Fire: How Cooking Made us Human, in which he posits all sorts of features of modern humans, including not only morphology (teeth and brain size), but also sociality, intersexual relationships, division of labor, and other forms of behavior were impelled by the discovery that we could cook food on fires. I haven’t read this book but I have read about it, including an interview with Wrangham at Edge. Based on this admittedly cursory reading, I think his hypothesis is a little simplistic (i.e., maybe easily digestible protein allows us more energy to evolved a larger brain, but there still must be selection pressures for that larger brain), but there may be something to it. However, there are many other alternative theories for bigger brains (my favorite is social interaction and language), several of them may have operated simultaneously, and it’s hard to discriminate among them.
One piece of evidence against Wrangham’s hypothesis was, until now, the finding that enlargement of human brains appeared to have begun well before hominins started cooking. Brain size enlargement in humans is traditionally throught to have begun its rapid phase with Homo erectus, about 1.5 million years ago, although there may have been an evolutionary speedup about 300,000 years ago. (Wrangham has also noted that human molar teeth underwent a relative reduction in size at that time, which one might expect if we were processing cooked rather than raw foods.) But the first evidence for controlled use of fire was about 400,000 years ago. There had been earlier reports of burned bones and vegetation associated with human presence from 1.5 million years ago, but those might have been due to wildfires rather than fires controlled by humans. (Controlling is important if cooking is to be frequent enough to affect our evolution.)
Now, however, a new paper in PNAS by Berna et al. (free download!) gives a pretty unequivocal report of controlled fire use about 1 million years ago, in “Wonderwerk Cave,” in northern South Africa:
The evidence for controlled fire use includes the fact that there are multiple findings well inside the cave of charred bone and burnt plant material. The source of the bones wasn’t identified, but presumably they’re animals rather than human (no cannibalism!), and reflectance spectra of the bones that match the spectra of newly heated bones and differ from those of unheated bones.
The burnt plant material was not wood but grasses, leaves, or brushes, indicating that humans hadn’t yet perfected the use of cooking over logs (this of course meant that they had to feed the fire constantly). The reflectance spectra suggests that bones were heated to a temperature between 400 and 500 degrees C, which is hot enough to cook stuff but not hot enough to cook a steak well done (but why would they want to?). The highest temperature suggested by bone and plant scans is about 700 degrees C—not hot enough to be produced by wood.
Here are a couple of pieces of charred bone for your inspection (these are from the paper):
The authors conclude this:
Thus, our data, although they do not show evidence of constructed combustion features, as listed by Roebroek and Villa as a criterion of controlled burning (3), demonstrate a very close association between hominin occupation and the presence of fire deep inside Wonderwerk Cave during the Early Acheulean. This association strongly suggests that hominins at this site had knowledge of fire 1.0 Ma. This is the most compelling evidence to date offering some support for the cooking hypothesis of Wrangham(1).
Note how carefully the authors hedge their conclusion; this is good science, showing that conclusions are tentative.
Berna, F., et al. 2012. Microstrategraphic evidence of in situ fire in the Archeulean strata of Wonderwerk Cave, Northern Cape province, South Africa. Proc. Nat. Acad. Sci. USA: published ahead of print April 2, 2012, doi:10.1073/pnas.1117620109
For the earlier suggestion that human morphology of a million years ago had already been evolutionarily molded by the use of fire, see:
Organ C, Nunn CL, Machanda Z, Wrangham RW. 2011. Phylogenetic rate shifts in feeding time during the evolution of Homo. Proc Natl Acad Sci U S A. 2011 Aug 30;108(35):14555-9. Epub 2011 Aug 22.