A central objection to the cooking hypothesis has been that there is no archaeological evidence of controlled fire as far back as the hypothesis predicts. Currently the oldest traces come from one-million-year-old deposits in Wonderwerk Cave in South Africa. But you have recently identified an independent line of evidence that humans tamed fire earlier than the archaeological record suggests. How does that work support your thinking?
Chimpanzees love honey, yet they eat very little of it because they get chased away by bees. African hunters and gatherers, in contrast, eat somewhere between 100 and 1,000 times as much honey as chimpanzees do because they use fire. Smoke interferes with the olfactory system of the bees, and under those conditions, the bees do not attack. The question is: How long have humans been using smoke to get honey? That's where the honeyguide comes in. The greater honeyguide is an African species of bird that is adapted to guiding humans to honey. The bird is attracted to human activity—sounds of chopping, whistling, shouting, banging and, nowadays, motor vehicles. On finding people, the bird starts fluttering in front of them and then leads them off with a special call and waits for them to follow. Honeyguides can lead humans a kilometer or more to a tree that has honey in it. The human then uses smoke to disarm the bees and opens the hive up with an ax to extract the honey inside. The bird gains access to the hive's wax, which it eats.
It used to be thought that the bird's guiding behavior [which is innate, not learned] originated in partnership with the honey badger and that humans moved in on this arrangement later. But in the past 30 years it has become very clear that honey badgers are rarely, if ever, led to honey by honeyguides. If there's no living species other than humans that has this symbiotic relationship with the bird, could there have been some extinct species of something that favored the honeyguide showing this behavior? Well, obviously, the most reasonable candidates are the extinct ancestors of humans. The argument points very strongly to our ancestors having used fire long enough for natural selection to enable this relationship to develop.
Claire Spottiswoode of the University of Cambridge discovered that there are two kinds of greater honeyguide females: those that lay their eggs in ground nests and those that lay in tree nests. Then she found that the two types of behavior are associated with different lineages of mitochondrial DNA [DNA that is found in the energy-producing components of cells and passed down from mother to offspring]. Based on a fairly conservative assessment of the rate of mutation, Spottiswoode and her colleagues determined that the two lineages had been separated for about three million years, [providing a minimum estimate for the age of the greater honeyguide species]. That doesn't necessarily mean that the guiding habit, which depends on humans using fire, is that old—it could be more recent—but at least it tells you that the species is old enough to allow for much evolutionary change.
If cooking was a driving force in human evolution, does this conclusion have implications for how people should eat today?
It does remind us that eating raw food is a very different proposition from eating cooked food. Because we don't think about the consequences of processing our food, we are getting a misunderstanding of the net energy gain from eating. One of the ways in which this can be quite serious is if people who are dedicated to a raw-food diet don't understand the consequences for their children. If you just say, “Well, animals eat their food raw, and humans are animals, then it should be fine for us to eat our food raw,” and you bring your children up this way, you're putting them at very severe risk. We are a different species from every other. It's fine to eat raw food if you want to lose weight. But if you want to gain weight, as with a child or an adult who's too thin, then you don't want to eat a raw diet.