In the next few days, some of us will be struggling with a turkey, consulting Delia about whether to parboil potatoes before roasting, wishing we'd taken Nigel Slater's advice and made the pudding in October, and wondering what to give vegetarians who don't like nuts. Invariably we'll eat too much, drink unhealthy amounts of alcohol and some of us may consider going on a detox in 2004. But although cooking is central to our lives and our festivities - whether it's Christmas, Hanukkah or Eid - few of us realise how much it has altered not only our physiology, but our psychology. Changes that took place in our bodies almost two million years ago could have reduced our ability to detoxify our foods, which may explain our unhealthy appearance in the New Year.
The idea that cooking has changed the human species fundamentally was dreamt up by Professor Richard Wrangham of Harvard University, Boston, as he sat in front of a dying fire in his living room one winter's night. Wrangham studies chimpanzees and, as he stared into the embers, he felt a pang of pity for his subjects, sleeping out in the cold and eating only raw food. He found himself wondering when it was that human beings created fire and learnt to cook. He realised that not only did he not know the answer, but that most textbooks on human evolution did not cover the subject. Out of 17 textbooks surveyed by one of his students, it was found that, while 10 of the textbooks(or 58 per cent) mentioned cooking, the sum total of space devoted to the subject from all of them amounted to less than two paragraphs.
"Cooking comes across as the equivalent of a piece of furniture," Professor Wrangham says. "If you've got it, you'll like it. But it isn't thought of as something that would have radically affected our ancestors' anatomy, or their social lives." Yet as long ago as 1773 James Boswell, Samuel Johnson's biographer, wrote: "My definition of Man is, a 'Cooking Animal'."
No one, perhaps unsurprisingly, took any notice. The great anthropologist Claude Lévi-Strauss quoted Edmund Leach on the subject: "Men do not have to cook their food, they do so for symbolic reasons to show they are men not beasts." Instead, current theories of human evolution use hunting to explain how we became human. The division of the sexes arose because women collected plants and looked after the children while men brought home the bacon. "Except that it wouldn't have been bacon, according to the proponents of the hunting hypothesis," says Wrangham. "It would have been a raw hunk of pig, and it would have stayed raw while being eaten."
One brave theorist, the anthropologist Loring Brace, argued in the Seventies that cooking was important: actually, not so much cooking as defrosting. During the ice ages it would have been necessary to thaw large hunks of frozen meat, which would have allowed humans to colonise glacial zones. But no one, according to Wrangham - not even Brace - has recognised how important cooking has been to our evolution.
The perfect experiment to test Wrangham's hypothesis has been carried out. Today some Westerners believe that raw food is healthier since cooking destroys enzymes and vitamins. In a study of Germans who practised this philosophy, researchers discovered that most long-term raw foodists (those who stuck to the diet for more than three years) were suffering from chronic energy deficiency. Many had lost a lot of weight and about half the women had ceased to menstruate.
A colleague of Wrangham's, Dr Nancylou Conklin-Brittain, calculated that an average woman on a raw-food diet would have to eat up to 10 kilograms (22 pounds) of food a day to gain enough calories to sustain her; this is almost a fifth of her body weight. Even Americans who, during Thanksgiving, can consume up to 7,000 calories, would not then eat more that 4.6kg of food. Neither, Wrangham thinks, would eating raw meat help. With considerable difficulty, he has observed how long it takes chimpanzees to eat a piece of raw meat. It usually takes them about an hour to absorb 400 calories - the equivalent of a sandwich - of flesh. For a human being to get his or her calorie intake from raw meat, they'd have to chew for six hours a day.
In contrast, cooked food is more edible; it's easier to digest because it's softer, uneatable food is rendered eatable and toxins are removed. Meat is tenderised by heat because the collagen holding the fibres together are softened and turned into gelatin. All known human populations have always cooked most of their food, be it the San bushmen of the Kalahari, or the Aché of the Americas. But the evidence for when our ancestors first used fire (and hence may have cooked) is patchy. Anthropologists estimate that it was between 400,000 to 1.6 million years ago.
Without fossil records for fire, Wrangham puts the date much earlier, between 1.6 and 1.9 million years ago. It is a dramatic step to take, posing a date for cooking without the usual scientific evidence, but Wrangham argues that it was at this point that Homo erectus (also called Homo ergaster) evolved. This species had a remarkably different figure from other hominids - it had a body very like our own.
By taming fire and learning how to cook, our ancestor would have had access to a superior diet. The reason that a better diet could have changed us is because first, we would not have needed such big teeth to grind all that raw food, and secondly, we could dispense with enormous guts. Modern human intestines (in particular, the colon) occupy only a fifth of our total gut volume, compared with more than 50 per cent in chimpanzees. To give a rather gruesome example, a chimp turned up at Jane Goodall's study site in Gombe, Tanzania, with a hand sticking out of its anus. It had swallowed a monkey arm whole. The students invented all kinds of unfortunate jokes, but Wrangham makes the point that this shows how much larger a chimp's gut is as only a human sword swallower might manage the same feat. He estimates that their faeces are twice as wide as ours - though they weigh less than two-thirds of the weight of an average person - because their food contains so much more fibre than ours.
Cooking may also have affected our psychological make-up. Having a hearth meant Homo ergaster had to store food and now had somewhere to sit and prepare and eat it. The larger males could have stolen females' food, leading Wrangham and his colleagues to put forward the Bodyguard Hypothesis: female Homo ergaster may have teamed up with one male to help her protect it. Thus being a domestic goddess 1.9 million years ago could have led to the rise of monogamy.
The knock-on effect is that since we evolved to allow cooking to take the toxins out of food and are not very good at disposing of them ourselves, the Christmas holidays will see an unhealthy level build up as we pour junk food no Homo ergaster could have imagined into our bodies. Just don't try the raw food diet as a detox.
Richard Wrangham features in a documentary directed by Sanjida O'Connell: 'Horizon - The Demonic Ape', 9pm, BBC 2, 8 January 2004Reuse content