Cooking up a new theory of evolution

What makes us human? It is a question for the ages, to be kicked about by scientists, philosophers and historians to name a few. And you can put together your own answer from a list including language, brain size, an ability to have sex pretty much all the time, love, culture, science, and so on.

Richard Wrangham, in his new book Catching Fire, asks a different, if related, question: what made us human? Most people would agree that human beings are different to other creatures (even those who would like to equate humans morally to the great apes). But why have we come to be so different? If we leave aside the religious explanation – an invisible supreme being made us this way – we are left to wonder what it is in our evolutionary past that set us on a different track from other apes.

Wrangham’s answer, while not entirely original, is still very interesting. He believes that the crucial turning point was neither controlling fire in itself (despite the book’s title) nor eating meat, but cooking. It is the change in our diets, and the improved ability to absorb nourishment that comes about through being able to cook food, that allowed ape-like creatures to evolve relatively quickly into recognisably human individuals, even if the finished product – Homo sapiens – was still a long way off.

Roughly 120,000 generations ago, the forebears of modern humans were chimpanzee-sized creatures called australopithecines. Apart from the fact that they walked upright, they were not very different to modern chimpanzees. Wrangham imagines the experience of meeting one: ‘Beneath a low forehead and big brow-ridge, bright dark eyes surmount a massive jaw. Her long, muscular arms and short legs intimate her gymnastic climbing ability.’

From at least 2.6million years ago, australopithecines were using tools in order to get at meat from dead animals, something beyond other apes, including modern chimpanzees. Around 2.3million years ago, a new species – habilines – seems to have emerged, the so-called ‘missing link’. While still the same overall size as modern, nonhuman apes, they had brains twice the size of our living ‘relatives’. Even then, the next step on the evolutionary road took hundreds of thousands of years, but somewhere around 1.9million years ago, some of these habilines evolved into Homo erectus, the first proper members of the genus Homo. Homo erectus had an anatomy, upright stance and pattern of walking similar to ours, but its brain was still smaller. Modern humans only emerged around 200,000 years ago.

The question for Wrangham is this: what changed to create Homo erectus? The common explanation is the eating of meat, the ‘Man-as-Hunter’ thesis. Australopithecines seem to have been, in dietary terms, similar to modern chimpanzees, who will eat monkeys, piglets or small antelopes when available, but who will also have a diet entirely free of meat for months on end. However, the upright australopithecines would have found chasing down prey much easier than a chimpanzee does on all fours. In turn, the development of such behaviour would itself have encouraged team work, larger bodies, increasing intelligence and cooperation.

However, Wrangham argues that the Man-as-Hunter thesis is inadequate in a number of ways. Most importantly, the thesis can’t explain why there are two forks in the evolutionary road – first habilines, then Homo erectus. How could both of these changes, hundreds of thousands of years apart, be caused by a single factor: eating meat? Meat-eating accounts for the first change well enough, but Wrangham points out that habilines looked markedly different from Homo erectus, ‘which had small jaws and small teeth that were poorly adapted for eating the tough, raw meat of game animals. These weaker mouths cannot be explained by Homo erectus‘s becoming better at hunting. Something else must have been going on.’

That something else, argues Wrangham, was cooking. Scientific research on those who choose to eat a mostly, or exclusively, raw-food diet gives us a clue as to why this might be the case. In 2006, nine volunteers took part in an experiment for BBC television where they spent 12 days eating like apes while living in a tented enclosure at Paignton Zoo in south-west England. The idea was to replicate the diet that we are supposed to have evolved to eat: mostly vegetables, with a little fish, and entirely raw. The volunteers consumed up to five kilogrammes of food per day, with nutritionists ensuring they consumed a healthy number of calories, yet they lost an average of 4.4 kilogrammes (about 10 pounds) in less than two weeks.

In another study in Germany of 513 raw-foodists, the average weight-loss over time was 12 kilogrammes (about 27 pounds) for women and 10 kilogrammes (22 pounds) for men. The researchers, quoted by Wrangham, concluded that ‘a strict raw-food diet cannot guarantee an adequate energy supply’. Among women eating totally raw-food diets, 50 per cent stopped menstruating, while a further 10 per cent suffered irregular cycles that were hardly conducive to reproduction.

Wrangham quotes another raw-foodist, Christopher Westra, describing his changing thoughts on sex. ‘In my experience, starting on living foods brought about a change in sexuality that was dramatic and completely unexpected. In just a few weeks, the number of times per day I thought about sex decreased tremendously.’ Westra seems to think this is a good thing, but Wrangham asks how a species could flourish on such a diet when over half of the women would be unable to become pregnant and the men lose interest in sex?

The effect of cooking, however, is dramatic, making it far easier for our bodies to obtain the nourishment from food. Wrangham notes that digestion comes in two parts: the first starts in the mouth and continues in the stomach, and is completed by the small intestine. The second part is done by the 400 or more species of ‘friendly bacteria’ that take up residence in our large intestines. So, the quicker we digest food, the more of its goodness we can grab for ourselves.

Cooking makes a big difference to this, as illustrated by patients who have had their large intestines removed, so that food is removed through a bag attached to the end of their small intestine, or ileum. These ileostomy patients can easily digest cooked starch – at least 95 per cent of oats, wheat, potatoes, plantains, cornflakes. A similar figure applies to a typical European or American diet of starchy foods, dairy products and meat. On the other hand, the figures for the ‘ileal digestibility’ of raw foods are much lower: wheat starch (71 per cent); potatoes (51 per cent); plaintains (48 per cent). This differential also seems to apply to protein. Wrangham points to the example of eggs, which are much better digested cooked rather than raw.

Why does this matter? Well, if nutrients are more easily extracted from food, then we can maximise their usefulness given our current digestive systems. But over the hundreds of thousands of years that evolution takes, this externalisation of digestion changed our digestive systems substantially. Compared to apes, humans have much shorter digestive tracts. And if we our using less of the energy available to us to digest food, that can be diverted to other areas of our bodies. Essentially, argues Wrangham, cooked food is brain food.

In the transition from australopithecines to habilines, brain volume rose by one third, from about 450 cubic centimetres to 612 cubic centimetres. In the earliest examples of Homo erectus (1.8million years ago), this had reached 870 cubic centimetres and went on to around 1,400 cubic centimetres with Homo sapiens around 200,000 years ago. Although we are about three times the size of australopithecines, our brains are bigger both absolutely and relatively in proportion to the rest of our bodies.

The change is not purely nutritional. Wrangham argues that it has social consequences, too. A sexual division of labour between male hunters and female gatherer/cooks only makes sense if eating is a relatively quick process. This is borne out by the fact that individuals in modern hunter-gatherer societies can spend as little as an hour per day eating, knowing that this will provide all the nutrition they need, freeing them to spend long periods finding and pursuing game. Without this free time, each individual would have to spend most of his or her time finding and consuming food for themselves, and a specialisation of labour would be impossible.

Wrangham compares this situation to the behaviour of chimpanzees and gorillas, who spend most of their time eating since they need to ingest relatively large quantities of fruit and leaves to survive. That process makes hunting, which chimpanzees will sometimes engage in for a few minutes at a time, a relatively risky business taking valuable time away from eating and digesting with no guarantee of success.

Wrangham’s ideas are fascinating and clearly have some substantial explanatory value. That said, they are often based on very small samples of fossils. Furthermore, there is no direct proof that humans began cooking 1.8million years ago. It could only be when cooking was done in well-established settings – like some kind of crude, constructed oven – that there would be any chance of evidence surviving. Such constructions clearly didn’t clearly begin until much later. As such, Wrangham must rely on indirect evidence to support his argument. There is also, as with many popular discussions of evolution, a storytelling aspect as Wrangham fills in gaps with educated speculations that provide plausible explanations for how society and anatomy develops, but are ultimately unprovable, for now at least.

One possible consequence of Wrangham’s ideas, however, is not at all academic and may be a useful avenue of research for a very modern problem: obesity. Could it be that our current, highly processed diet means that we are effectively consuming far more food than we think? Wrangham points out that the traditional method of counting calories in food, the Atwater convention, may be misleading in this regard. Wilbur Atwater was a nineteenth-century professor of chemistry in Connecticut, USA. He argued that the amount of energy in food could be calculated by completely burning it in a device called a bomb calorimeter and measuring the heat produced. It’s still, give or take a few tweaks, the way we calculate calorie content today.

However, we don’t burn food, we digest it – and digestion is a costly process. How much energy is consumed in digestion varies from one food type to another. Protein is harder to digest than carbohydrate, which is in turn harder to digest than fat. But the nature of the food also has an effect. Soft food in small particles will be easier to digest than bigger lumps of tough food – which is where cooking, and food processing, may have a significant impact. Furthermore, Atwater assumed that only about 10 per cent of food would pass all the way through, undigested. But roughly milled flour, for example, is much more likely to remain undigested than finely milled flour. Protein consumed with high-fibre foods is also less likely to be digested than if it is eaten on its own or with low-fibre foods.

Wrangham concludes, following food writer Michael Pollan, that we should choose ‘real foods’ over ‘nutrients’: ‘The less processed our food, the less intense we can expect the obesity crisis to be.’ In effect, Wrangham is arguing that – in one respect at least – processed food is actually too nutritious because we can digest it significantly more easily. It’s an interesting point, but it also seems a little throwaway, tacked on to the end of a much more developed argument about human evolution. Wrangham’s thesis would help to explain why obesity rates have shot up in recent years, despite the fact that calorie intakes appear not to have changed much: all calories are not the same. On the other hand, is our food today really very different from what we ate 50 years ago in terms of digestibility? And is eating more traditional food really the answer? A dozen pages at the end of a book on a rather different subject is not enough evidence to decide.

Nonetheless, Catching Fire is a great example of the popular science book: take a Big Idea and serve in bite-sized, easily digested portions, leaving the reader well satisfied.

Rob Lyons is deputy editor of spiked. He edits spiked’s What’s the Future of Food? online debate.

Catching Fire: How Cooking Made Us Human, by Richard Wrangham, is published by Basic Books. (Buy this book from Amazon(UK).)