2008 : WHAT HAVE YOU CHANGED YOUR MIND ABOUT? WHY?

Like people since even before Darwin, I used to think that human origins were explained by meat-eating. But three epiphanies have changed my mind.  I now think that cooking was the major advance that made us human.

First, an improved fossil record has shown that meat-eating arose too early to explain human origins. Significant meat-eating by our ancestors is initially attested in the pre-human world of 2.6 million years ago, when hominids began to flake stones into simple knives. Around the same time there appears a fossil species variously calledAustralopithecus habilis or  Homo habilis. These habilis presumably made the stone knives, but they were not human. They were Calibans, missing links with intricate mixture of advanced and primitive traits. Their brains, being twice the size of ape brains, tell of incipient humanity: but as Bernard Wood has stressed, their chimpanzee-sized bodies, long arms, big guts and jutting faces made them ape-like. Meat-eating likely explains the origin of habilis.

Humans emerged almost a million years later when habilis evolved intoHomo erectus. At 1.6 million years ago Homo erectus were the size and shape of people today. Their brains were bigger than those ofhabilis, and they walked and ran as fluently as we do. Their mouths were small and their teeth relatively dwarfed — a pygmy-faced hominoid, just like all later humans. To judge from the reduced flaring of their rib-cage they had lost the capacious guts that allow great apes and habilis to eat large volumes of plant food. Equally strange for a “helpless and defenceless” species they had also lost their climbing ability, forcing them to sleep on the ground — a surprising commitment in a continent full of big cats, sabretooths, hyenas, rhinos and elephants. 

So the question of what made us human is the question of why a population of habilis became Homo erectus. My second epiphany was a double insight: humans are biologically adapted to eating cooked diets, and the signs of this adaptation start with Homo erectus. Cooked food is the signature feature of human diet. It not only makes our food safe and easy to eat, but it also grants us large amounts of energy compared to a raw diet, obviating the need to ingest big meals. Cooking softens food too, thereby making eating so speedy that as eaters of cooked food, we are granted many extra hours of free time every day.

So cooked food allows our guts, teeth and mouths to be small, while giving us abundant food energy and freeing our time. Cooked food, of course, requires the control of fire; and a fire at night explains howHomo erectus dared sleep on the ground.

 Cooked food has so many important biological effects that its adoption should be clearly marked in the fossil record by signals of a reduced digestive system and increased energy use. While such signs are clear at the origin of Homo erectus, they are not found later in human evolution. The match between the biological merits of cooked food and the evolutionary changes in Homo erectus is thus so obvious that except for a scientific obstacle, I believe it would have been noticed long ago. The obstacle is the insistence of archaeologists that the control of fire is not firmly evidenced before about a quarter of a million years ago. As a result of this archaeological caution, the idea that humans could have used fire before about 250,000 to 500,000 years ago has long been sidelined. 

But I finally realized that the archaeological record decays so steadily that it gives us no information about when fire was first controlled. The fire record is better at 10,000 years than at 20,000 years; at 50,000 years than 100,000 years; at 250,000 years than 500,000 years; and so on. Evidence for the control of fire is always better when it is closer to the present, but during the course of human evolution it never completely goes away. There is only one date beyond which no evidence for the control of fire has been found: 1.6 million years ago, around the time when Homo erectus evolved. Between now and then, the erratic record tells us only one thing: the archaeological evidence is incapable of telling us when fire was first controlled. The biological evidence is more helpful. That was my third epiphany.

The origin of Homo erectus is too late for meat-eating; the adoption of cooking solves the problem; and archaeology does not gainsay it. In a roast potato and a hunk of beef we have a new theory of what made us human.