Catching Fire: How Cooking Made Us Human by Richard Wrangham

perceive not only taste but also particle size and texture. Some of the brain cells (neurons) responsive to texture converge with taste neurons in the amygdala and orbito-frontal cortex of the brain, allowing a summed assessment of food properties. This sensory-neural system enables primates to respond instinctively to a wide range of food properties other than merely taste, including such factors as grittiness, viscosity, oiliness, and temperature.

    In 2004 such abilities in the human brain were reported for the first time. A team led by psychologist Edmund Rolls found that when people had foods of a particular viscosity in their mouths, specific brain regions were activated. Those regions partly overlapped with regions of taste cortex that register sweetness. The picture emerging from such studies is that hard-wired responses to properties such as taste, texture, and temperature are integrated in the brain with learned responses to the sight and smell of food. So the mechanisms that allow animals to assess the quality of raw foods directly apply to cooked foods and allow them to choose foods of a good texture for easy digestion.

    Rolls’s studies suggest that the proximate reasons chimpanzees and many other species like their meat and potatoes cooked may be the same as in humans. We identify foods that have high caloric value not just by their being sweet, but also by their being soft and tender. Our ancestors were surely prepared by their preexisting sensory and brain mechanisms to like cooked foods in the same way. A long delay between the first control of fire and the first eating of cooked food is therefore deeply improbable.

     
     
     
    A long delay between the adoption of a major new diet and resulting changes in anatomy is also unlikely. Studies of Galapagos finches by Peter and Rosemary Grant showed that during a year when finches experienced an intense food shortage caused by an extended drought, the birds that were best able to eat large and hard seeds—those birds with the largest beaks—survived best. The selection pressure against small-beaked birds was so intense that only 15 percent of birds survived and the species as a whole developed measurably larger beaks within a year. Correlations in beak size between parents and offspring showed that the changes were inherited. Beak size fell again after the food supply returned to normal, but it took about fifteen years for the genetic changes the drought had imposed to reverse.

    The Grants’ finches show that anatomy can evolve very quickly in response to dietary changes. In the case of the drought year in the Galapagos, the change in diet was temporary and therefore so was the change in anatomy. Other data show that if an ecological change is permanent, the species also changes permanently, and again the transition is fast. Some of the clearest examples come from animals confined on islands that have been newly created by a rise in sea level. In fewer than eight thousand years, mainland boa constrictors that occupied new islands off Belize shifted their diets away from mammals and toward birds, spent more time in trees, became more slender, lost a previous size difference between females and males, and were reduced to a fifth of their original body weight. According to evolutionary biologist Stephen Jay Gould, this rate of change is not unusual. Drawing from the fossil record, he suggested that fifteen thousand to twenty thousand years may be about the average time one species takes to make a complete evolutionary transition to another. While a species that takes many years to mature, such as our ancestors, would take longer to evolve than a rapidly growing species, such rapid rates of evolution are sharply inconsistent with some previous interpretations of the effects of cooking. Loring Brace suggested that the use of fire for softening meat began around 250,000 to 300,000 years ago, followed by a supposed drop in tooth size that began about 100,000 years