Chimps and other large primates subsist mainly on fruits and leaves. These are such low-calorie foods that the animals have to forage for most of their waking hours. Modern humans get most of their energy from starchy grains or plant roots. Over the past 6 million years our diet must have undergone several shifts, when we started using stone tools, learned to cook with fire, and settled down as farmers.
Some of these changes are hard to date. There is an ongoing debate over what constitutes the first evidence for cooking hearths. And digging sticks, used to unearth tubers and bulbs, do not fossilise. An alternative way of tracking dietary changes is to look at the genes involved in digestion.
A digestive enzyme called salivary amylase plays a key role in breaking down starch into simple sugars so it can be absorbed in the gut. Humans have much higher levels of amylase in their saliva than chimpanzees, and recently it was discovered how this came about.
While chimps have only two copies of the salivary amylase gene (one on each of the relevant chromosome pair), humans have an average of six, with some people having as many as 15 (Nature Genetics, vol 39, p 1256). DNA copying errors during the production of sperm and eggs must have led to the gene being repeatedly duplicated.
To find out when the duplications happened, the gene was sequenced in people from several countries, as well as in chimps and bonobos. “We were hoping to find a signature of selection about 2 million years ago,” says Nathaniel Dominy, a biological anthropologist now