Science Article

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 at Dartmouth College in Hanover, New Hampshire, who led the work. That is around the time our brains underwent significant growth, and one theory is that it was fuelled by a switch to a starchier diet.

But the team found the gene duplications had happened more recently - some time between 100,000 years ago and the present day. The biggest change in that period was the dawn of agriculture, so Dominy thinks the duplications happened when we started farming cereals. "Agriculture was a signal event in human evolution," he says. "We think amylase contributed to it."

It was the advent of agriculture that allowed us to live in larger settlements, which led to innovation, the cultural explosion and, ultimately, modern life. If we consider all the mutations that led to these pivotal points in our evolution, human origins begin to look like a trail of unfeasible coincidences. But that is only because we do not see the harmful mutations that were weeded out, points out John Hawks at the University of Wisconsin-Madison. "What we're left with is the ones that were advantageous." It is only from today's viewpoint that the mutations that give us our current physical form appear to be the "right" ones to have. "It's hindsight," says Hawks. "When we look back at the whole process, it looks like a stunning series of accidents."