A DRAMATIC shift in diet sometime during the evolution of modern humans has left its imprint on our genome. The discovery could provide some of the strongest evidence to date in support of a controversial hypothesis that purports to explain why humans, alone among all the apes, suddenly evolved such big brains.
One plausible reason is that early hominins suddenly stumbled on a new, rich food source capable of fuelling a large, energetically expensive brain. For many years, anthropologists presumed the crucial food source was meat, which became more accessible as our ancestors began to use stone tools for hunting or cutting. More recently, however, others have proposed an alternative 鈥� starchy tubers. Proponents of this view argue that early hominins had teeth better suited to grinding plant matter than tearing flesh. Recent studies of isotope ratios in hominin fossils also suggest a plant-rich diet.
鈥淓arly hominins may have stumbled on a new, rich food source capable of fuelling a large brain鈥�
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But definitive proof is hard to come by. 鈥淲e鈥檙e talking millions of years ago, we鈥檙e talking perishable food items. We鈥檙e just not going to find archaeological evidence for it,鈥� says Nathaniel Dominy, an evolutionary anthropologist at the University of California, Santa Cruz.
So Dominy and his colleagues decided to look for evidence in an unusual place: our genome. They focused on a gene called AMY1, which codes for salivary amylase, a starch-digesting enzyme. They already knew that the number of copies of AMY1 varies widely from person to person, and when the researchers surveyed 50 American college students of European descent, they found anywhere from 2 to 15 copies. Moreover, individuals with more copies had higher levels of amylase in their saliva. By contrast, chimpanzees, whose natural diet contains very little starch, have just two copies and very little salivary amylase.
The researchers then compared the genes of ethnic groups that traditionally eat a high-starch diet 鈥� such as Europeans, Japanese and the African Hadza people 鈥� with those whose traditional diet is very low in starch, such as the African Datog and Asian Yakut. Those from a high-starch background averaged 6.72 gene copies, significantly higher than the 5.44 copies carried by those from a low-starch background (Nature Genetics, ). 鈥淲e think that selection is strongly favouring more copies in populations with more starch in the diet,鈥� says Dominy. The study is one of the first to show that natural selection can lead to an increase in gene copy numbers.
If that increase coincided with the dramatic expansion in our ancestor鈥檚 brain size about 1.8 million years ago, that would be the strongest possible evidence that roots and tubers, not meat, fuelled our intelligence.
Unfortunately, the researchers have been unable to pin down the date with any precision. 鈥淎ll we know is it鈥檚 within the time frame of modern human origins,鈥� says Dominy. The researchers are working to gather more corroborating evidence.
That leaves those in the meat camp plenty of room for scepticism. 鈥淚t would only be relevant if the mutation happened between 1.5 and 3 million years ago,鈥� says Craig Stanford, an anthropologist at the University of Southern California in Los Angeles. 鈥淚f you can鈥檛 nail down the time frame better, this doesn鈥檛 have any bearing on the meat-eating versus tuber-eating question.鈥�
