What traits make us uniquely human? Anthropologists have been occupied with this question for decades in an effort to understand the origins of human evolution and our eventual domination of the planet. Several traits of modern humans-- language, bipedalism, hairlessness, incredibly strong molar enamel, and large brains-- distinguish us from our primate relatives. But the precise story of why and how it all came to be still remains unresolved. Now we can add one more trait to our list of things distinctly human, and its evolution may have pre-dated all others. The evidence comes from a surprising place-- our saliva.
Our saliva contains the enzyme amylase, responsible for cleaving starch molecules into smaller glucose units. Without amylase, we would not be able to digest starch and other complex carbohydrates-- and that means we would not be able to enjoy many of the foods that constitute a large part of the global human diet-- foods such as corn, potatoes, rice, wheat, and sorghum.
Multiple lines of evidence now indicate that the ability to digest large quantities of starch may have been a crucial adaptation in human evolution-- providing the calories needed to grow large, cognitively-sophisticated brains capable of complex language and social cooperation. This idea is a serious departure from the leading hypothesis that carnivory (via hunting) was the dietary shift needed to support large brains in early humans.
The breakthrough study, lead by George Perry of Arizona State University and Nathaniel Dominy of UC Santa Cruz (http://www.nature.com/ng/journal/v39/n10/abs/ng2123.html), first demonstrates that individuals with more copies of the AMY1 gene tend to have higher levels of amylase in their saliva. The researchers then sampled a suite of high- and low-starch populations spanning cultures world-wide-- Hadza hunter-gathers who survive primarily on roots and tubers, and two agricultural populations (Japanese and European Americans) comprised the high-starch sample. Low-starch populations, of which there are considerably few, included rainforest hunter-gatherers (Biaka and Mbuti) and pastoralists (Datog and Yakut). In line with expectations, mean AMY1 copy number was greater in the high-starch compared to low-starch populations.
Notably, there was no geographic pattern in AMY1 copy number to suggest that populations closer to one another have more similar AMY1 copy numbers than populations that are further apart-- this pattern would be expected if variation in AMY1 is driven largely by neutral genetic changes (genetic drift). Instead, the results suggest that variation in AMY1 is related to ecological adaptations in diet. Perry and Dominy hypothesize that natural selection is driving differences in AMY1 copy number. Their results do provide some compelling evidence for natural selection at the AMY1 locus, but the authors cautiously note that the jury is still out on this question-- pending additional data of course.
Shedding some light on the evolutionary history of AMY1, Perry and Dominy also looked at AMY1 variation in chimps and bonobos, our close genetic relatives. Their primary diet-- ripe fruit-- contains very little starch, leading the researchers to predict low numbers of AMY1 in these apes. Indeed, the data indicate that chimps and bonobos have, at most, 2 functional copies of AMY1. The researchers report that humans have 3 times more AMY1 copies compared to chimps, on average-- and bonobos may not have any functional AMY1 copies at all. These findings support the conclusion that elevated AMY1 copy numbers arose in the human lineage, not before it.
If this doesn't convince you, Dominy and colleagues have also found evidence that Homo erectus, an early human progenitor, specialized on eating high-starch corms and tubers. In this sister study, Dominy used stable isotope analysis, a common method to assess diet composition. In a nutshell, the stable isotope signatures of consumers will resemble the stable isotope signatures of their food sources-- after some corrections for fractionation. As it turns out, Homo erectus has a stable isotope signature that is consistent with a high-starch diet, and decidedly not consistent with a carnivorous one.
All of these lines of evidence suggest that having many copies of AMY1 is likely to have evolved early in the human lineage-- indeed it may have been critical to launching humans on our own immensely successful, starch-filled, evolutionary path.