Saccharides are a food source rich in energy. Large polymers such as starch are partially hydrolyzed in the mouth by the enzyme amylase before being cleaved further into sugars. Many mammals have seen great expansions in the copy number of the amylase gene. These duplications allow for the pancreatic amylase AMY2 to re-target to the salivary glands, allowing animals to detect starch by taste and to digest starch more efficiently and in higher quantities. This has happened independently in mice, rats, dogs, pigs, and most importantly, humans after the agricultural revolution.
Following the agricultural revolution 12,000 years ago, human diet began to shift more to plant and animal domestication in place of hunting and gathering. Starch has become a staple of the human diet.
Despite the obvious benefits, early humans did not possess salivary amylase, a trend that is also seen in evolutionary relatives of the human, such as chimpanzees and bonobos, who possess either one or no copies of the gene responsible for producing salivary amylase.
Like in other mammals, the pancreatic alpha-amylase AMY2 was duplicated multiple times. One event allowed it to evolve salivary specificity, leading to the production of amylase in the saliva (named in humans as AMY1). The 1p21.1 region of human chromosome 1 contains many copies of these genes, variously named AMY1A, AMY1B, AMY1C, AMY2A, AMY2B, and so on.
However, not all humans possess the same number of copies of the AMY1 gene. Populations known to rely more on saccharides have a higher number of AMY1 copies than human populations that, by comparison, consume little starch. The number of AMY1 gene copies in humans can range from six copies in agricultural groups such as European-American and Japanese (two high starch populations) to only two to three copies in hunter-gatherer societies such as the Biaka, Datog, and Yakuts.
The correlation that exists between starch consumption and number of AMY1 copies specific to population suggest that more AMY1 copies in high starch populations has been selected for by natural selection and considered the favorable phenotype for those individuals. Therefore, it is most likely that the benefit of an individual possessing more copies of AMY1 in a high starch population increases fitness and produces healthier, fitter offspring.
This fact is especially apparent when comparing geographically close populations with different eating habits that possess a different number of copies of the AMY1 gene. Such is the case for some Asian populations that have been shown to possess few AMY1 copies relative to some agricultural populations in Asia. This offers strong evidence that natural selection has acted on this gene as opposed to the possibility that the gene has spread through genetic drift.
Variations of amylase copy number in dogs mirrors that of human populations, suggesting they acquired the extra copies as they followed humans around. Unlike humans whose amylase levels depend on starch content in diet, wild animals eating a broad range of foods tend to have more copies of amylase. This may have to do with mainly detection of starch as opposed to digestion.