The ability to digest starch may have given our human ancestors an evolutionary advantage in certain circumstances, according to an article by George Perry et al. in Nature Genetics.[1] The enzyme amylase, secreted by salivary glands (and also by the pancreas), helps to hydrolyse or break down starch in the diet when mixed with water. The gene that produces the enzyme in saliva, called AMY1, is on chromosome 1, other authors have theorised that the salivary amylase gene evolved from the pancreatic amylase gene via an upstream retrovirus insertion.[2]

Perry's team took salivary and DNA samples from people of Datog, Hadza, Mbuti, Biaka, Japanese, Yakut and European-American populations and analysed their typical dietary protein and starch intake. The results indicate that geographically distinct groups of humans tend to have variable levels of the AMY1 gene according to the level of starch in their diets: Those whose diets consist of higher protein levels tended to have on average fewer copies of the gene, and vice versa. More copies of the gene leads to a higher level of salivary amylase. Fruit-eating chimpanzees however have few copies of the gene. The authors suggest that: "This behavioral variation raises the possibility that different selective pressures have acted on amylase", i.e. it could be evidence for genetic adaptation to the availability of starch in the environment.

Another study in the American Journal of Human Genetics[3] published just a month prior to Perry's article suggests that the gene for lactase persistence (LP, the ability to digest the lactose sugar in milk after childhood) came about through evolution of two groups geographically and chronologically distinct, and that "there is a still-ongoing process of convergent evolution" of the LP alleles in humans".

It appears that we are still evolving according to what we eat, but can our genes keep up with the pressures inflicted on us by modern western diets?

Follow up:

References:

1. Perry, G., et al. "Diet and the evolution of human amylase gene copy number variation" Nature Genet doi: 10.1038/ng2123 9 Sept. 2007.

2. Meisler MH, Ting CN. "The remarkable evolutionary history of the human amylase genes."Crit Rev Oral Biol Med. 1993;4(3-4):503-9. PMID 7690604

3. Enattah NS, Trudeau A, Pimenoff V, et al. "Evidence of still-ongoing convergence evolution of the lactase persistence T-13910 alleles in humans." Am J Hum Genet 2007 Sep;81(3):615-25. PMID 17701907