Sunday, August 30, 2009

Primal Potatoes, part 1

Although in overall form the human gut appears distinctly carnivorous, humans have one unusual feature for a carnivore: we have salivary glands that produce amylase, which has the the sole function of digesting starch. Since meat contains essentially no starch, this feature could not have arisen as an adaptation to an exclusive meat diet. It clearly represents an adaptation to starch consumption. Did this arise recently, in response to the Neolithic shift to cereal-based diets, or does it have deeper roots?

Meet AMY1

AMY1 is the gene that codes for the production of salivary amylase. Perry et al compared the AMY1 copy number variation humans from low-starch diet populations, humans from high-starch diet populations, and wild-born chimpanzees. They also examined the amount of nucleotide sequence divergence among the three AMY1 gene copies found in the human genome reference sequence.

The high-starch diet populations included Hadza hunter-gatherers and two agricultural groups, European Americans and Japanese. The low-starch diet populations included Yakut pastoralists and two rainforest hunter-gatherer groups, Biaka and Mbuti. The high-starch diet people carried a median of 7 and a mean of 6.72 copies of AMY1, in a range of 2 to 15 copies. The low-starch diet people carried a median of 5 and a mean of 5.44 copies of AMY1, and exhibited a range of 2 to 13 copies. Thus, AMY1 copy number correlated positively with diet.

When Perry et al examined 15 wild-born chimpanzees, they found evidence of only two diploid AMY1 copies. Thus, whereas the humans from high- and low- starch diet groups vary but the average human, regardless of high- or low-starch diet, has about 3 times as many copies of this gene as the chimpanzee. As a result, humans produce 6 to 8 times as much salivary amylase as chimpanzees.

Perry et al found a low amount of nucleotide sequence divergence among the three AMY1 gene copies found in the human genome reference sequence, which “implies a relatively recent origin that may be within the time frame of modern human origins.” Assuming that humans and chimps diverged from a common ancestor about 6 million years ago, they estimate that the increase in AMY copy number occurred within the last 200,000 years, the time period during which modern humans (H. sapiens sapiens) emerged.

This strongly suggests that starchy food has had a significant role in human diets for substantial time periods, starting in Paleolithic times. Most likely, this adaptation arose as an advantage in digestion of tubers, corms, and bulbs. It implies that these foods played a large enough role in human evolutionary diets to give a selective survival advantage to individuals who had extra copies of AMY1 (compared to chimps) and hence an enhanced ability to digest starch. I see no other way of explaining our unusual production of an otherwise useless salivary enzyme.

As I noted in The Garden of Eating, in 1996 anthropologist Melissa Darby, M.A., of Lower Columbia Research and Archaeology (Oregon) demonstrated that Northern Hemisphere paleolithic humans had access to arrowhead (Sagittaria latifolia) a prolific wetlands plant that produces a tuber very similar to the white potato. Pollen data indicates that this tuber —called “wapatos” by the Chinook tribe – thrived in the last Ice Age throughout North America, the North American Great Basin, Siberia, and Northern Europe – overlapping the time that during which Perry et al estimate that humans started showing extra copies of the AMY1 gene.

Darby has harvested approximately 5,418 calories per hour gathering wapatos from a knee-deep pond. The tuber is most abundant from late fall through spring, when other plant foods are scarce. People can eat these tubers without grinding or mashing, and they cook thoroughly in a bed of hot ashes in 10 minutes, no oven required. They keep fresh in a cool place, and also dried.

American Prairie Indian women also gathered and cultivate the starchy camas bulb. Darby says that a woman gathering camas could net 5,279 calories per hour. This would consist primarily of starch: nearly 1000 g of it.

Thus, we know that humans had access to tubers—I’ll call them primal potatoes—even during the Ice Ages. It seems fairly certain that we have descended from a long line of tuber-eaters extending back at least a quarter of a million years. I have more to say on this in my next post.