But a new analysis of fossil feces from an Arizona cave suggests that the evolution of thrifty genes had little to do with famine and much more to do with the nature of the ancient feast. The research, reported in the August issue of Current Anthropology, shows that prehistoric hunter-gatherers in the Southwest lived on a diet remarkably high in fiber, low in fat, and consisting largely of foods with extremely low glycemic indices. That diet alone, the researchers say, could have been enough to fix fat-hoarding genes in place.
"What we're saying is we don't really need to look to feast or famine as a basis for thrifty genes," said Karl Reinhard, an archaeologist at the University of Nebraska-Lincoln and one the study's authors.
Native Americans have some of the highest rates of type 2 diabetes of any ethnic group. In some populations, up to half of adults suffer from the disease. The idea that this prevalence might be tied to ancient cycles of feast and famine first emerged in the 1960s and has been debated ever since.
"To understand the basis of these high rates of diabetes, one has to look at the best dietary data one can find," Reinhard said. "That comes from coprolites," the technical term for fossil dung. "By looking at coprolites we're seeing exactly what people ate."
Over the years, Keith Johnson, an archaeologist at California State University, Chico, directed excavations at Antelope Cave. Johnson and other archaeologists have collected nearly 200 coprolites from Antelope Cave, a deep cavern located in northern Arizona, just across the border from St. George, Utah. For as long as 4,000 years, the cave has been host to people from various cultures, including Pueblo and Virgin Anasazi. Reinhard and Johnson selected 25 coprolites from the cave they suspected were of human origin and examined them in the lab for food remnants.
Four of the coprolites turned out to be from dogs or other canids, based on the presence of parasites specific to dogs. One turned out to be nothing more than a clump of sediment. But 20 were found to be human and provided the researchers a wealth of dietary data.
The analysis suggests a diet dominated by maize and high-fiber seeds from sunflowers, wild grasses, pigweed, and amaranth. These were usually ground into a fine flour and often showed signs of having been cooked. The researchers also found bones from small mammals, likely rabbit. The flour and meat were likely cooked together with water in stew, the researchers say.
Prickly pear, a desert succulent, was also found repeatedly in the samples and was probably an important part of the ancient diet.
"These plant foods are very, very high in fiber," Reinhard said. "The seeds have thick shells, and they ate the whole thing, ground into a fine meal. That maximizes the fiber content."
These foods also have very low glycemic indices (GI), the measure of how fast a food causes blood sugar to increase. Recent research suggests that foods with high GIs may increase risk of obesity and diabetes. Foods with indices of 70 or above are considered high GI. Those with indices of 50 or below are considered low.
"Modern, cultivated sunflower achenes have a GI of 10," the researchers write. "Modern, cultivated amaranth has a glycemic index of 25. Prickly pear was a very important prehistoric food. It has a glycemic index of 7, which is the lowest recorded for southwestern plant food, and one of the lowest values for any recorded human food."
Traditional maize has a GI of 57, the researchers say, and was "probably the highest GI for available foods at Antelope Cave."
Reinhard and Johnson conclude that this high-fiber, low-GI diet could have been the evolutionary pressure that fixed a thrifty genotype in place, leaving modern populations susceptible to disease when they moved to a modern diet of high fat, sugary foods that cause rapid spikes in blood glucose.
"The feast or famine scenario long hypothesized to be the pressure for thrifty genes isn't necessary, given the dietary evidence we've found," Reinhard said.
EurekAlert. 2012. "Feces fossils yield new insights into ancient diets and 'thrifty genes'". EurekAlert. Posted: July 24, 2012. Available online: http://www.eurekalert.org/pub_releases/2012-07/uocp-ffy072412.php