Digesta, newly discovered food source of Ice Age people

Digesta, newly discovered food source of Ice Age people
Bison skulls recovered from an archaeological site near New Mexico. Photo: University of Michigan Museum of Anthropological Archaeology
Research at the University of Michigan has found that Ice Age food theories and models fail to take into account an important source of calories and carbohydrates.

Digesta, a type of partially digested plant debris seen in bison and other large game herbivores’ stomachs and digestive tracts, may have served as a primary source of nutrition for early human foragers.

Digesta will help academics better address important evolutionary anthropology topics and will be incorporated into nutrient models. According to author Raven Garvey, associate professor of anthropology and affiliate of the Research Center for Group Dynamics at the University of Michigan Institute for Social Research, it even calls into question the notion that “hunting and gathering,” on which all prehistoric people depended until roughly 10,000 years ago, was segregated by sex.

Early foragers may, in some contexts, have consumed their required portion of “vegetables” in the form of digesta, according to Garvey. Eating not only the herbivores’ meat and organs but also digesta would net a person a significantly higher number of calories, and also would expand the kinds of macronutrients such as protein, fat and carbohydrates available to the forager.

“Failure to account for this underappreciated resource could have important consequences in studies that address major questions in evolutionary anthropology. Accounting for digesta as a source of both kilocalories and carbohydrates leads to predictions that differ from foraging models that do not include this resource.” she said.

The role of digesta in two of these issues—sex-divided subsistence work and archaeologically noted changes in plant use and sedentism, or the transition to more permanent settlements—is examined in Garvey’s study, which was published in the journal Evolutionary Anthropology.

Using estimates of available protein and carbohydrates in the native tissues and digesta, respectively, of a large ruminant herbivore (Bison), Garvey shows that, with digesta included, a group of 25 adult foragers could meet the USDA’s average recommendations for proteins and carbs for three days without additional supplementation.

In some situations, such as those when plants were rare or inedible to people, such a resource may have been essential. Since a single resource could be used to provide all of the necessary sustenance, it might have also reduced the need to hunt and gather separately.

Dating from 11,000 to 10,000 years ago, Folsom points are tools associated with prehistoric bison hunting. (Folsom points are projectile points associated with the Folsom tradition of North America.) Photo: University of Michigan Museum of Anthropological Archaeology

Garvey’s “bison model” challenges the notion that human subsistence labor was always sharply sex-divided as well as the conventional view of large-bodied prey being exclusively provided by men. Hunting by women is more likely when high-energy resources can be obtained with little danger. In times and places where large-bodied herbivores were abundant, a group’s overall hunting success could have been improved, and plant-animal scheduling conflicts reduced through women’s hunting.

There is some archaeological evidence to suggest female hunting (and the child care provisioning that would have entailed) was more common during prehistory than in the later ethnographic period, Garvey says.  For instance, a study of “grave goods” discovered that between 30% and 50% of all large-game hunters in the Americas between 8,000 and 13,000 years ago—during the late glacial (late Pleistocene and early Holocene) era—might have been female.

This strategy might have been influenced both by the relative abundance during that period of large-bodied herbivores and by people’s high mobility. Following migratory game, groups increased the reliability of hunting, but created conditions that reduced the accessibility of edible plants. Consuming digesta would have filled in this resource shortage.

According to Garvey, Digesta consumption may have significantly influenced dietary breadth and human movement in the Americas. Her “bison model” suggests that while large-bodied herbivores could temporarily meet the needs of small human groups in terms of overall nutrition, the ratio of protein to carbs in a single animal renders digesta an unsustainable source of carbohydrates over the long term.

To put it another way, a 1,000-pound bison could supply three days’ worth of protein and carbs to a group of 25 adult foragers, but if they sought a new animal every fourth day, they would leave around six days’ worth of protein unconsumed with each bison kill.

As Garvey notes, the positive effect of Digesta on nutrient profiles may have reduced the demand for fresh plants, perhaps freeing up time and energy for other activities.

University of Michigan News

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