A Cultural Context for Human-Microbe Symbiosis  3
microbes within our gut. In particular, the gut microbiome appears to play an
important role in our adaptability to new environments. Understanding the
changes in composition of gut microbes since Homo sapiens diverged from
other species provides helpful insight into current health problems faced by
the modern Western world.
Early Humans and the Foraging Diet
The fossil record shows the evolution of the body in response to variations in
diet. For example, the current shape and size of our gastrointestinal tract, as
well as the formation of our teeth, originate from dietary shifts that took place
during the Stone Age. Adaptations in gut shape and size likely led to altera-
tions in the gastrointestinal tract’s internal environment, conditions that may
have selected for different types of bacterial life within the gut. Dietary intake,
physiological adaptations, and gut bacterial composition evolved simultane-
ously and depended on one another. Thus, the human microbiome coevolved
with the human genome and with human cultures (including diet, agriculture,
food preparation, etc.). Looking into the evolution of the human diet gives
clues to how our gut flora has changed over time.
Prior to learning how to cultivate plants for food, humans survived by for-
aging, searching their habitat for wild food sources. This foraging diet was far
from static and depended on the availability of foods, which varied across sea-
sons, weather conditions, and local environments. In order to survive in these
variable habitats, early hunter-gatherer populations had to develop flexibility
in the foods they ate.
One of the first major dietary shifts in early human history occurred
approximately 4.4 million years ago, when humans split from prehistoric apes.
Modern humans evolved from earlier hominins, a group that includes extinct
human species, our recent ancestors, as well as modern humans. This evolu-
tionary split is reflected in a number of recognizable physical adaptations that
point to changes in both diet and habitat. Archeologists have uncovered fossil
evidence revealing that hominins gradually developed larger, thickly enameled
teeth. These changes in tooth structure emerged during a time when hominins
were learning to use their premolars on harder foods, and thus were likely
linked to the introduction of new foods. This dental adaptation is indirect evi-
dence for the introduction of starch-rich underground storage vegetables such
as bulbs, corms, or tubers. Similar underground vegetables continue to play an
important role in the diets of the few remaining modern-day hunter-gatherer
With diverse foods of both plant and animal origin, subsequent homi-
nins adapted to various habitats and eventually transitioned to more open
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