As humans, one of our biggest evolutionary advantages is our chompers. Our combination of molars, canines, and incisors help us to consume a diverse and omnivorous diet, unlike many other animals. This dramatically expands the available menu for energy sources, and thus increases nutritional options and the chance of survival. But according to a study published on July 31 in the journal Science, our early hominin ancestors’ desire to bite off more than they could literally chew is the likely reason for our dental evolution.
The findings come from a team led by researchers at Dartmouth College, and offer the human fossil record’s first known evidence of what’s known as behavioral drive—the tendency for a species to develop routines that are helpful for their survival before their physiology evolves to meet these demands.
“We can definitively say that hominins were quite flexible when it came to behavior and this was their advantage,” anthropologist and study lead author Luke Fannin said in a statement.
Fannin explained that anthropologists often discuss hominin behavioral and morphological changes as if they evolved in tandem with one another. Instead, his team’s analysis indicates that early human behavior was its own evolutionary force—one with large implications for our physical and dietary journey.
Fannin and colleagues reached their conclusions by examining multiple sets of fossilized teeth from various hominin and primate species, starting with one of our distant relatives from 3.9 to 2.9 million years ago called Australopithecus afarensis. Researchers were particularly interested in their levels of graminoids, the carbon and oxygen isotopes left behind from eating grasses and sedges. They also looked at fossil teeth from two extinct primate species who were contemporaries of A. afarensis—huge terrestrial monkeys that looked similar to baboons called theropiths, as well as smaller herbivorous monkeys known as colobines.
The team discovered that while A. afarensis, theropiths, and colobines all initially preferred a diet heavy in insects, fruits, and flowers, they started shifting to the tougher, graminoid-heavy sedges and grasses 3.4 to 4.8 million years ago. However, these changing palettes came at least 700,000 years before their teeth and digestive systems optimized for the vegetation.
Then, about 2.3 million years ago, hominin teeth began exhibiting significantly smaller amounts of oxygen and carbon isotopes. According to researchers, this implies that the era’s human ancestor—Homo rudolfensis—lessened their grass intake for oxygen-depleted water. They offered three possible theories for this. In one scenario, H. rudolfensis simply drank far more water than other primates and African animals. Another hypothesis is that they adopted behavior similar to today’s hippopotamuses, spending their days submerged in water and eating at night. The explanation most consistent with early-human behavior, however, is that the hominins started harvesting underground, carbohydrate-rich plant parts like tubers and bulbs.
The latter argument tracks with a species rapidly growing in physical size and numbers. Energy-rich plants were everywhere, they didn’t fight back like prey, and were more nutritious. At this point in history, ancient humans were also beginning to form stone tools that made searching for underground plants even easier.
“We propose that this shift to underground foods was a signal moment in our evolution,” Fannin said. “It created a glut of carbs that were perennial—our ancestors could access them at any time of year to feed themselves and other people.”
According to study senior author Nathaniel Dominy, one of anthropology’s remaining “burning questions” is “What did hominins do differently that other primates didn’t do?”
“This work shows that the ability to exploit grass tissues may be our secret sauce,” he said.
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