One of humans’ significant evolutionary advantages over other animals is our exceptional chewing ability. Our combination of incisors, canines, and molars allows us to ingest a wide variety of foods, creating a diverse, energy-rich diet.
However, according to a new study published in the journal Science on July 31, 2024, this ability may not have been perfect from the start.
In fact, our early ancestors likely began tackling tougher, more difficult-to-chew plant foods before they were physiologically ready.
The study, led by a team of researchers at Dartmouth College, is the first to find clear evidence in the fossil record of so-called “behavioral drives.”
This concept refers to the practice of adopting behavioral strategies to improve survival before an organism’s organs or systems have evolved adaptively.
“We can clearly say that behavioral flexibility was one of the key advantages of early humans,” said anthropologist Luke Fanning, the study’s lead author.
The researchers found key clues by analyzing the fossilized teeth of early humans and other primates, including Australopithecus afarensis, dating between 3.9 and 2.9 million years ago.
Their analysis focused on the carbon and oxygen isotopes in the teeth, which reveal whether the hominins consumed a diet primarily composed of grasses, such as sedges and grass roots. These plants are tough, fibrous, and difficult to chew and digest.
By comparing the teeth of Theropithecus and Parapapius, which lived at the same time as Australopithecus afarensis, the study found that these species began gradually shifting to a more structured diet based on grasses between 3.4 and 4.8 million years ago, but their dental adaptations lagged far behind these dietary changes. In other words, they “ate more than their teeth could handle.”
Even more striking, the study revealed a significant decrease in oxygen and carbon isotopes in the teeth of the early human Homo rudolfensis around 2.3 million years ago.
This suggests that they reduced their intake of above-ground grasses and likely began shifting to more hidden, energy-rich, underground plants, such as tubers and bulbs, as sources of carbohydrates.
The researchers suggest that this shift in dietary behavior coincided with the emergence of stone toolmaking, which facilitated the excavation of underground resources.
“We believe the shift to underground carbohydrate plants was a key turning point in human evolution,” Fanning noted. “These plants were available year-round, provided a durable source of energy without consuming them, and made social behavior and reproduction possible for ancient humans.”
Nathaniel Dominy, senior author of the study, added that understanding the differences between early humans and other primates has always been a core issue in anthropology.
“Our study suggests that our consumption of grass tissues was likely a decisive advantage—our secret weapon.”