Cravings preceded evolution, according to a study of fossil teeth.

New isotopic and fossil evidence suggests that early humans began eating plants before they had the ideal teeth for it.

Led by Dartmouth College in the United States, the research, published in the journal Science, supports the long-hypothesized concept of "behavioral drive," in which behaviors beneficial to survival emerge before physical adaptations that make it easier.

This concept has long shaped evolutionary theory , yet detecting it in the fossil record is challenging because behaviors are often inferred from physical traits , making it difficult to assess them independently of their associated morphologies, the journal notes.

To overcome this, the researchers examined fossilized hominid teeth for carbon and oxygen isotopes left over from eating plants known as gramineae, which include grasses and sedges. They found that hominids gravitated toward these carbohydrate-rich plants long before their teeth evolved to chew them efficiently.

According to the authors, at least three lineages of Pliocene primates, including early hominins, independently transitioned to graminivorous diets, despite lacking these specialized traits.

It wasn't until 700,000 years later that evolution finally caught up with them in the form of longer molars, like those that allow modern humans to easily chew tough plant fibers.

"We can say with certainty that hominins were quite flexible in their behavior and that this was their advantage ," emphasizes Luke Fannin, for whom the findings suggest that their success was due to their ability to adapt to new environments despite their physical limitations.

To reach their conclusions, the team analyzed teeth from several species , starting with Australopithecus afarensis, to follow the evolution of consumption of different parts of grasses over millennia.

For comparison, they also analyzed the fossilized teeth of two extinct primate species that lived at roughly the same time.

All three species switched from eating fruits, flowers, and insects to grasses and sedges between 3.4 and 4.8 million years ago, despite lacking the teeth and digestive systems optimal for eating these tougher plants.

Hominins and the two primates displayed similar plant diets until 2.3 million years ago, when the carbon and oxygen isotopes in the former's teeth changed abruptly.

This drop in the proportions of both isotopes suggests that the human ancestor of the time, Homo rudolfensis, reduced their consumption of grasses and drank more oxygen-depleted water, according to a university statement.

Later hominins gained regular access to underground plant organs known as tubers, bulbs, and corms.

Oxygen-depleted water is also found in these bulging appendages that many grasses use to store large amounts of carbohydrates safe from herbivores.

"We propose that this shift toward underground foods was a defining moment in our evolution ," Fannin says. "It created a surplus of carbohydrates that were perennial: our ancestors could access them at any time of year to feed themselves and others."

"Even now, our global economy revolves around just a few grass species: rice, wheat, corn, and barley," says Nathaniel Dominy, who believes that "our ancestors did something completely unexpected that changed the game for the history of species on Earth."

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