Long before their teeth evolved to handle tough, fibrous plants, early humans were already digging up and eating grasses, sedges, and starchy underground foods.

A new fossil-tooth isotope study shows this behavior began about 700,000 years before longer molars emerged -- revealing that behavioral innovation, not anatomy, drove the change.

Grains Before Grinders: A Taste Ahead of Evolution

As early humans moved from the dense forests of Africa into open grasslands, they began relying on quick, reliable sources of energy. This shift in habitat led them to favor grassy plants, especially grains and the starchy tissues stored underground.

A new study led by Dartmouth researchers reveals that hominins started eating these carbohydrate-rich foods long before they had the dental structures best suited for them. The research offers the first fossil evidence of what scientists call "behavioral drive," in which survival-boosting behaviors emerge before the physical traits that make them easier. The team reports their findings in Science.

Behavior Before Biology: A Fossilized Clue

Researchers examined fossilized hominin teeth for carbon and oxygen isotopes left behind by eating graminoids, a group that includes grasses and sedges. Their analysis showed that ancient humans began consuming these plants far earlier than their teeth evolved to process them effectively.

It took another 700,000 years for evolution to produce longer molars like those in modern humans, which are well-suited for breaking down tough plant fibers.

"We can definitively say that hominins were quite flexible when it came to behavior, and this was their advantage."

Evolution Lags as Diet Leaps Forward

The findings suggest that the success of early humans stemmed from their ability to adapt to new environments despite their physical limitations, says Luke Fannin, Guarini '25, a postdoctoral researcher at Dartmouth and lead author of the study.

"We can definitively say that hominins were quite flexible when it came to behavior, and this was their advantage," Fannin says. "As anthropologists, we talk about behavioral and morphological change as evolving in lockstep. But we found that behavior could be a force of evolution in its own right, with major repercussions for the morphological and dietary trajectory of hominins."

Chemistry Over Morphology: Reading Ancient Meals

Nathaniel Dominy, the Charles Hansen Professor of Anthropology and senior author of the study, says isotope analysis overcomes the enduring challenge of identifying the factors that caused the emergence of new behaviors -- behavior doesn't fossilize.

"Anthropologists often assume behaviors on the basis of morphological traits, but these traits can take a long time -- a half-million years or more-to appear in the fossil record," Dominy says.

"But these chemical signatures are an unmistakable remnant of grass-eating that is independent of morphology. They show a significant lag between this novel feeding behavior and the need for longer molar teeth to meet the physical challenge of chewing and digesting tough plant tissues," he says.

Monkey See, Monkey Chew: A Shared Dietary Shift

The team analyzed the teeth of various hominin species, beginning with the distant human relative Australopithecus afarensis, to track how the consumption of different parts of graminoids progressed over millennia. For comparison, they also analyzed the fossilized teeth of two extinct primate species that lived around the same time -- giant terrestrial baboon-like monkeys called theropiths and small leaf-eating monkeys called colobines.

All three species veered away from fruits, flowers, and insects toward grasses and sedges between 3.4 million to 4.8 million years ago, the researchers report. This was despite lacking the teeth and digestive systems optimal for eating these tougher plants.

A Watery Puzzle in Ancient Isotopes

Hominins and the two primates exhibited similar plant diets until 2.3 million years ago when carbon and oxygen isotopes in hominin teeth changed abruptly, the study found. This plummet in both isotope ratios suggests that the human ancestor at the time, Homo rudolfensis, cut back on grasses and consumed more oxygen-depleted water.

The researchers lay out three possible explanations for this spike, including that these hominins drank far more water than other primates and savanna animals, or that they suddenly adopted a hippopotamus-like lifestyle of being submerged in water all day and eating at night.

The explanation most consistent with what's known about early human behavior, they report, is that later hominins gained regular access to underground plant organs known as tubers, bulbs, and corms. Oxygen-depleted water also is found in these bulging appendages that many graminoids use for storing large amounts of carbohydrates safely away from plant-eating animals.

Rooting for Carbs: The Brain Food Revolution

The transition from grasses to these high-energy plant tissues would make sense for a species growing in population and physical size, Fannin says. These underground caches were plentiful, less risky than hunting, and provided more nutrients for early humans' expanding brains. Having already adopted stone tools, ancient humans could dig up tubers, bulbs, and corms while facing little competition from other animals.

"We propose that this shift to underground foods was a signal moment in our evolution," Fannin says. "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."

Shrinking Teeth, Growing Adaptability

Measurements of hominin teeth showed that while they became consistently smaller -- shrinking about 5% every 1,000 years -- molars grew longer, the researchers report. Hominins' dietary shift toward graminoids outpaced that physical change for most of their history.

But the study found that the ratio flipped about 2 million years ago with Homo habilis and Homo ergaster, whose teeth exhibited a spurt of change in shape and size more suited to eating cooked tissues, such as roasted tubers.

Grass: Humanity's Secret Weapon

Graminoids are ubiquitous across many ecosystems. Wherever they were, hominins would have been able to maximize the nutrients derived from these plants as their teeth became more efficient at breaking them down, Dominy says.

"One of the burning questions in anthropology 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," Dominy says.

"Even now, our global economy turns on a few species of grass -- rice, wheat, corn, and barley," he says. "Our ancestors did something completely unexpected that changed the game for the history of species on Earth."

Reference: "Behavior drives morphological change during human evolution" by Luke D. Fannin, Chalachew M. Seyoum, Vivek V. Venkataraman, Justin D. Yeakel, Christine M. Janis, Thure E. Cerling and Nathaniel J. Dominy, 31 July 2025, Science.

DOI: 10.1126/science.ado2359