As early humans migrated from lush African forests to open grasslands, they needed new, reliable energy sources. This shift spurred a taste for grassy plants – especially grains – and for starchy plant tissues hidden underground.
A new Dartmouth-led study, published in Science, reveals that hominins began eating these carbohydrate-rich foods long before their teeth evolved to handle them efficiently. The research provides the first human fossil evidence of “behavioral drive,” where survival-driven behaviors emerge well before the physical adaptations that support them.
Early diet before dental adaptation
To investigate, the team analyzed carbon and oxygen isotopes in fossilized hominin teeth, reflecting diets heavy in graminoids – a group that includes grasses and sedges. Surprisingly, early humans were consuming these plants far earlier than their teeth were suited to grinding them down.
Hominins
Hominins are the group consisting of modern humans, extinct human species and all our immediate ancestors. They are distinguished from other apes by characteristics such as bipedalism and a larger brain relative to body size.
Yet, it wasn’t until roughly 700,000 years later that evolution produced the longer molars necessary for efficiently chewing tough plant fibers.
This delayed adaptation underscores how early humans thrived despite their physical limitations.
“We can definitively say that hominins were quite flexible when it came to behavior, and this was their advantage,” said Dr. Luke Fannin, a postdoctoral researcher at Dartmouth and the study’s lead author.
“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,” he continued.
Dr. Nathaniel Dominy, the Charles Hansen Professor of Anthropology and senior author, emphasized the value of isotope analysis in uncovering ancient behavior: “Anthropologists often assume behaviors based on morphological traits, but these traits can take a long time – a half-million years or more – to appear in the fossil record. 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.”
Tracking ancient diets
The researchers analyzed teeth from several hominin species, beginning with Australopithecus afarensis, and compared them to fossilized teeth from two contemporaneous primates: giant ground-dwelling, baboon-like monkeys called theropiths and smaller, leaf-eating colobines.
All three species shifted away from fruits, flowers and insects to graminoids between 3.4 million and 4.8 million years ago – even though their teeth and digestive systems were ill-suited for these plants.
However, by 2.3 million years ago, isotope signatures in hominin teeth abruptly changed, diverging from the other primates. This drop in both carbon and oxygen isotopes suggests that Homo rudolfensis – the human ancestor at the time – reduced grass consumption and began accessing oxygen-depleted water.
The researchers propose three possible explanations:
- Hominins drank far more water than other primates and savanna animals.
- They adopted a semi-aquatic, hippopotamus-like lifestyle.
- They began regularly eating underground plant organs such as tubers, bulbs and corms.
The third explanation fits best. These carbohydrate-rich underground stores, also rich in oxygen-depleted water, were plentiful, safe from herbivores and accessible year-round. With stone tools already in use, hominins could dig them up easily.
“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.”
Teeth catch up to diet
Over time, hominin teeth shrank by roughly 5% every 1,000 years, even as molars lengthened. For much of their history, their dietary reliance on graminoids outpaced dental adaptation. But around 2 million years ago, species such as Homo habilis and Homo ergaster developed teeth better suited to processing tougher and even cooked plant tissues, like roasted tubers.
Graminoids are ubiquitous in many ecosystems, meaning early humans could capitalize on their availability.
“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 said.
“Even now, our global economy turns on a few species of grass – rice, wheat, corn and barley,” he added. “Our ancestors did something completely unexpected that changed the game for the history of the species on Earth.”
Reference: Fannin LD, Seyoum CM, Venkataraman VV, et al. Behavior drives morphological change during human evolution. Science. 2025;389(6759):488-493. doi: 10.1126/science.ado2359
This article is a rework of a press release issued by Dartmouth. Material has been edited for length and content.