As early humans migrated from verdant African forests to grasslands, their need for accessible energy supplies prompted them to cultivate a preference for herbaceous plants, particularly grains and subterranean starchy tissues.

However, a recent study led by Dartmouth indicates that hominins commenced consuming these carbohydrate-dense foods prior to developing the optimal dentition for such dietary practices. The research presents the inaugural evidence from the human fossil record of behavioural drive, indicating that survival-enhancing behaviours arise before to the physical adaptations that facilitate them, as reported by the researchers in Science.

Behavioral Adaptation Preceded Physical Evolution

The researchers examined fossilised hominin teeth for carbon and oxygen isotopes derived from the consumption of graminoids, encompassing grasses and sedges. Ancient people began ingesting these plants significantly earlier than their teeth adapted for efficient mastication. Evolution did not progress for 700,000 years until it resulted in the development of elongated molars, which enable modern humans to efficiently masticate resilient plant fibres.

According to Luke Fannin, a postdoctoral researcher at Dartmouth and the study’s primary author, the findings indicate that the success of early humans sprang from their capacity to adapt to novel surroundings despite their physical constraints.

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

Nathaniel Dominy, the Charles Hansen Professor of Anthropology at Dartmouth and senior author of the paper, asserts that isotope analysis addresses the persistent difficulty of determining the conditions that led to the genesis of novel behaviours, as behaviour itself does not fossilise.

“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.

Underground Carbohydrates Mark a Turning Point in Diet

“But these chemical signatures are an unmistakable remnant of grass-eating that is independent of morphology,” he stated. “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.”

The team examined the dentition of multiple hominin species, starting with the ancient human relative Australopithecus afarensis, to trace the evolution of graminoid eating across millennia. For comparative analysis, they examined the fossilised dentition of two extinct primate species that coexisted during the same epoch—giant terrestrial baboon-like monkeys known as theropiths and diminutive folivorous monkeys referred to as colobines.

The researchers report that all three species shifted from consuming fruits, flowers, and insects to grasses and sedges between 3.4 million and 4.8 million years ago. This occurred despite the absence of appropriate dentition and digestive systems suited for consuming these more resilient plants.

The study revealed that hominins and the two primates shared comparable plant diets until 2.3 million years ago, when there was a sudden alteration in the carbon and oxygen isotopes present in hominin teeth. The decline in both isotope ratios indicates that the human progenitor, Homo rudolfensis, reduced its grass consumption and increased its intake of oxygen-depleted water.

The researchers provide three potential explanations for this increase, suggesting that these hominins consumed far more water than other primates and savanna fauna, or that they abruptly embraced a hippopotamus-like existence, remaining submerged in water throughout the day and foraging at night.

The explanation that aligns most closely with current understanding of early human behaviour is that later hominins obtained consistent access to subterranean plant structures referred to as tubers, bulbs, and corms. Oxygen-deficient water is also present in the swollen appendages that numerous graminoids utilise for securely storing substantial quantities of carbohydrates away from herbivorous animals.

According to Fannin, the shift from grasses to these high-energy plant tissues is logical for a species experiencing population growth and increased physical stature. These subterranean caches were abundant, less perilous than hunting, and supplied greater nourishment for the developing brains of early humans. Ancient people, having already utilised stone tools, could excavate tubers, bulbs, and corms with minimal competition from other creatures.

“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.”

Research indicates that hominin teeth measurements revealed a continuous reduction in size, around 5% every millennium, whereas molars exhibited an increase in length. The nutritional transition of hominins towards graminoids exceeded the rate of their physical evolution over the most of their history.

The study revealed that approximately 2 million years ago, the ratio reversed with Homo habilis and Homo ergaster, whose dentition had a significant alteration in morphology and dimensions, more conducive to the consumption of cooked materials, such as roasted tubers.

Graminoids are prevalent in numerous habitats. Regardless of their location, hominins could optimise the nutrients obtained from these plants as their dentition evolved to more effectively process them, according to Dominy.

“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.”

Key Points

• Early humans ate tough plants before teeth evolved for it.
• Behavior changes preceded physical evolution.
• Isotope analysis confirmed early grass-based diet.
• Shift to underground carbs aided brain and body growth.
• Molars evolved later to match new dietary needs.

Original Publication
Authors: Luke D. Fannin, Chalachew M. Seyoum, Vivek V. Venkataraman, Justin D. Yeakel, Christine M. Janis, Thure E. Cerling and Nathaniel J. Dominy.
Journal: Science
DOI: 10.1126/science.ado2359
Method of Research: Experimental study
Subject of Research: People
Article Title: Behavior drives morphological change during human evolution
Article Publication Date: 31-Jul-2025

Original Source: https://home.dartmouth.edu/news/2025/07/changes-diet-drove-physical-evolution-early-humans