As early hominins transitioned from the shelter of dense African forests to expansive grasslands, their dietary landscape changed dramatically. This shift demanded new sources of readily available energy, and grassy plants—particularly grains and the starchy subterranean tissues of plants—became increasingly vital to their survival. A groundbreaking study led by researchers at Dartmouth College now reveals that hominins adopted these carbohydrate-rich foods long before their dental architecture was optimally adapted for consuming such tough plant matter. This discovery sheds new light on the complex interplay between behavior and biology throughout human evolution.
The research team employed isotopic analyses of fossilized hominin teeth, focusing specifically on carbon and oxygen isotope signatures embedded in the dental enamel. These chemical remnants provide a direct window into the diets of our ancient ancestors. By tracing isotopic ratios characteristic of graminoids—a group encompassing grasses and sedges—the scientists could pinpoint when early humans began incorporating these plants into their diets. Surprisingly, these chemical markers indicate a pronounced behavioral shift toward graminoid consumption several hundred thousand years before the morphological adaptations in teeth emerged, challenging the long-held assumption that physical traits and behaviors evolve concurrently.
This temporal lag between behavior and morphology exemplifies what evolutionary biologists term “behavioral drive.” Behavior often acts as a leading force in evolutionary trajectories, enabling organisms to exploit new ecological niches and resources prior to the full suite of anatomical specializations necessary for such exploitation. In the context of early hominins, this means that despite possessing teeth that were not yet ideal for processing gritty, fibrous plant tissues, they nonetheless intensified their consumption of graminoids, relying on cultural and technological innovations such as rudimentary tools and fire to aid in food acquisition and preparation.
Detailed analyses of molar morphology reveal that while hominin teeth generally diminished in overall size—shrinking by roughly five percent every 1,000 years—their molars simultaneously grew longer. This elongation is interpreted as an adaptive modification facilitating more efficient breakdown of tough vegetative fibers. However, this dental evolution significantly lagged behind the initial dietary transition toward grass consumption, with substantial changes in tooth shape and size only materializing approximately two million years ago. Species such as Homo habilis and Homo ergaster exemplify this dental shift, their evolved molars better suited for chewing and processing cooked plant materials.
Isotope data also pinpoint a dramatic dietary pivot around 2.3 million years ago involving Homo rudolfensis. During this period, carbon and oxygen isotope ratios in their teeth shifted sharply, indicative of reduced direct grass consumption and increased intake of oxygen-deprived water sources. This unique signature aligns closely with the exploitation of underground plant storage organs like tubers, bulbs, and corms, which contain starch-rich carbohydrates encased within oxygen-limited tissues. The consistent availability and high nutritional value of such underground resources likely played a critical role in supporting the growing energetic demands of larger brains and bodies in these hominins.
The team’s findings suggest that early humans’ dietary flexibility was not just a passive response to environmental change, but an active driver of evolutionary processes. The strategic reliance on underground plant organs provided a dependable carbohydrate reservoir that required less physical risk than hunting and offered abundant calories year-round. Access to these subterranean resources, facilitated by manual digging tools, allowed hominins to thrive in open grassland ecosystems where fruits and insects were less plentiful or seasonal.
Perhaps most compellingly, this research argues that early human innovation and cultural adaptation spontaneously prompted major evolutionary shifts in physiology. The precedence of behavior over morphology challenges the traditional anthropological narrative which posits that bodily adaptations are prerequisites for novel behaviors. Instead, hominins’ early embrace of tough, grassy plants and underground starches motivated the gradual dental transformations that eventually optimized their diets, highlighting the co-evolution of culture and biology.
The implications of this study extend beyond understanding prehistoric diets; they inform the fundamental question of what set early humans apart from other primates. While many primates adapted to specific dietary niches with corresponding anatomical specializations, hominins demonstrated unmatched behavioral plasticity by exploiting grass-based food sources ahead of physical device. This flexibility likely conferred significant survival advantages, enabling them to colonize diverse habitats and develop complex social systems.
Moreover, this evolutionary narrative resonates with the centrality of grasses in modern human civilizations. Contemporary global economies heavily depend on a handful of grass species—such as rice, wheat, corn, and barley—that constitute primary calorie sources. The ancestral behavior of embracing graminoids as a staple food thus represents a foundational step in the trajectory leading to agriculture and civilization itself. Recognizing this deep history reinforces the notion that human success derives not only from biological innovation but also from strategic behavioral choices.
Lead author Luke Fannin encapsulates this perspective: early hominins’ willingness to integrate suboptimal foods into their diet, despite morphological inadequacies, reveals a core human trait—adaptive behavioral flexibility. This flexibility, rather than morphologically deterministic traits alone, illuminated a pathway for evolutionary success. As anthropologists revisit long-standing assumptions about the synchrony of morphological and behavioral evolution, these findings herald a paradigm shift in interpreting the hominin fossil record.
Senior author Nathaniel Dominy emphasizes the power of isotope analysis as a revolutionary tool in paleoanthropology. Since behavior rarely fossilizes, reliance on physical traits alone has obscured the timing and nature of ancient behavioral innovations. Chemical signatures preserved in teeth emerge as irrefutable evidence for dietary shifts, underscoring the critical role of new methodologies in elucidating human evolutionary history.
Collectively, the evidence from this study reframes behavioral change as a potent evolutionary force capable of driving morphological adaptations. By embracing behavior as both a survival strategy and an evolutionary catalyst, we gain a richer understanding of how our ancestors navigated and transformed their environments. This nuanced view illuminates the dynamic feedbacks between culture, biology, and environment that shaped the human lineage.
In conclusion, the revelation that behavior led morphological evolution in early hominins underscores a unique hallmark of our species’ emergence. The early adoption of graminoid consumption millions of years ago, preceding dental evolution by hundreds of thousands of years, not only exemplifies the principle of behavioral drive but also marks a pivotal juncture in the evolution of human diets. It is a testament to the ingenuity and adaptability that have propelled our species from the African savannas to global dominance.
Subject of Research: People
Article Title: Behavior drives morphological change during human evolution
News Publication Date: 31-Jul-2025
Web References: https://doi.org/10.1126/science.ado2359
Image Credits: L to R: Public domain; Don Hitchcock; Fernando Losada Rodríguez (rotated)
Keywords: Early humans, Human evolution, Hominins, Social sciences, Anthropology, Anthropogenesis, Human adaptation, Applied anthropology, Homo sapiens, Physical anthropology, Research methods, Adaptive evolution, Evolution, Isotopes
