New isotopic and fossil evidence suggests that early primates, including hominins, began eating grasses long before their bodies evolved the traits needed to process them efficiently – findings that support the long-hypothesized concept of behavioral drive, researchers report. The concept of behavioral drive – the idea that major shifts in animal behavior can lead to new selective pressures and, ultimately, catalyze the evolution of new physical traits – has long shaped evolutionary theory. However, detecting behavioral drive 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. To overcome these challenges, Luke Fannin and colleagues examined an unusual dietary shift in primate evolution – the repeated adoption of grass-based diets (graminivory) without the typical physical adaptations seen in other grazing mammals, such as high-crowned teeth or complex digestive systems. According to the authors, at least three Pliocene primate lineages, including early hominins, independently transitioned towards graminivorous diets, despite lacking these specialized traits. Fannin et al. analyzed carbon and oxygen stable isotope ratios from fossil primate specimens to determine behavioral shifts in diet towards C₄ plants, and compared the timing of these shifts to the development of dental traits linked to efficient grass eating. The authors found that in several species – including early hominins – changes to dental morphology lagged behavioral dietary shifts by roughly 700,000 years. What’s more, Fannin et al. show that the stem ancestors of Paranthropus and Homo had significantly greater dietary flexibility than their descendants, supporting the idea that behavioral variability precedes morphological specialization. According to the authors, the findings not only show that behavioral shifts in early primates predated morphological adaptation – supporting behavioral drive in early hominins – they also show that behavioral shifts may have initiated key evolutionary trajectories, particularly the dietary divergence of Homo around 2.3 million years ago.

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