Behavior drives morphological change during human evolution.

Dietary shifts and corresponding morphological changes can sometimes evolve in succession, not concurrently—an evolutionary process called behavioral drive. Detecting behavioral drive in the fossil record is challenging because it is difficult to measure behaviors independently from corresponding morphologies. To solve this problem, we focused on a puzzling behavior in the fossil record of some primates: eating graminoid plants. We report carbon and oxygen isotope ratios from fossil cercopithecid monkeys and integrate the data into a view of hominin dietary evolution, finding that changes in graminivorous behavior preceded corresponding changes in dental morphology by ~700,000 years. Decoupling diets and morphologies in time was conducive to determining when and to exploring why dietary changes helped to propel human evolution. Editor's summary: The wide array of diverse life-forms on this planet are a result of millions of years of selection and adaptation driving morphological change. We can look back at fossils and identify changes over time, but it is difficult to identify another key component of adaptation in the fossil record: that of behavior. The theory of behavioral drive posits that behavioral change can expose animals to new selective regimes, leading to adaptive morphological change over time. Fannin et al. looked at isotopes in hominin fossils to characterize a dietary shift to graminoid plants, finding that this shift predated adaptive tooth changes by 700,000 years and confirming the presence of behavioral drive. —Sacha Vignieri [ABSTRACT FROM AUTHOR]