East African grassy woodlands appeared 10 million years earlier than thought, influencing hominoid evolution

Challenging a longstanding paleoecological theory, two new related studies reveal that savannah grasses existed in East Africa over 10 million years earlier than previously thought – and describe how those grasses led to open landscapes that drove skeletal adaptations in hominoids during the early Miocene. Savannahs and lowlands are usually home to grasses that photosynthesize via the C₄ carbon fixation pathway. As well as being able to withstand drier conditions and high temperatures, C₄ grasses survive better in direct sun than their shade-loving predecessors, C₃ grasses. Scientists have long sought to pin down when C₄ grasses first emerged in eastern Africa, because the spread of such plants would have reflected a transformation of the once-forested region and likely contributed to evolution of early hominoid features. Past research has indicated that C₄ grasses appeared less than 10 million years ago. Yet now, the study by Daniel Peppe and colleagues pushes this date back to 21 million years ago. While analyzing ancient soil samples from 9 sites in present-day Kenya and Uganda where early Miocene mammalian fossils have been found, the team identified abundant biomass signatures of C₄ grasses dating from 21 million years ago to 17 million years ago. Their findings challenge the canonical theory that East Africa was mostly forest at this time. Instead, the land likely consisted of mixed, open habitats like grassy woodlands.

Building upon Peppe et al.’s work, Laura MacLatchy and colleagues assert that the need to forage in grassy woodlands drove hominoids to develop versatile musculoskeletal systems. In the past, it was thought that fruit foraging in canopied, dense forests caused these locomotor changes. By examining dental and skeletal fossils, including a Morotopithecuis femur, from the Moroto site in Uganda, the group evaluated what hominoids ate and how their skeletons supported foraging in eastern Africa 21 million years ago. Dental patterns showed that hominids consumed leaves with lower water content, like drier stalks of C₄ grasses, rather than eating juicier forest fruits. Skeletal records revealed femurs, vertebrae, and torso characteristics akin to those of living non-human primates that inhabit savannahs and other open ecosystems today. These results reinforce that the rise of C₄ grasses and development of grassy woodlands adaptively propelled hominoid evolution towards a new form of locomotion. “Documenting the early stages of hominoid evolution as the lineage diverged from other [primates] is critical for interpreting the adaptive significance of traits that ultimately define modern hominoids, including humans,” MacLatchy et al. write.