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Revisting the Savanna Hypothesis

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Understanding the environmental context in which early (and later) hominins lived remains a crucial issue in human origins research, as not only does it provide the wherewithal and limits for life’s necessities, but more broadly it helps to shape evolutionary pathways. The “Savanna Hypothesis” grew out of observations that early hominins typically dwelt in open and arid landscapes unlike those of their closest great ape relatives who preferred moist forests. It emerged before we had developed a fuller comprehension of the hominin phylogenetic record, or its age, and lacking in fuller records for associated faunal, climate and environmental shifts. It has thus been repeatedly challenged, to be followed by an alternative “Forest Origins Model”, which now holds sway. The debate concerns both the nature and timing of the emergence of open environments in Africa as a broad influence, and also early hominins’ habitat and foraging preferences. New carbon isotope evidence for Miocene and Pliocene fauna contributes to resolving the first issue. A more secure (although still patchy) understanding of vegetation cover has emerged showing that in East Africa C4 grasses, as typical, essential components of savannas, first appeared as significant elements of vegetation structure just before 9Ma. First equids, and then elephantids and Gomphotheres, quickly adapted to the new resources, while bovids, suids, and hippopotamids showed more gradual, mixed adaptations. There are some hints, from faunal and possibly from other isotopic sources, for low frequency fluctuations in the proportions C4 vegetation from the Late Miocene onwards, suggesting perhaps some expansion of C3-dominated vegetation in the earlier Pliocene, and a later expansion of C4 again by ca. 2 Ma. If that is the case it may help to explain the evidence for forest habitat associations in early Pliocene sites and isotope data for the associated hominins. Published carbon isotope evidence for hominin diets suggests that hominins began to exploit C4 resources only after ca. 4Ma, more than 5Ma after the emergence of C4 grasses. These results are consistent with the woodland/forest model as they suggest a preference for C3 resources in spite of the presence of open vegetation elements elsewhere on the landscape. However, new carbon isotope data for Sahalenthropus individuals from Chad, Central Africa, indicate mixed diets that include highly variable C4 resources, not dissimilar to co-existing suids and bovids. Viewed from the longer perspective it would seem that neither model is a good fit; unsurprisingly, evolutionary trends are extremely complex and highly variable.

This talk is part of the Biological Anthropology Seminar Series series.

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