Inside-of-Africa: How landscape openness shaped Homo sapiens evolution by facilitating dispersal and gene-flow in Middle and Late Pleistocene Africa

Homo sapiens as a clade originated c. 500 thousand years before present (500 ka) as it diverged from Homo neanderthalensis. The topic of early H. sapiens evolution and dispersal since this cladogenesis has long been of interest in scientific literature and public debate. The development of this field has been significantly accelerated in recent years by the advances made in the scientific fields of archaeological, anthropological and genetic research; exemplified by the publication of the earliest observed fossil belonging to the archaicH. sapiens clade at Jebel Irhoud, Morocco, dated at c. 315 ka in 2017. Recent evidence from these fields opposes the long-held view that anatomically modern humans (AMH) evolved linearly from a single population. Instead, a pan-African model of evolution is proposed, whereby geographically isolated H. sapiens populations,
possibly shaped and maintained by ecological boundaries, evolved independently with fluctuating degrees of gene-flow over time. A thorough understanding of the ecological context these hominins experienced has long been hampered by spatial and temporal gaps in the African palaeovegetation record. Records of past vegetation that cover timescales relevant to the
emergence of AMHs now exist that are relevant to environmental change in northen, southern, eastern, western and central Africa. This means it is becoming possible to explore how hominin evolutionary development coincided with the changing vegetational (habitat) context.We present the idea of a three-stage ‘Inside-of-Africa’ environmental framework for hominin evolution: (i) a predominance of hospitable vegetation 500–400 ka facilitating initial dispersal of archaic H. sapiens, (ii) a predominance of ecological barriers (e.g. deserts and rainforests) 400–250 ka limiting dispersal and gene-flow, causing independent evolution, and (iii) a predominance of hospitable vegetation 250–100 ka (re-)connecting populations and resulting in the combination of the full suite of contemporary AMH characteristics. To test this framework we review and synthesize all available long (>c. 100 ka) lacustrine palynological records relevant to past vegetation change across Africa. We find the past vegetation data supports the waxing and waning of hospitable vegetation and ecological barriers during the last 500 ka in line with the timings proposed in our environmental framework.