African Brain Size

Human evolution is characterised by speciation, extinction and dispersal events that cannot currently be explained by global or regional paleoclimate records [1]–[3]. Despite the widespread distribution of hominins across Africa and Eurasia after two million years ago, the majority of hominin species originated in East Africa and subsequently colonised the rest of Africa and Eurasia [4]–[5]. Arguably the most important episode in hominin evolution occurred in this region during the period bracketing 1.8–1.9 Ma when hominin diversity reached its highest level with the appearance of the Homo (sensu stricto) and new species of the Paranthropus genera. In addition to speciation, a second major process that begins during this period is the episodic migration of hominins out of the Rift Valley and into Eurasia. This period also witnessed the most dramatic increases in hominin brain size; early representatives of the Homo erectus sensu lato (Homo erectus and H. ergaster) in Africa had a brain that was >80% larger than the gracile australopithecine Australopithecus afarensis and ~40% larger than Homo (Australopithecus) habilis (Figure 1). However, there remains no consensus for what conditions or mechanisms drove these major changes.

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Figure 1. Top panel shows the East African Rift valley lake variability shown both as the number of Basin containing deep or shallow lakes and the calculated normalised lake index.

The putative hominin dispersals ‘D’ (red arrows out of Africa, dotted within Africa only) are shown above. Middle panel shows African hominin species diversity over time. Bottom panel shows hominin brain estimates for Africa and Eurasia. Hominin specimen dates and brain size estimates were taken from Shultz et al [3]. East African hominin diversity at each 100 kyrs interval were estimated using first (FAD) and last appearance dates (FAD) from the literature [2],...