Modern human evolution in East Africa
East Africa played a central role in all phases of human evolution, and current evidence suggests that it was critical in shaping the modern human lineage as well – both in terms of the environmental context of the first modern humans, and in terms of the direction and extent of subsequent dispersals.
The complex ecological and topographic structure of East Africa promotes niche subdivision and thus diversity, and rifting and volcanism expose and geologically fingerprint the sedimentary sequences in which fossils are found. The wealth of earlier hominin fossils and archaeological sites has meant that the later phases of human evolution in the area have received comparatively less attention. Very few sites and fossil remains that are known to date to the period of modern human origins (MIS7-MIS6, or 240,000 – 130,000 years ago) have been found in East Africa so far. The fossils of Ngaloba (LH18, Tanzania)1, Eliye Springs (KNM-ES11693, Turkana, Kenya)2 and Singa (Sudan)3 may date to this period, but lack secure context or chronometric dates, while the 270 Kyr ϒ-ray spectrometry date for the fossil of Guomde (KNM-ER3884, Turkana, Kenya)4 places it at the earliest margin. Very early East African Middle Stone Age (MSA) sites, such as Baringo5, record the transition from late Acheulean to MSA industries, but pre-date the first anatomically modern human fossils by nearly 100,000 years6.
The available dates from MSA sites suggest that humans were present in the Rift Valley, at least intermittently, throughout the period, with the fossils from Omo Kibish and Herto in Ethiopia7-12 representing the best evidence for early humans and their behaviour in the region. MSA sites that date to the MIS5-MIS4 period (130,000 – 60,000 years ago) in East Africa are also few13,14, limiting our understanding of this critical phase in human evolutionary history. Nevertheless, sites such as Kapedo/Silali place humans at the heart of the Rift Valley during the last interglacial15, while sites in Rusinga14 and possibly Kabua in West Turkana extend this presence to the first half of the Upper Pleistocene.
MSA sites in East Africa in the context of the southern and northern extensions of the ITCZ
The transition between the MSA and LSA in East Africa (and elsewhere) is still poorly defined, although new studies are offering fascinating glimpses of the potential asynchronous and localised nature of the shift from one industry to another. Dates of ca. 60,000 and 41,000 years have been obtained for the earliest LSA levels at the sites of Mumba in Tanzania and Enkapune Ya Muto in Kenya, respectively16,17, but the majority of LSA sites seem to be younger than this as it is also the case in the rest of the continent. In most of East Africa, the transition from hunting and gathering to pastoralism and farming took place in the last 5,000-4,000 years, and although well documented archaeologically, the extent to which this major socio-economic change was through cultural or demic diffusion remains unclear18, making it difficult to assess its biological consequences.
Very few groups remain who practice a foraging subsistence in East Africa today. Among these, the Hadza and Sandawe in Tanzania have been the focus of long-term ethnographic and ethnoarchaeological studies19-22, as well as of a series of recent genetic publications23-25. The results of these studies are fascinating, indicating that, as well as click languages, the Sandawe share a unique maternal ancestry with the Khoisan hunter-gatherers of southern Africa, while the Hadza contain genomic diversity that stands out at a global level. These results point to the great time-depth of genetic differentiation in these groups, supported by other studies26, and suggest the long-term survivorship of early human populations in eastern Africa until the recent expansion of food-producing societies assimilated or replaced them. These findings – archaeological and genetic – provide the framework underpinning the importance of the period targeted by the In Africa Project in reconstructing the processes and events that shaped human diversity in Africa between 200,000-5,000 years ago.