Why East Africa could be driving modern human evolution?
Since the work of Emiliani and Shackleton, the paradigm in palaeoclimate research has been that the northern hemisphere response to orbital forcing mechanisms had a global effect on temperature, precipitation and sea-level. Recent work on tropical palaeoenvironmental records has challenged these views, changing our interpretations of the environmental context for the evolution of modern man.
The general principles that explain the world’s Quaternary palaeoclimate1-4 (originally suggested by Milankovic) predict that during glacial phases and growth of continental ice the tropics would be dry and lake-levels low, while during glacial terminations and early interglacials the opposite would be true. However, recent research, especially studies of deep lake cores in Africa, has revealed a range of local conditions in the tropics that make certain areas not follow these predictions, or at least, to do so with a greater level of variance than expected. Tropical patterns of climate change appear to respond variously to orbital-scale precessional insolation changes, or sea-surface temperature (SST) variation, or shifts in the position of the Intertropical Convergence Zone (ITCZ) and monsoon strength5-7. In particular, monsoon intensity, itself shaped by precession-modulated insolation patterns, seems to affect African tropical climate8. In other words, different micro-regions in the tropics respond to different climatic processes to a different extent at any one time, resulting in localised patterns and directionality of change. For example, we now know that during most of the last interglacial period (between 135,000 -127,000 years ago, 110,000-85,000 years ago, and 78,000-74,000 years ago), the area around Lake Malawi and Lake Tanganyika experienced extreme aridity, to the point that Lake Malawi became a shallow saline lake surrounded by semi-deserts9,10. However, at a similar time, Lake Naivasha had periods of extremely high water levels (during 146,000-120,000 years ago, with peak high stands between 139,000-133,000 years ago, and again at 113,000-108,000 years ago and around 91,000 years ago), interspersed by phases in which the lake even dried completely11,12. High water levels at around 135,000-130,000 years ago have also been suggested for the Turkana-Suguta and Magadi-Natron systems13,14. Recent studies further suggest that tropical responses to climate change in the last 70,000 years tracked northern hemisphere changes closely, and that some of the intense temperature fluctuations on millennial and even centennial scales observed in the Greenland records (Heinrich Events 1 and 4-6, and some Dansgaard-Oeschger events) may be expressed as sharp peaks of aridity and moisture in the tropics15,16. This suggests that the tropics as a whole were likely to be neither in or out-of-phase with high latitude climate change, and that latitudinal variation in response to both eccentricity-enhanced precessional forcing mechanisms and variation in SSTs/ITCZ position in the tropics might be on a scale of degrees or shaped by a marked boundary at the Equator17.
This regional palaeoenvironmental complexity is critical for understanding the ecology of early human populations in East Africa. Current anthropological and archaeological evidence does not allow us to establish with certainty where in Africa the ancestral human population lived – the earliest known modern human fossils are from Ethiopia, but as these are the only hominin fossil from that time (c. 200,000 years ago) in sub-Saharan Africa, neither its local ancestry, or the presence/absence of similarly modern populations elsewhere can be confirmed. Unfortunately, the number of hominin fossil specimens and chronologically dated archaeological sites for Middle Pleistocene Africa does not allow establishing whether any one region of the continent supported human populations continuously for the last 250,000 years.
Both East Africa and the southern Cape have been proposed as the ecological refugium for early humans18,19. In a detailed analysis of the palaeoecology of the Cape and its relationships with climate and sea-level changes suggests that this area may well have been a refugium for late Pleistocene humans19. However, the area’s potential for also promoting demographic growth and expansive strategies beyond southern Africa is less clear. It is likely that the southern Cape, and indeed other small ecological enclaves of Africa acted as refugia, with pulses of growth and local expansion20 with fascinating cultural and behavioural consequences21. However, the long-term destiny of the people who lived in these refugia remains to be established, one of the exciting challenges ahead. In a comprehensive study of the geography and palaeoanthropology of East Africa, Basell suggested the area would have acted as a core refugium for hominins through time18. A fascinating new study has simulated the different dynamic response to climate change across Africa using a wide platform of datasets – both a range of palaeoclimatic data and archaeological distributions, and assessed the bearing of such regional climatic effects on late Quaternary human populations22. This study highlights the importance of the tropics as hotspots of population endemism, consistent with Basell’s analysis of eastern Africa as the geographical core for later human evolution.
At a continental level, East Africa seems to have acted as the region of Africa where hominin populations survived. However, in terms of human population foraging parameters the area is too large to be treated at that scale. The complex environmental and topographic conditions in East Africa suggest that the region as a whole may not have been one refugium, but several, each shaped by very localised responses to changes in climate. This heterogeneous ecological outcome at a regional level would have created what is named here a refugium network - in other words, a set of independent, potentially asynchronous refugia linked by semi-contiguous geographic corridors. This refugium network would have favoured an adaptation based on a rapid demographic response to times of plenty or famine, while allowing mobility between lake basins and river valleys and plains at a regional level to offset localised environmental change. The concept of a refugium network lies at the heart of the In Africa Project and the parallel study of lake basins in the Kenyan Rift Valley in the lake Quaternary.
The importance of understanding where the refugia were, and who lived in them, lies in the difficulty of interpreting human diversity in Africa through time. Although humans descend genetically from a small, likely localised population, many others lived at any one time, whose size, behaviour and adaptations shaped the competitive environment of the one(s) who survived. Also, many of these other groups probably survived differentially over prehistoric time, and are part of the human archaeological and fossil records of late Pleistocene Africa. The recent re-assessment of the remains from Iwo Ileru in Nigeria23 highlights the presence of comparatively archaic features in this late Pleistocene/early Holocene fossil. Equally, preliminary assessments of the newly discovered remains from West Turkana (Late Quaternary Human Evolution in West Turkana Project) suggest greater morphological variation in African human populations at the Pleistocene/Holocene boundary. Together, these findings reinforce the view that when Africa was a continent of hunter-gatherers it supported significantly greater levels of human diversity than it does today.