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Humans did not stem from a single ancestral population in one region of Africa, as is often claimed. Instead, our African ancestors were diverse in form and culture, and scattered across the entire continent.
The Museum’s Prof Chris Stringer, and collaborators led by Dr Eleanor Scerri of the University of Oxford and the Max Planck Institute for the Science of Human History, have found that human ancestors were scattered across Africa, and largely kept apart by a combination of diverse habitats and shifting environmental boundaries, such as forests and deserts. Millennia of separation gave rise to a great diversity of human forms, whose mixing ultimately shaped our species.
While it is widely accepted that our species originated in Africa, less attention has focused on how we evolved within the continent. Many had assumed that early human ancestors originated as a single, relatively large ancestral population, and exchanged genes and technologies, such as stone tools, in a more or less random fashion.
In a paper published in Trends in Ecology and Evolution next week, this view is challenged, not only by the usual study of bones (anthropology), stones (archaeology) and genes (population genomics), but also by new and more detailed reconstructions of Africa’s climates and habitats over the last 300,000 years.
‘When we look at the morphology of human bones over the last 300,000 years, we see a complex mix of archaic and modern features in different places and at different times,’ said Prof. Chris Stringer, researcher at the Natural History Museum and co-author on the study. ‘We do see a continental-wide trend towards the modern human form, but some archaic features are present until remarkably recently.’
The material culture tells a similar story. ‘Stone tools and other artefacts have remarkably clustered distributions in space and through time,’ said Dr. Eleanor Scerri, researcher at the University of Oxford, the Max Planck Institute for the Science of Human History, and lead author of the study. ‘While there is a continental-wide trend towards more sophisticated material culture, this ‘modernization’ clearly doesn’t originate in one region or occur at one time period.’
The genes concur. ‘It is difficult to reconcile the genetic patterns we see in living Africans, and in the DNA extracted from the bones of Africans who lived over the last 10,000 years, with there being one ancestral human population,’ said Prof. Mark Thomas, geneticist at University College London and co-author on the study. ‘We see indications of reduced connectivity very deep in the past, some very old genetic lineages, and levels of overall diversity that a single population would struggle to maintain.’
To understand why human populations were so subdivided, and how these divisions changed through time, the researchers looked at the past climates and environments of Africa, which give a picture of shifting and often isolated habitable zones. Many of the most inhospitable regions in Africa today, such as the Sahara, were once wet and green, with interwoven networks of lakes and rivers, and abundant wildlife. Similarly, some tropical regions that are humid and green today were once arid. These shifting environments drove subdivisions within animal communities and numerous sub-Saharan species exhibit similar phylogenetic patterns in their distribution.
The shifting nature of these habitable zones means that human populations would have gone through many cycles of isolation – leading to local adaptation and the development of unique material culture and biological makeup – followed by genetic and cultural mixing.
‘The evolution of human populations in Africa was multi-regional. Our ancestry was multi-ethnic. And the evolution of our material culture was, well, multi-cultural,’ said Dr Scerri. ‘We need to look at all regions of Africa to understand human evolution.’
Prof Chris Stringer comments on the complexity of the subject, and the research that has led to this point saying, ‘Although I am one of the researchers who originally helped to develop the view that our species, Homo sapiens, had originated in Africa, I have increasingly come to the realisation that our African origin was a complex process. The great diversity of African fossils between 200,000 and 400,000 years ago suggests that multiple lineages existed on the African continent at that time. In southern Africa there was a very primitive human species called H. naledi. In Central Africa, it looks like the larger-brained species H. heidelbergensis, exemplified by the famous Broken Hill cranium from Zambia, still survived. And our own lineage H.sapiens was represented by fossils scattered all the way from South Africa (Florisbad) to Ethiopia (Omo Kibish) to Morocco (Jebel Irhoud). These Homo sapiens specimens display great variation in shape, and in 2002 this led me to argue that our African origins might be 'multiregional', with populations in different parts of Africa evolving independently during times of climatically-driven isolation, and at other times mixing with each other.’
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