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Fractured African roots

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 were scattered across the entire continent, according to a study led by a Maltese archaeologist.

Multiregional evolution: The patchwork of diverse fossils, artefacts and environments across Africa indicate that modern humans emerged from the interactions between a set of interlinked populations living across the continent and whose connectivity changed through time. Photo: Yasmine Gateau/Max Planck Institute for the Science of Human HistoryMultiregional evolution: The patchwork of diverse fossils, artefacts and environments across Africa indicate that modern humans emerged from the interactions between a set of interlinked populations living across the continent and whose connectivity changed through time. Photo: Yasmine Gateau/Max Planck Institute for the Science of Human History

A scientific consortium, led by Maltese archaeologist Eleanor Scerri of the University of Oxford and the Max Planck Institute for the Science of Human History, has found that human ancestors were scattered across Africa and largely kept apart by a combination of diverse habitats along with various shifting environmental boundaries.

Millennia of separation gave rise to a staggering diversity of human forms, whose mixing ultimately shaped our species.

While it is widely accepted that our species originated in Africa, less attention has been paid to how man evolved within the continent. Many had assumed that early human ancestors originated as a single, relatively large ancestral population, exchanged genes and technologies like stone tools in a more or less random fashion.

In a paper published in Trends in Ecology and Evolution this 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.

 “Stone tools and other artefacts – usually referred to as material culture – have remarkably clustered distributions in space and through time,” said Scerri, researcher at the University of Oxford and the Max Planck Institute for the Science of Human History, and lead author of the study.

“While there is a wide trend towards more sophisticated material culture, this ‘modernisation’ clearly doesn’t originate in one region or occur at any one period.”

Human fossils tell a similar story.

The researchers looked at past climates and environments of Africa

“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 Chris Stringer, researcher at the London Natural History Museum and co-author on the study.

“As with the material culture, we do see a continental-wide trend towards the modern human form, but different modern features appear in different places at different times and some archaic features are present until remarkably recently.”

The study of genes concurs.

“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 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.”

Evolutionary changes of braincase shape from an elongated to a globular shape. The latter evolves within the Homo sapiens lineage via an expansion of the cerebellum and bulging of the parietal. Left: micro-CT scan of Jebel Irhoud 1 (circa 315,000 years old, Africa); Right: Qafzeh 9 (circa 95,000 years old, the Levant). Photo: Philipp GunzEvolutionary changes of braincase shape from an elongated to a globular shape. The latter evolves within the Homo sapiens lineage via an expansion of the cerebellum and bulging of the parietal. Left: micro-CT scan of Jebel Irhoud 1 (circa 315,000 years old, Africa); Right: Qafzeh 9 (circa 95,000 years old, the Levant). Photo: Philipp Gunz

To understand the reasons 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.

Archaeologist Eleanor ScerriArchaeologist Eleanor Scerri

Many of the most inhospitable regions in Africa today, such as the Sahara, were once wet and green, with interwoven networks of lakes, 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 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 make-up – followed by genetic and cultural mixing.

 “Convergent evidence from these different fields stresses the importance of considering population structure in our models of human evolution,” said co-author Lounes Chikhi of the CNRS in Toulouse and Instituto Gulbenkian de Ciência in Lisbon. “This complex history of population sub-division should thus lead us to question current models of the ancient population size changes and perhaps reinterpret, as changes in connectivity,” he added.

Dr Scerri concluded: “The evolution of human populations in Africa was multiregional.

“Our ancestry was multi-ethnic and the evolution of our material culture was, well, multicultural. We need to look at all regions of Africa to understand human evolution.”

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