It’s a historical knot that scientists have been trying to unravel for many years: that of the exact family tree of our species, Homo sapiens, and how it evolved.
To solve this puzzle, they examine the genome – i.e. the entire genetic information of an individual contained in each of its cells – of current populations. Using models, they can estimate at what point in time the common ancestors of populations that carried certain genetic variations still shared the same genes before they split. And thus get valuable information about our distant ancestors.
An international research team led by the Universities of McGill (Canada) and California at Davis (USA) presents a new theory of human origins in Africa: Different groups of people living in different regions migrated through Africa spanning hundreds of thousands of years mixed together for a long time. A hypothesis presented in the journal Nature on May 17, 2023, which contradicts some of the prevailing theories.
Traces of a “phantom species” in our genome?
The most classical model posits that about 150,000 years ago there existed a single ancestral population in eastern or southern Africa – the stem of our family tree – from which other populations split off – the branches.
Only among the genetic differences between the populations that the specialists identified did some appear to be particularly ancient. Much earlier than the assumed date, when our common ancestors would have split into several groups. It has subsequently been suggested that the central ancestral population may have been the result of an occasional admixture of modern humans with another hominid, as would have occurred in Eurasia with Neanderthals (Homo neanderthalensis) or Denisovans (Homo denisovensis). A “ghost species” as no trace of this “mysterious human” has been found: neither fossils nor DNA.
“[…] it was difficult to reconcile them [deux] “Thermal theories using the rare fossil and archaeological evidence of human habitation from places as far away as Morocco, Ethiopia and South Africa show that Homo sapiens lived across the continent at least 300,000 years ago,” said Brenna Henn, co-lead author of the study New study and population geneticists at the University of California’s Department of Anthropology and Center for Genome, in a statement.
Branches that separate and cross
Recent research therefore offers an entirely new explanation. For this purpose, the genome material of 290 individuals from four geographically and genetically different African groups was analyzed: the Nama of Namibia and South Africa; the Mende of Sierra Leone; the Gumuz of Ethiopia and Sudan; the Amhara and Oromo of Ethiopia. The similarities and differences between these populations have been tracked over the past million years to better understand genetic relationships across the continent. Eurasian genetic material was also included to identify genetic traces resulting from colonial incursions.
By computer modeling other scenarios based on a new algorithm, the researchers discovered that the individuals of the famous ‘common core’ may not be as homogeneous as previously thought. Branches of some Homo sapiens groups seemed to occasionally separate, only to later reunite. Distances and approaches that the authors explain in terms of probable climate changes between about 1 million years and 100 years. They would have “resulted in population expansion or divergence into new regions of the world, but also in decline or mixing with other populations,” explains Brenna Henn, this time interviewed by National Geographic.
As an example, she cites an event that happened about 120,000 years ago: the end of an ice age would have marked the beginning of a transition from cold and dry conditions to hot and humid conditions in certain regions of Africa. Rising sea levels may also have pushed the individuals inland. “It is at this point that we find that two branches of the human family tree merged to become the ancestors of today’s Khoisan, related but culturally distinct groups now confined to southern Africa and boasting the greatest genetic diversity on the planet,” develops the Experts.
“We find that weakly structured strain models provide more likely explanations for polymorphism previously attributed to contributions from archaic hominins in Africa.”
A weakly structured tribe of human origin in Africa https://t.co/5gGLw0CYbU pic.twitter.com/6Kavd92KKo
— Odyssey to the Far East (@motherlander) March 24, 2022
⋙ According to a Harvard study, the Vikings left Greenland due to sea level rise
An “incredible variety” and old
If these numerous genetic exchanges did indeed take place between the ancestors of Homo sapiens, the gray areas that previous models have struggled to clarify can finally be explained without invoking the famous “ghost species”. Some fossils discovered with surprising properties could also be interpreted differently: their mix of ancient and modern traits, previously considered evidence of interbreeding with a “mysterious hominid,” could be explained by the complex gene flow that has occurred between humans of different parts of the continent. And that was long before human expansion outside of Africa.
“This is consistent with recent notions in paleoanthropology that the ancestry of the Homo sapiens group that left Africa would consist of multiple African populations. It also shows that we should be more specific because talking about African descent is not enough. The variety is huge. Incredible,” concludes Brenna Henn.
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