An article published in the journal “Nature” reports a study that relaunches the idea that the Homo sapiens species emerged not from a single population that had e linear evolution but from different groups of genetically similar hominins that diversified and then interbred again. A team of researchers led by Brenna Henn of the University of California-Davis, USA, and Simon Gravel of McGill University in Montreal, Canada, conducted a genetic analysis and a comparison with early Homo sapiens fossils to try and locate the origin of modern humans. The researchers tested different evolutionary and migratory models and the one that best matches the data points to ramifications of African populations that subsequently interbred again until they merged.
The idea that modern humans evolved from genetic reshuffles of populations that migrated to different regions of Africa and then came together again is not new. The problem is that the amount of early Homo sapiens fossils is limited and often only partial skeletons and the temperature and humidity conditions in Africa tend to destroy DNA relatively quickly. This makes it difficult to conduct comparative anatomical studies and impossible to hope to obtain pieces of DNA. An article published in the journal “Trends in Ecology & Evolution” in July 2018 supports the multiregional thesis of the different populations based on anthropological, archaeological, genetic, and climatic elements.
Advances in genetic analysis techniques are giving a big hand in the reconstruction of human history. Paradoxically, many studies so far mainly offer information on interbreedings with Neanderthals and Denisovans, which are considered separate species. The issue is complex for various reasons.
Some studies of modern African humans suggest that there was an interbreeding in the distant past with a hominin species of which there are no separate genetic traces that can be linked to available fossils. Now this new genetic study offers an alternative explanation in which those traces of interbreeding involve different populations of early Homo sapiens that had already diversified and then reunited.
For this new study, the researchers inferred detailed demographic models for various African populations and used existing genetic data to which they added DNA from 44 Mana, a group that belongs to the Khoisan, or Khoe-San, population of South Africa who has a genetic diversity higher than that of other modern ethnic groups. With this data, they tested different evolutionary models that go beyond those typically used that indicate that Homo sapiens evolved from a single African population considering only an interbreeding with another hominin species.
The image (Courtesy Ragsdale, A.P., Weaver, T.D., Atkinson, E.G. et al. All rights reserved) shows a schematic of the various evolutionary patterns of Homo sapiens including also an interbreeding with Neanderthals: (a) illustrates the classic model of evolution from a single African population; (b) illustrates a variant with regional branches that interbreed again; (c) illustrates the classical model with the interbreeding of another African archaic hominin species and (d) illustrates a multiregional model in which various branches diversify and then interbreed again.
The multiregional model is the one that best explains all the genetic data used in this study. The analysis suggests that around 120,000 years ago, during a period of climate change, two diversified populations came together to become the ancestors of today’s Khoisan. About 100,000 years ago a new fusion of diverse pro populations gave rise to the Eastern and Westerns African populations.
This sort of multiregional mosaic of African populations that may have helped form Homo sapiens could explain both the fossils of early modern humans and the genetic anomalies found in some populations. It would confirm that, from the very beginning, the history of humanity was marked by migrations and crossbreedings between different populations. Certainly, the discussions will continue and the continuous advances in genetic techniques will offer new information to shed light on the complex history of humanity.
