An international team led by scientists from the Harvard School of Medicine has produced the first sequences of ancient human DNA from the entire genome of West and Central Africa and confirms that the origin of the modern human being was produced from four different African populations that lived within different time frames, between 300,000 and 200,000 years ago.
This finding reinforces the new argument made by archaeologists and geneticists that human origins in Africa could have involved very divergent and geographically separate populations.
The remote origins of humanity
DNA analysis of the children of Central-West Africa, whose remains come from a rocky site called Shum Laka, in a grassland area of Cameroon, indicates that at least three main human lineages, ancestors of the hunters-Collectors from Central Africa today, hunter-gatherers from Southern Africa or present-day humans, diverged genetically from each other some 200,000 years ago.
“Our analysis shows the existence of at least four major deep human lineages that contributed to the current populations, and that diverged from each other 250,000 to 200,000 years ago,” stated David Reich of Harvard Medical School.
A fourth human population, previously unknown, also emerged in that time span and left a small genetic mark on modern West and Eastern Africans, according to the study in the magazine Nature. Said ancestral lineage possessed a small amount of DNA from hominid populations that had originated before the emergence of the human species, indicating that they were possibly Neanderthals. It indicates the existence of a lineage of archaic humans that have not yet been identified.
It is a "ghost lineage," - a group for which we have no physical evidence - that does not seem to have survived to the present-day as a distinct population.
The researchers attempted to obtain DNA from 18 different skeletons but were only successful with four: a small child and a teenager from the same tomb, which are 8,000 years old, and the neighboring tombs of two children which are approximately 3,000 years old. The last date is more or less similar to the beginning of the Bantu expansion, suggesting that these skeletons could reveal many details about their origin.
The mitochondrial genome, inherited from an individual’s mother, was not particularly enlightening, as it coincides with the variants found widely in Africa. It was for one of the Y chromosomes, a rare version that is found only in a few modern populations in Africa and seems to have been introduced into modern humans by crossing with an archaic human.
This rare haplogroup (A00) was discovered seven years ago when an African-American descending from slavery went to a genetic analysis company and obtained results that surprised everyone.
A species on the move
A previous genetic study, led by evolutionary geneticist Pontus Skoglund of the Francis Crick Institute in London, identified a human population that had its origin more than 200,000 years ago and that was ancestral to hunter-gatherers from the tropical rainforest of West and Central Sub-Saharan Africa. The new study provides further evidence of that ancestral line, demonstrating that the history of humanity would not be marked by inaction and isolation, but by movement and mixture.
While this represents an interesting advance in our understanding of human history, we still have a long way to go before a diverse sample of genomes of African populations is obtained. Will there be any more surprises? Possibly.
Reference: Ancient West African foragers in the context of African population history, Nature (2020). DOI: 10.1038/s41586-020-1929-1 , https://www.nature.com/articles/s41586-020-1929-1