Cambridge University researchers have made a groundbreaking discovery that reshapes our understanding of human evolution. This research reveals a significant event in our ancestry—a genetic "divorce" and "remarriage" of two ancient hominin populations that occurred over a million years ago. These populations were separated for an extensive period before reuniting approximately 300,000 years ago, leaving a lasting imprint on the DNA of every human alive today.
This pioneering study, published in Nature Genetics, utilizes innovative computational techniques to uncover genetic patterns indicative of an ancient mixing event. The timing of this event coincides with the emergence of the earliest anatomically modern humans on Earth, presenting a missing chapter in our evolutionary history.
The research team, consisting of Trevor Cousins, Aylwyn Scally, and Richard Durbin from the Department of Genetics at Cambridge, developed a new computational tool known as “cobraa” (coalescence-based reconstruction of ancestral admixture). This tool enables the detection of population separations and mixing events through analysis of our DNA patterns. Their findings indicate that modern humans are a genetic amalgamation, with approximately 80% of our ancestry traced back to one ancient population and 20% to another.
Durbin, a computational biologist and professor at Cambridge, emphasizes that this research unveils the complexity of our evolutionary origins. Unlike other known mixing events, such as the interbreeding between Neanderthals and non-African humans around 50,000 years ago, this ancient admixture is part of the genetic heritage shared by every person on Earth today.
An intriguing aspect of the study is the evidence of a severe genetic bottleneck following the split of the two populations around 1.5 million years ago. This bottleneck resulted in a drastic reduction in genetic diversity for the lineage that contributed the majority of modern human ancestry. Furthermore, the genetic material from the minority population is not evenly distributed throughout the genome, hinting that natural selection favored the genetic traits of the majority population.
“We inferred regions of the present-day genome derived from each ancestral population, finding that material from the minority correlates strongly with distance to coding sequences,” the authors explain. This suggests that the genetic contributions from the minority population were less advantageous in the context of the majority's genetic background.
The researchers also established a significant connection between regions derived from the majority ancestral population and patterns of genetic divergence seen between modern humans and our extinct relatives, the Neanderthals and Denisovans. This indicates that the majority population likely served as the primary ancestral source for these archaic humans as well.
So, who were these ancient populations? The fossil record points to various human-like species that existed approximately 1.5 million years ago, including different populations of Homo erectus and later Homo heidelbergensis. The researchers suggest that the bottleneck observed in the majority lineage may represent a founder event linked to migrations and physical separation.
“What’s becoming clear is that the idea of species evolving in clean, distinct lineages is too simplistic,” notes Dr. Cousins. “Interbreeding and genetic exchange have likely played a significant role in the emergence of new species repeatedly across the animal kingdom.”
To validate their findings, the researchers applied their model to genetic data from other species, including bats, dolphins, gorillas, and chimpanzees. The results varied, with some species showing little evidence of ancient genetic structures, while others displayed entirely different patterns. This variability across species enhances the credibility of the distinct patterns discovered in humans.
Moreover, the analysis identified genes with unusually high or low contributions from the minority population. Genes with rich minority ancestry were often linked to functions related to neural development, while those with minimal minority ancestry were typically involved in RNA processing, cell structure, and immune functions. These patterns suggest that the two ancestral populations may have adapted to different environments prior to their reunion.
This discovery contributes to a growing body of evidence indicating that human evolution is far more intricate than previously understood. Instead of a straightforward tree-like branching of populations, it appears that our evolution involved repeated separations and reintroductions of genetic material. The ancient admixture event identified in this study predates more recent interbreeding occurrences, such as those involving Neanderthals.
“The fact that we can reconstruct events from hundreds of thousands or millions of years ago just by analyzing DNA today is astonishing,” says Scally. “And it reveals that our history is richer and more complex than we ever imagined.”
