Sometimes, primate species not only diverge but also converge.
A recent genetic study published in Nature Genetics has identified traces of an ancient population in human DNA. This population diverged from humans’ main lineage around 1.5 million years ago but later converged with it again. The study estimates that the genetic legacy of this second population, referred to as Population B, accounts for about 20% of our DNA today.
The research team also suggests that this genetic legacy may have been beneficial for our species. They propose that these genes could have contributed to the enhancement of our cognitive abilities.
“The question of where we come from is one that has fascinated humans for centuries. For a long time, it’s been assumed that we evolved from a single continuous ancestral lineage, but the exact details of our origins are uncertain,” co-author Trevor Cousins said in a press release.
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The separation of these two evolutionary groups likely led to a parallel evolution of Populations A and B. The evolution of Population A is largely consistent with what experts already knew, but the new study provides a more detailed account of its evolutionary history.
According to the team, after the two populations separated, Population A experienced a “severe bottleneck.” While Population B thrived, Population A underwent a significant decline in numbers, resulting in a drastic reduction in genetic diversity.
From Population A emerged the primary human species of the past, including our own, as well as a different branch that resulted in the Neanderthals and Denisovans. This human species (or subspecies) inhabited Central Asia hundreds of thousands of years ago.
Cross-Species Reunion
The history of Population B remains unclear. The study doesn’t uncover any indications of events such as population bottlenecks. Experts believe that the core of this population eventually disappeared. However, at some point, its members crossed paths with Population A, leading to genetic exchange. Scientists estimate that this “reunion” between the populations occurred more than a million years after their separation, roughly 300,000 years ago. Following this reunion, Population B vanished without leaving a trace.
The genetic legacy each population left in modern humans is unequal. Around 80% of our genetic material can be traced back to Population A, while the remaining 20% comes from Population B. The study reveals that the genes from Population B are situated apart from those of Population A.
Researchers suggest that these two populations were likely poorly compatible. Over time, incompatibilities were eliminated through a process known as purifying selection. This is a natural selection mechanism that removes harmful mutations from a population.
Regarding this genetic legacy, the team emphasizes that the Population B genes are closely linked to brain function and neural processing. This could indicate that the small amount of genetic inheritance from Population B may have significantly influenced the evolution of human intelligence and the development of our species.
The analysis focused on DNA from contemporary humans rather than skeletal remains of prehistoric populations. The team used the 1000 Genomes Project initiative, which provided a vast genetic database containing information about the inhabitants of Africa, Asia, Europe, and the Americas.
The history of convergent evolution is extensive. One of the most well-known cases is that of the Neanderthals. This now-extinct species inhabited Europe and Asia tens of thousands of years before Homo sapiens left Africa. Once these two species encountered each other, they coexisted in several regions of Eurasia.
The Neanderthals’ genes vary among populations but typically represent around 1% or 2% in non-African descendants. Thanks to that genetic legacy, scientists discovered that Homo sapiens and Neanderthals produced hybrids whose lineages eventually merged with those that gave rise to contemporary humanity.
Image | Patrick Hendry
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