The human genome is hiding secrets that point to a mystery ancestor alongside our hominid cousins the Neanderthals and Denisovans, according to AI software.
Homo sapiens, the only surviving species in the homo genus, once bred with its extinct relatives Homo neanderthalensis and Homo denisova hundreds of thousands of years ago.
The evidence is in our genes today, and from fossil records unearthed in caves across the world. It is estimated that people of European and Asian descent have about two per cent Neanderthal DNA, while people from the Pacific Islanders are more likely to have inherited some Denisovan DNA. But there are gaps in our knowledge, for example, some fragments of our genetic blueprint don’t match up to any known hominid species, and these have left scientists scratching their heads. Where did this DNA come from?
The idea of a third ancestor has been floating around for a while. Now, a team of researchers from the University of Tartu in Estonia, and Universitat Pompeu Fabra and the Barcelona Institute of Science and Technology in Spain, believe they have more evidence to support that hypothesis of a third source. Their results were published in Nature Communications this week.
They built a machine-learning model, specifically a “deep learning in an Approximate Bayesian Computation framework,” to find a third source unaccounted for in their dataset taken from the 1000 Genomes Project that maps out human genes. It all sounds very fancy and complicated – and it is, the paper is filled with all sorts of statistical algorithms – but essentially it all boils down to some clever pattern matching to compare genetic models from different demographics.
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“About 80,000 years ago, the so-called Out of Africa occurred, when part of the human population, which already consisted of modern humans, abandoned the African continent and migrated to other continents, giving rise to all the current populations”, said Jaume Bertranpetit, coauthor of the study and a researcher at the Institute of Evolutionary Biology at Universitat Pompeu Fabra.
“We know that from that time onwards, modern humans cross bred with Neanderthals in all the continents, except Africa, and with the Denisovans in Oceania and probably in South-East Asia, although the evidence of cross-breeding with a third extinct species had not been confirmed with any certainty.”
The team reckons that if a third ancestor existed, it's either a descendant of Neanderthals and Denisovans mating, or diverged early from the Denisova lineage. Although no conclusive evidence has been found, another team of scientists did find evidence of an early hominid with a hybrid of half Neanderthal and half Denisovan DNA last year. ®