The oldest cross between hominid species occurred 700,000 years ago

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In 1856, a group of miners who worked in a cave in the Neander Valley in western Germany discovered strange human-looking bones. They belonged to a specimen that would be labeled Neanderthal-1 , the first recognized as belonging to a species close to ours. Since then, science tries to recompose the path of Homo neanderthalensis until its disappearance, about 40,000 years ago.

To do this, researchers have relied on the bones and stone tools they left behind, from the Iberian Peninsula to the Altai massif, but also on the paleogenetics - the analysis of ancient DNA - which in recent years is providing a New look at the origins of the different human lineages.

Sometimes, genomic models and fossil evidence fall into contradictions that are difficult to explain. In 2016, an analysis prepared by researchers from the Max Planck Institute from DNA, concluded that Neanderthals and Denisovans separated as a species, after evolving from a common ancestor, some 400,000 years ago . The problem is that this figure did not coincide with fossils such as those found in the Sima de los Huesos , in Atapuerca, much earlier than that period and that already point to the existence of a Neanderthal lineage. The chronology did not fit.

This Thursday, the journal Science Advances publishes the details of a new model that solves the problem, since its results push back the division between species until 600,000 years ago . That earlier separation is important because the interpretation of many fossils, such as those of Homo heidelbergensis and Homo antecessor, may vary.

"That divergence between Neanderthals and Denisovans around 600,000 years is consistent with the findings of the Sima of Bones (dated around 430,000 years ) belong to the Neanderthal lineage, as the morphology of fossils and DNA suggested ", comments the paleanthropologist María Martinón-Torres, director of the National Center for Research on Human Evolution (CENIEH).

Common ancestor

But the new study also reveals that common ancestors of Neanderthals and Denisovans shared offspring with members of another supercaic hominid population , which had separated from the rest of hominids two million years ago. This hybridization constitutes the oldest documented episode of genetic exchange between different human groups.

The analyzes allow us to predict that this remote population was large, based on its genetic diversity, with an effective size of between 20,000 and 50,000 individuals, but there is very little more data available on them. "It is true that it could be Homo erectus , but the truth is that it could also be Homo antecessor , or some taxon that has not yet been named," explains Alan Rogers, an anthropologist at the University of Utah and responsible for research.

"It is interesting that, in this study, Rogers and his team delay the divergence between the lineage of the sapiens and the one that gave rise to Neanderthals and Denisovans up to 700,000 years," says Martinón-Torres. That date would support the idea that Homo antecessor could be a common ancestor of both lineages, an advanced hypothesis in 1997 and that it had been rejected by some specialists, claiming that its antiquity - around 860,000 years - seemed to contradict genetic estimates.

"Now we cannot rule out that this archaic population that hybridized with the Neandersovans is our beloved Homo antecessor or is very closely related to him."

Sporadic hybridization

Previous work had already established that modern Europeans and Asians have in their DNA inheritance of Neanderthals and Denisovans. Between 2% and 4% of the genes of the current non-African populations are the product of sporadic crossings between sapiens and Neanderthal; in the case of the Denisovans - of which there are hardly any fossils - it is known that they bequeathed between 4% and 6% of the genetic material to current native populations of Papua New Guinea, Australia and Melanesia.

In this new model, the authors have analyzed the frequency with which the mutations are shared, both in the genome of Africans and modern Europeans, as in that of the ancient Neanderthals and Denisovans. "To do this, we have used modern human genome sequences, obtained from the Simons Genome Diversity Project and sequences from the old genome of the Max Planck Institute for Evolutionary Anthropology," explains Rogers. One of its main novelties is that it can be applied independently of population fluctuations , "which often interfere with efforts to study the most remote past."

From the results obtained, the North American researchers also propose that there were three great waves of migration to Europe and Asia: the first two million years ago , at which time that first supercatic population arrived and prospered to form a mass of population; the second, 700,000 years ago , with Neanderthal / Denisovan ancestors; Finally, modern humans, who left Africa about 50,000 years ago . These three large population movements in turn drove five major episodes of hybridization. The last one, coinciding with the arrival of modern sapiens .

Doubts about the origin

This reconstruction of the complex history of Neanderthals repeats a pattern similar to what some recent discoveries are revealing about the first anatomically modern populations of sapiens , and how they spread throughout Europe and Asia. About 50,000 years ago, populations of sapiens departed from Africa, experienced a demographic bottleneck period, in which their population declined markedly, and then split into regional populations across the two continents.

These fluctuations leave a mark on genetic diversity. "And it seems that the same thing happened 600,000 or 700,000 years ago " with Neanderthals and Denisovans, Rogers explained. "There was another diaspora from outside Africa that nobody had imagined before."

However, this last point raises doubts in other specialists . "I do not agree that the origin has to be African, it is not a hypothesis derived from genetic analysis," says Martinón-Torres. The director of CENIEH points to an Eurasian origin for the Neanderthal lineage.

"Once that first expansion occurred, documented at least 1.8 million years ago in Dmanisi (Georgia), hominids can now move east and west without invoking a new African dispersion. In fact, our analysis of the 1.2 million year old jaw of Sima del Elefante (Atapuerca) and the fossils of Homo antecessor of the Gran Dolina (860,000 years) point to that possibility. "

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