DNA analysis provided insight into the intimate life of Neanderthals

Illustrative image created with artificial intelligence

Neanderthal DNA fragments were first extracted from fossils in the 1990s, and since then, genetics has helped to more accurately reconstruct the shared history of the two species.

Both Neanderthals and modern humans (Homo sapiens) originated from a population in Africa about a million years ago; their paths diverged approximately 600,000 years ago, and Neanderthals spread across Europe and Western Asia, where they lived until about 40,000 years ago. Around 250,000 years ago, one of the early groups of Homo sapiens left Africa and engaged in relationships with Neanderthals.

Descendants of this wave dispersed across the world, retaining up to several percent of Neanderthal heritage in their genome. Today, it is present in people of non-African descent. Over generations, these genes have fragmented into tiny pieces that vary among individuals. Some of these fragments disappeared over time, likely because they offered no advantages or were harmful and thus did not become established in the population.

Scientists paid particular attention to the X-chromosome — one of the two sex chromosomes: in modern humans, it contains significantly fewer Neanderthal fragments than other parts of the genome. This seemed unusual and required an explanation.

To understand the possible reason, researchers from the University of Pennsylvania (USA) turned to the Neanderthal genome itself. The X-chromosome is inherited in a specific way: a mother always passes an X to her child, while a father passes either an X or a Y.

Scientists hypothesized that if Homo sapiens genes on this chromosome were detrimental to hybrid children, they should have disappeared from the Neanderthal population. However, the opposite was true: the Neanderthal X-chromosome contained a disproportionately large share of Homo sapiens DNA fragments compared to other chromosomes.

Theory of sexual attraction

To find the reason for this genetic anomaly, the authors of the study, whose results are published in the journal Science, proposed a hypothesis of strong partner selection.

In their opinion, this is the most likely explanation: Neanderthal men might have preferred women of mixed ancestry — for example, daughters or granddaughters from earlier mixed unions. It was through such women that Homo sapiens X-chromosomes could have gradually become established in the Neanderthal population.

At the same time, in the communities of our ancestors who left Africa, the situation was similar, but with one important genetic nuance.

Homo sapiens women probably also found Neanderthals attractive and formed couples with them. However, here the biology of heredity intervened. The fact is that a father can only pass his X-chromosome to his daughter, but never to his son (sons receive the Y-chromosome from their father).

This means that if a boy was born in a "human woman + Neanderthal man" pair, the chain of transmission of the Neanderthal X-chromosome was immediately broken. Due to this peculiarity, even with mutual attraction, Neanderthal DNA in humans gradually "washed out" and disappeared from the population, while the proportion of Homo sapiens X-chromosomes in Neanderthals could gradually increase.

Researchers emphasize that for such a distinct genetic pattern to emerge, a stable behavioral model is needed, not random contacts.

However, some scientists call for caution: the observed effect might be partly explained by specific analysis methods and requires further verification.

Furthermore, material evidence from such a distant time is very scarce, and there is currently no clear "cultural trace" of mixed communities. New findings in the future — skeletons, tools, or ancient DNA samples — could significantly clarify or even change current interpretations.

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