When early Homo sapiens began migrating out of Africa, they entered regions already inhabited by Neanderthals. These encounters did not remain distant or hostile alone — genetic traces show that the two groups mixed, leaving a legacy still visible in human DNA today.
A new genetic study now sheds light on how that interbreeding likely happened. By analysing patterns found on the X chromosome, researchers say the evidence points to a strong imbalance: Neanderthal males and Homo sapiens females were the most common pairing.
The X chromosome plays a special role in inheritance. Women carry two X chromosomes, while men carry one X and one Y. This difference allows scientists to trace sex-specific ancestry patterns more precisely than with other chromosomes.
Most people outside sub-Saharan Africa carry between 1% and 4% Neanderthal DNA, yet very little of it appears on their X chromosomes. This long puzzled scientists, who previously believed Neanderthal genes on the X chromosome were biologically incompatible and gradually eliminated over time.
However, when researchers examined ancient Neanderthal genomes, they found the opposite pattern — a surprisingly high presence of Homo sapiens DNA on Neanderthal X chromosomes. This contrast suggested a directional flow of genes, best explained by mating between Neanderthal men and modern human women
According to co-lead author Alexander Platt of the University of Pennsylvania, the genetic signals do not explain why this pattern occurred. The interactions could have been peaceful, coercive, or shaped by population dynamics rather than deliberate choice.
Further analysis showed that early interbreeding may have begun as far back as 250,000 years ago, with a second, more intense wave occurring around 47,000 years ago as Homo sapiens expanded deeper into Eurasia.
The findings also challenge earlier assumptions that Neanderthal DNA vanished from certain parts of the genome due to harmful effects. Mathematical models instead showed that repeated mating bias alone could produce the genetic patterns seen today.
Neanderthals and Homo sapiens split from a common ancestor roughly 600,000 years ago. Despite physical differences, Neanderthals were intelligent, socially complex and capable of art, tool-making and possibly language.
As Homo sapiens populations grew much larger, their genes gradually overwhelmed Neanderthal lineages. Rather than disappearing entirely, researchers suggest Neanderthals may have been absorbed into the modern human gene pool — surviving not as a species, but as part of us.
The research was published in the journal Science, offering one of the clearest genetic insights yet into how two ancient human groups became biologically intertwined.
