Today, most people of non-African ancestry carry around two per cent Neanderthal DNA. These ancient traces are scattered across our genome, a reminder of encounters between the two groups roughly 50,000 to 60,000 years ago. (Image: Reuters)A new study has shed light on a little-known chapter of human history – and it suggests that love and attraction may have shaped our DNA in surprising ways.Researchers have found that when Neanderthals and early humans interbred tens of thousands of years ago, the pairings were heavily one-sided. Most unions appear to have been between Neanderthal men and human women.The findings, published in the journal Science on Thursday, help explain a long-standing genetic mystery: why Neanderthal DNA is almost completely missing from the human X chromosome.A genetic gapToday, most people of non-African ancestry carry around two per cent Neanderthal DNA. These ancient traces are scattered across our genome, a reminder of encounters between the two groups roughly 50,000 to 60,000 years ago.Also read | Ancient ape fossils offer new clues to humanity’s first stepsBut scientists have been puzzled over one detail. Large portions of the human X chromosome – one of the two chromosomes that determine biological sex – show almost no Neanderthal ancestry.For years, the common explanation was that Neanderthal genes on the X chromosome were harmful. Experts believed these genes may have caused health or fertility problems, and were gradually removed from the human population over time.The new study challenges that idea.A research team from the University of Pennsylvania, led by geneticist Sarah Tishkoff and researcher Alexander Platt, compared modern human DNA with genetic data from Neanderthal fossils.Story continues below this adThey found that Neanderthals carried significant amounts of human DNA on their own X chromosomes. If human and Neanderthal genes had been biologically incompatible, researchers argue, this exchange would not have worked both ways.The evidence suggests the two groups were capable of producing healthy offspring, meaning something else explains the missing Neanderthal DNA in human X chromosomes.How inheritance shaped the outcomeThe answer lies in the way X chromosomes are passed down.Women have two X chromosomes, while men have one X and one Y chromosome. Mothers pass one X chromosome to all of their children. Fathers pass their X chromosome only to daughters and their Y chromosome to sons.Story continues below this adAlso Read | Oldest hominin fossils in East Asia found to be nearly 1.8 million years oldIf a Neanderthal man had a child with a human woman, their son would inherit a Y chromosome from his Neanderthal father and an X chromosome from his human mother. That son would carry no Neanderthal DNA on his X chromosome.Over time, repeated pairings of this kind would mean very little Neanderthal X chromosome DNA would enter the broader human population.Daughters from such unions would inherit one Neanderthal X chromosome, but if they later had children with human men, that genetic material would become diluted across generations.The researchers also found that Neanderthals had far more human DNA on their X chromosomes than expected. This confirms the theory that Neanderthal men had more involvement with human women than the other way around.Story continues below this adAncient mating was not random. It was a social process that took place over thousands of years.Rather than being about genetic compatibility, it seems that social processes and migrations were key to our evolution.Now, scientists hope to uncover whether these genetic findings could shed even more light on how ancient societies were organised, perhaps even how males moved from group to group.What is known is that our evolution was not just about surviving and being selected for. Love, relationships, and social processes were part of it too – and it is evident in our DNA today. © IE Online Media Services Pvt LtdTags:humansNeanderthals