This caused great surprise when, in 2010, a team led by Svante Pääbo proved that modern humans and Neanderthals interbred in the past. However, how this genetic mixing took place has remained a mystery to this day. A research team led by Alexander Platt from the University of Pennsylvania has provided a concrete explanation for the hybridization process for the first time by conducting genetic analyzes focusing on the X chromosome of the two species.
According to the results of the research published in the journal Science, when Neanderthals and sapiens mated, the matches mostly occurred between Neanderthal men and sapiens women. According to the researchers, this finding explains why the genetic inheritance of Neanderthal origin in today’s humans is not evenly distributed. Modern humans have low levels of Neanderthal ancestry, and this genetic heritage is not distributed homogeneously throughout the genome.
Why is the X chromosome important?
The X chromosome is critical because its inheritance is directly linked to biological sex. While other chromosomes are inherited equally from both parents, the X chromosome is passed down differently: women have two X chromosomes, while men have one X and one Y chromosome. If matings between two populations are not gender balanced, the amount of admixture on the X chromosome will be markedly different from the rest of the genome.
“Neanderthal desert”
When scientists examined the modern human genome, they saw that the X chromosome was like a “Neanderthal desert”; that is, there was significantly less Neanderthal DNA in this chromosome. In contrast, 62% more modern human DNA was found in the X chromosome of Neanderthals compared to other chromosomes. Daniel Harris, one of the study’s co-authors, describes this as a “staggering imbalance.”
This pattern can be explained in two ways:
Neanderthal X chromosome variants were disadvantaged in modern humans and were eliminated by natural selection.
Early interbreeding occurred mostly between Neanderthal men and modern human women; this led to limited introduction of Neanderthal X chromosome DNA into the human gene pool.
Using mathematical simulations, the researchers showed that a model based solely on female migration could explain only part of this difference. They therefore concluded that the most likely explanation was a distinct mating preference between Neanderthal men and modern human women. According to Platt, “the simplest explanation is mate preferences.”
Previous studies
Two studies previously published in Nature and Science showed that hybridization occurred 43,050,000 years ago, that some lineages mixed more than once, and that only the adaptive part of Neanderthal genes persisted.
Alternative explanations
According to paleoanthropologist Antonio Rosas from the National Museum of Natural Sciences of Spain, this study seeks to answer an important question: What exactly happened in interspecies hybridization events? Rosas states that for the difference in the X chromosome to appear, there must be a certain standard pattern in the sexual profile. However, he thinks that an explanation based solely on mate preference may not be sufficient.
The lower likelihood of survival of hybrid offspring between sapiens men and Neanderthal women could also explain this imbalance, Rosas says. In this case, the hybrids in question may not have left a trace in the gene pool.
Similarly, Gemma Marfany, professor of genetics at the University of Barcelona, notes that survival rates of hybrid offspring may have varied depending on the type of mother. If male hybrids born to Neanderthal mothers had a low survival rate, this may have led to the reduction of the Neanderthal X chromosome by natural selection.
According to Marfany, although the study is remarkable and headline-making, it needs to be supported by more Neanderthal genomes, mitochondrial DNA analysis and deeper genome analysis.
Prehistory expert Ignacio Martín Lerma from the University of Murcia reminds that the study offers an important perspective, but it is genetic inference, not direct behavioral evidence.
José Yravedra from the Complutense University of Madrid points out an alternative possibility: There may have been fewer women in the last Neanderthal groups. This may have increased mobility and led sapiens women to have more contact with Neanderthals.
The study shows once again that the relationships between Neanderthals and modern humans are complex and multidimensional; but more research is needed for definitive answers.
