Newspaper article The Christian Science Monitor

What Missing Neanderthal Y-Chromosomes Say about Human Ancestry

Newspaper article The Christian Science Monitor

What Missing Neanderthal Y-Chromosomes Say about Human Ancestry

Article excerpt

Neanderthals and Homo sapiens interbred tens of thousands of years ago, leaving us with a bit of the archaic humans' DNA. But scientists aren't clear how viable such hybrid offspring would have been.

Since about 2 percent of all non-African modern human genomes today come from Neanderthals, it's clear that some of these hybrid children were successful. But geneticists conjectured that the hybrid males may have faced more challenges than the hybrid females.

And a new study suggests their suspicion was right. It turns out that the Neanderthal Y-chromosome appears in no known H. sapiens genomes still around today.

Both H. sapiens and Neanderthals are believed to be descended from Homo heidelbergensis, a human species that emerged in Africa some 700,000 years ago and spread to Europe and Western Asia. Scientists believe that the African branch began to evolve into H. sapiens while the Eurasian branch evolved into Neanderthals. The two lineages became entwined again, possibly as far back as 100,000 years ago, when H. sapiens began to migrate out of Africa and encountered their stocky, barrel-chested relatives.

This new study sheds light on a longstanding debate among scientists over how similar Neanderthals were to H. sapiens, sharpening our picture of an intriguingly tangled group of branches within the human family tree.

"The fact that the Neanderthal Y we describe has never been observed in modern humans suggests that the lineage is most likely extinct," the study authors write in their paper published Thursday in the American Journal of Human Genetics.

So what drove the Neanderthal Y-chromosome to extinction?

It's possible that the archaic sequences simply slipped away over time, as generations of hybrids interbreeding with modern humans diluted those genes. But those Neanderthal genetics might have been disadvantageous for hybrids too, suggests study lead author Fernando Mendez, a geneticist at Stanford University.

"There is the idea that maybe there were some genetic variants that when combined in hybrids had negative effects," he tells The Christian Science Monitor in an interview. So his team dug into the Neanderthal Y-chromosome hunting for genes that could affect the fitness of the hybrid offspring.

Dr. Mendez and his team found three mutations in the Neanderthal sequence that could affect the viability of male hybrid offspring, one of which, he says, would cause a pregnant mother's body to reject any male fetus that carried that gene. …

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