Why human-Neanderthal sex is tricky to prove





A bundle of recent genetic studies have suggested modern humans had sex with Neanderthals thousands of years ago when the two populations roamed the planet alongside each other. However, the bones left behind by the two species don't bear any obvious traces of interbreeding, and a new study of monkeys in Mexico shows why we shouldn't expect them to.

Researchers examined blood samples, hair samples and measurements collected from mantled howler monkeys and black howler monkeys that were live-captured and released in Mexico and Guatemala between 1998 and 2008. The two monkey species splintered off from a common ancestor about 3 million years ago; today they live in mostly separate habitats, except for a "hybrid zone" in the state of Tabasco in southeastern Mexico, where they coexist and interbreed.

Through an analysis of genetic markers, from both mitochondrial DNA (the DNA in the cells' energy-making structures that gets passed down by mothers) and nuclear DNA, the researchers identified 128 hybrid individuals that were likely the product of several generations of interbreeding. Even so, these hybrids shared most of their genome with either one of the two species and were physically indistinguishable from the pure individuals of that species, the team found....



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