Homo sapiens and Neanderthals are believed to have met over 30,000 years ago; a new study maps their history.
Neanderthals branched off from modern humans a few thousand years before they met up with Homo sapiens, who migrated from Africa, a Howard Hughes Medical Institute (HHMI)news release reported. The two groups are believed to have looked past their differences and had children together.
A research team set out to pinpoint what areas of the human genome were associated with Neanderthal DNA.
"The goal was to understand the biological impact of the gene flow between Neanderthals and modern humans," David Reich, an HHMI investigator at Harvard Medical School and the lead scientist on the new research. "We reasoned that when these two groups met and mixed, some new traits would have been selected for and remained in the human genome, while some incompatibilities would have been selected against and removed."
The team also looked for what regions of the human genome completely lacked Neanderthal DNA.
"The most interesting findings were about the places in the genome that are devoid of Neanderthal genes - 'Neanderthal ancestry deserts'," Reich said.
The team found the X chromosome lacked Neanderthal DNA. Women have two X chromosomes while men have an X and a Y. This finding suggested a phenomenon had occurred called "hybrid sterility"; this can occur when two fertility-related genes inherited from the X chromosomes could react poorly with other's in the genome. This can leave males, who only have one X chromosome, infertile.
"When you have populations that have sufficiently diverged, this male-only sterility can occur," Reich said.
The team looked for signs of Neanderthal DNA in the testes to determine if hybrid sterility occurred. They found genes expressed in the testes were low in Neanderthal DNA. This means that men possessing a Neanderthal X chromosome may have been sterile and unable to pass it down.
"It tells us that when Neanderthals and modern humans met and mixed, they were at the very edge of being biological compatible," Reich said.
The information could be used to understand sterility in general. The information also led to insight into the genetic changes associated with diseases such as lupus and diabetes.
"The other direction we want to go is to use this information as a tool for understanding human disease genes," Reich said.
