50,000 Year-Old Toe Bone Reveals Interbreeding, Inbreeding among Early Humans

A toe-bone of a woman who lived nearly 50,000 years ago has revealed a long history of interbreeding between modern humans, Neanderthals, Denisovans and a mysterious ancient relative.

The latest research also shows that smaller population of Neanderthals and their close cousins inbred.

Back in 2006, Svante Pääbo of the Max Planck Institute in Leipzig, Germany began a project to sequence the Neanderthal genome. His team had sequenced the genome of Neanderthal using bones of three female Neanderthals from Europe who lived about 40,000 years ago.

The toe-bone, which was found in 2010 in Denisova Cave in Southern Siberia, provided scientists with a high-quality genetic material, helping the team get a better look at Neanderthal genome.

Both Neanderthals and the Denisovans lived in the same cave in the Altai Mountains. Previous research has shown that human ancestors interbred with the Denisovans.

Montgomery Slatkin, Fernando Racimo and Flora Jay from UC Berkeley were part of an international team that worked on the project. Genome analysis of Neanderthal, Denisovans and modern humans has shown that there was a "gene flow" among the three ancient human groups.

The research team also estimates that modern non-African humans share at least 1.5 to 2.1 percent of their genomes with Neanderthals. One region of the Neanderthal DNA that relates with sunlight adaptation has already been found in East Asians.

According to the scientists, the Denisovans have also left their mark on modern humans. But its genes are mostly restricted to Oceanic and Asian populations. Han Chinese along with Asian populations and Native Americans have 0.2 percent Denisovan genes.

The Mystery Cousin

Genome comparison of the three groups also point to Denisovans interbreeding with a yet-to-be-identified relative of the modern humans.

Researchers place their bets on human ancestor known as Homo erectus as the fourth group in the entwined breeding history of early humans. Homo erectus is one of the oldest known humans.

"The paper really shows that the history of humans and hominins during this period was very complicated," said Slatkin, a UC Berkeley professor of integrative biology, according to a news release . "There was lot of interbreeding that we know about and probably other interbreeding we haven't yet discovered."

The study was published in the journal Nature.

Ancient Siberian Woman Born of Incest

Inbreeding might have been common in early human relatives, especially in smaller populations of Neanderthals and the Denisovan that had a limited gene pool.

The woman, whose toe completed the picture of early human evolution, was born out of inbreeding, researchers found. According to her genome, she was a daughter of a closely related couple such as half-siblings, uncle-niece or an aunt and nephew.

The genes that set Modern Humans Apart

Racimo and colleagues were also able to identify at least 87 genes found only in modern humans that are different from the related genes in Neanderthals and Denisovans. Scientists say that these genes may explain how modern humans were different from their cousins.

"There is no gene we can point to and say, 'This accounts for language or some other unique feature of modern humans,'" Slatkin said. "But from this list of genes, we will learn something about the changes that occurred on the human lineage, though those changes will probably be very subtle."