DNA Study Proves Early Humans Were Inbred, And Also Bred Liberally With Neanderthals And Other Hominid Ancestors

By Ajit Jha on December 18, 2013 1:08 PM EST

DNA
The most complete DNA sequencing of the Neanderthal genome to date shows that there was inbreeding and interbreeding with at least 4 different species of early hominids, a new study claims. (Photo: Shutterstock)

A new study claims that researchers have performed the most complete sequencing of the Neanderthal genome to date. The results of the sequencing revealed several intriguing facts about this group of human ancestors, including the fact that the Neanderthals and Denisovans more closely related than was previously thought. Though the two hominid species split about 300,000 years ago, they are among the group of pre-humans with a long history of interbreeding.   

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Even before this study, it was known that two hominid cousins, Neanderthals and Denisovans, accompanied our ancestors in Africa until they migrated out about 60,000 years ago. Neanderthals migrated out of Africa about 300,000 years ago, and came to settle in Europe and western Asia.

Denisovans are a relatively new addition to our family history tree, discovered by paleoanthropologists in 2008. They unearthed a 40,000 year-old adult tooth and a preserved fossilized pinkie bone while digging a cave in southern Siberia. The bone was that of a young girl between five and seven years old. The DNA analysis of this bone showed that although the skeleton was genetically close to Neanderthals, there was still distinct enough to merit new nomenclature  - ultimately, the term "Denivosan"  was chosen after the name of the cave where the pinkie bone was found.

The new genome analysis study, which will be published in the Dec. 19 issue of the journal Nature, study was performed by an international team of researchers based out of University of California, Berkeley consisting of population geneticist Montgomery Slatkin, graduate student Fernando Racimo, and postdoctoral student Flora Jay among others.

After analyzing DNA extracted from a 50000 year old woman's bone, the team concluded that at least four different types of early humans in Asia and Europe interbred over a long period. While Denisovans and Neanderthals eventually died out, they left a bit of their genetic heritage among modern humans. Modern non-Africans have between 1.5 to 2.1 percent Neanderthal genomes, according to the research team. While earlier studies had revealed the presence of about 6 percent Denisovans genes in the DNA of Australian aborigines, New Guineans, and some Pacific Islanders, the present study reveals about 0.2 percent Denisovan genes among Han Chinese and other mainland Asian populations, as well as Native Americans.

The genome comparison further reveals the presence of a mysterious fourth group of early humans living in Eurasia - perhaps Homo erectus, a species that split about a million years ago - with whom Denisovans interbred.    

The sample DNA from the toe bone of the Neanderthal woman indicated that she was highly inbred. She was probably an offspring of a closely related mother and father, who could possibly be half siblings. The upshot of the study is that inbreeding was more common among Neanderthals in comparison to modern populations. 

Does the retention of Neanderthal DNA help modern humans? A recent, but unconnected study recently published in Molecular Biology and Evolution offers evidence of UV-light adaptation associated with the accumulation of a Neanderthal DNA region on chromosome 3, the site of 18 genes including Hyal2 gene. That's just 18 genes - the new U.C. Berkeley study identifies at least 87 genes in modern humans that are different from related Denisovans and Neanderthals, all of which hold the possibilities to reveal how we came to be different from the early human populations that went extinct. 

Image above courtesy of Shutterstock.

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