5.3.5 The Denisovans

The Denisovans, like Homo naledi, are archaic Homo. There are not a lot of specimens—just one finger bone, three teeth, some long bone fragments, a partial jawbone, and a parietal bone skull fragment. Because of this lack of evidence, very little is known of their anatomical features. Some of the specimens come from Denisova Cave in Siberia, Russia, and are dated to between 500,000 and 30,000 years ago. These dates are arrived at based on the few fossils that exist, inferences made from genetic studies, and sediment analysis. More recently another specimen was found on the Tibetan plateau. In 1980 a jaw and two teeth were uncovered in the Baishiya Karst Cave by a monk, but it wasn’t until 2010 that scientists were able to study the jaw. Dating placed the specimen at approximately 160,000 years ago. Protein analysis determined the jaw to be of Denisovan origin and from a member of a population who were most likely well adapted to living in high altitudes (Chen et al. 2019).

Because so few bones have been found, most understanding of this species comes from genetic analyses. According to nuclear DNA studies, Denisovans and Neanderthals were more closely related to each other than they were to modern humans. DNA evidence suggests that the Denisovans interbred with modern humans and with local Neanderthal populations over multiple time periods. Tracing the male Y chromosome, one study indicated that interbreeding between early humans and Neanderthals actually replaced the ancient Denisovan Y chromosome once found in Neanderthals. The time of divergence of the Denisovan is estimated to be around 700,000 years ago, with modern humans diverging from the Neanderthal around 370,000 years ago (Petr et al. 2020). H. heidelbergensis is typically considered to have been the direct ancestor of both Denisovans and Neanderthals, and sometimes also of modern humans.

One specimen is a first-generation hybrid, Denisova 11—nicknamed “Denny”—that had a Denisovan father and a Neanderthal mother (Slon et al. 2018). Denisova 11 was found in Denisova Cave in Russia and provides evidence that Late Pleistocene Homo species interbred when the groups met. Comparison of the DNA of these three groups suggest that most modern-day Europeans and Asians inherited about 1–4 percent of their DNA from Neanderthals, with no Denisovan ancestry in Europe and 0.1 percent in China. The genetics found in Tibetans, Melanesians, and Indigenous Australian are currently being challenged; originally, they were thought to be about 3–5 percent Denisovan and 2.74 percent Neanderthal. Statistical geneticist Ryan Bohlender and his team have investigated the percentages of extinct hominin DNA in modern humans. They concluded that Neanderthals and Denisovans are not the whole story and that there could be a third group yet unknown contributing to the Pacific Islander genome (Rogers, Bohlender, and Huff 2017). Statistical and genetic evidence can serve as indicators of the existence of a group for which no fossils have yet been found. These are referred to as ghost populations. For example, there are indications that 2–19 percent of the DNA of four West African populations may have come from an unknown archaic hominin that split from the ancestor of humans and Neanderthals between 360 KYA and 1.02 MYA (Durvasula and Sankararaman 2020). The hypothesis of a third lineage in the genus Homo appears to have received further confirmation with a discovery in China.