The Denisova hominin (/dɨˈnɪsəvə/) is the name given to the remains of a member of the genus Homo that may be a previously unknown species based on an analysis of its mitochondrial DNA (mtDNA). In March 2010, discovery was announced of bone fragments of a juvenile that lived about 41,000 years ago found in Denisova Cave (Altai Krai, Russia), a region also inhabited at about the same time by Neanderthals and modern humans.^[1][2] The mtDNA of the Denisova hominin is distinct from the mtDNAs of Neanderthals and modern humans.^[3] In December 2010, an international team of scientists determined the sequence from the nuclear genome of this group (known as the Denisovans) from this finger bone. According to their analysis, this group shares a common origin with the Neanderthals and interbred with the ancestors of modern Melanesians.^[4]

Anatomy and lineage

Little is known of the precise anatomical features of the Denisovans since the only physical remains discovered thus far are the finger bone from which only mitochondrial genetic material was gathered. A tooth found in Denisova Cave carries a mtDNA very similar to that of the finger bone and shares no derived morphological features with Neanderthal or modern humans.^[5] The Siberian bone's mtDNA differs from that of modern humans by 385 bases (nucleotides) in the mtDNA strand out of approximately 16,500, whereas the difference between modern humans and Neanderthals is around 202 bases. In contrast, the difference between chimpanzees and modern humans is approximately 1,462 mtDNA base pairs. Analysis of the specimen's genome shows it to be due to a common branch of ancestors with Neanderthal lineage, but, after they diverged from one another, Denisovans and Neanderthals had largely separated population histories.^[1][4][5][6]

Discovery

In 2008, Russian archeologists working at the site of Denisova Cave in the Altai Mountains of Siberia uncovered a small bone fragment from the fifth finger of a juvenile hominin, dubbed the "X-woman" (referring to the maternal descent of mitochondrial DNA^[6]), or the Denisova hominin. Artifacts, including a bracelet, excavated in the cave at the same level were carbon dated to around 40,000 BP.

A team of scientists led by Johannes Krause and Swedish biologist Svante Pääbo from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, sequenced mtDNA extracted from the fragment. Because of the cool climate in the location of the Denisova Cave, the discovery benefited from DNA's ability to survive for longer periods at lower temperatures.^[2] The analysis indicated that modern humans, Neanderthals, and the Denisova hominin last shared a common ancestor around 1 million years ago.^[3] Some studies suggest that modern humans coexisted with Neanderthals in Europe, and the discovery raises the possibility that Neanderthals, modern humans and the Denisovan hominin may have co-existed.^{[citation needed]}

The DNA analysis further indicated that this new hominin species was the result of an early migration out of Africa, distinct from the later out-of-Africa migrations associated with Neanderthals and modern humans, but also distinct from the earlier African exodus of Homo erectus.^[3] Professor Chris Stringer, human origins researcher at London's Natural History Museum and one of the leading proponents of the recent single-origin hypothesis, remarked: "This new DNA work provides an entirely new way of looking at the still poorly understood evolution of humans in central and eastern Asia." Pääbo noted that the existence of this distant branch creates a much more complex picture of humankind during the Late Pleistocene.^[6]

Later in 2010, a second paper from the Svante Pääbo group reported the prior discovery, in 2000, of a third upper molar from a young adult, dating from about the same time (the finger was from level 11 in the cave sequence, the tooth from level 11.1). The tooth differed in several aspects from those of Neanderthals while having archaic characteristics similar to the teeth of Homo erectus. They again performed mitochondrial DNA analysis on the tooth and found it to have a different but similar sequence to that of the finger bone, indicating a divergence time about 7,500 years before, and suggesting it belonged to a different individual from the same population.^[5]

Nuclear genome analysis

In the same second 2010 paper, the authors report the isolation and sequencing of nuclear DNA from the Denisova finger bone. This specimen showed an unusual degree of DNA preservation and low level of contamination. They were able to achieve near-complete genomic sequencing, allowing a detailed comparison with Neanderthal and modern humans. From this analysis, they concluded that in spite of the apparent divergence of their mitochondrial sequence, the Denisova population along with Neanderthal shared a common branch from the lineage leading to modern African humans. The estimated time of divergence between Denisovans and Neanderthals is 640,000 years ago, and that between both these groups and modern Africans is 804,000 years ago. They suggest that the divergence of the Denisova mtDNA results either from the persistence of a lineage purged from the other branches of humanity through genetic drift or else an introgression from an older hominin lineage.^[5]

Interbreeding with modern humans

In addition to genetic studies linking approximately 4% of non-African modern human DNA to Neanderthals, these tests comparing the Denisova hominin genome with those of six modern humans whose genome has been sequenced, a ǃKung from South Africa, a Nigerian, a French person, a Papua New Guinean, a Bougainville Islander and a Han Chinese showed that between 4% and 6% of the genome of Melanesians (represented by the Papua New Guinean and Bougainville Islander) derives from a Denisovan population, possibly introduced during the early migration of the ancestors of Melanesians into Southeast Asia. This history of interaction suggests that Denisovans once ranged widely over eastern Asia.^[5]

References

Further reading

• Fox, Maggie (2010-03-24), "Possible new human ancestor found in Siberia", Reuters, http://www.reuters.com/article/idUSTRE62N4VS20100324 .
• Ghosh, Pallab (2010-12-22), Ancient humans, dubbed 'Denisovans', interbred with us, BBC News online, http://www.bbc.co.uk/news/science-environment-12059564, "The study shows that Denisovans interbred with the ancestors of the present day people of the Melanesian region north and north-east of Australia. Melanesian DNA comprises between 4% and 6% Denisovan DNA." 
• Hawks, John (2010-12-22), "The Denisova genome FAQ", John Hawks Weblog, http://johnhawks.net/weblog/reviews/neandertals/neandertal_dna/denisova-nuclear-genome-reich-2010.html .
• Rincon, Paul (2010-03-25), DNA identifies new ancient human dubbed 'X-woman', BBC News online, http://news.bbc.co.uk/1/hi/sci/tech/8583254.stm .
• Wade, Nicholas (2010-03-24), "Bone May Reveal a New Human Group", The New York Times, http://www.nytimes.com/2010/03/25/science/25human.html .
• Zimmer, Carl (2010-03-24), Hybrid speculation (blog), Discover, http://blogs.discovermagazine.com/loom/2010/03/24/the-x-womans-fingerbone/ .
• Zimmer, Carl (2010-12-22), Meet the Denisovans, the newest members of the human tree of life (blog), Discover, http://blogs.discovermagazine.com/loom/2010/12/22/meet-the-denisovans-the-newest-members-of-the-human-tree-of-life/ .