Denisova Hominin



The Denisova hominin is the name given to the remains of a member of the genus Homo which 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.

The mtDNA of the Denisova hominin is distinct from the mtDNAs of Neanderthals and modern humans. In December 2010, an international team of scientists determined sequence from the nuclear genome of this group (known as the Denisovans) from this finger bone. Based on their analysis, this group was related to the Neanderthals and interbred with the ancestors of modern Melanesians.




Anatomy and Lineage

Little is known of the anatomical features of the individual in question since the only physical remains discovered thus far are the finger bone from which only mitochondrial genetic material was gathered. 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 nuclear DNA shows it to be a branch of the Neanderthal lineage. Even though the Denisova hominin's mtDNA lineage predates the divergence of modern humans and Neanderthals, coalescent theory does not preclude a more recent divergence date for her nuclear DNA.




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), or the Denisova hominin. Artifacts, including a bracelet, excavated in the cave at the same level were carbon dated to around 40,000 BC.

A team of scientists led by Johannes Krause and Swedish biologist Svante Pbo 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.

The analysis indicated that modern humans, Neanderthals, and the Denisova hominin last shared a common ancestor around 1 million years ago. Modern humans are known to have overlapped with Neanderthals in Europe for more than 10,000 years, and the discovery raises the possibility that Neanderthals, modern humans and the Denisovan hominin may have co-existed.

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.

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." Paabo noted that the existence of this distant branch creates a much more complex picture of humankind during the Late Pleistocene.

In 2010, a second paper from the Svante Paabo group reported the prior discovery, in 2000, of a 3rd 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 Neanderthal, 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 from the bone, indicating a divergence time about 7500 years before, and suggesting it belonged to a different individual from the same population.




Nuclear Genome Analysis

  Ancient humans, dubbed 'Denisovans', interbred with us   BBC - December 23, 2010

In the same 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. 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.




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 modern humans from Africa, Europe, East Asia and Melanesia showed that between 4% and 6% of the genome of Melanesians derives from a Denisovan population, introduced after the lineage leading to Melanesians diverged from that leading to other Eurasians.

Denisova Hominin  Wikipedia







In the News ...





A world map of Neanderthal and Denisovan ancestry in modern humans   Science Daily - March 28, 2016
Most non-Africans possess at least a little bit Neanderthal DNA. But a new map of archaic ancestry suggests that many bloodlines around the world, particularly of South Asian descent, may actually be a bit more Denisovan, a mysterious population of hominids that lived around the same time as the Neanderthals. The analysis also proposes that modern humans interbred with Denisovans about 100 generations after their trysts with Neanderthals.




Do we owe our thick hair and tough skin to Neanderthals?   Daily Mail - March 28, 2016
World map of prehistoric ancestry shows how interbreeding has changed and even helped modern humans. The study has unearthed some surprising new benefits these illicit encounters have gifted to modern humans living today.For example genetic variants inherited from Denisovans appear to have given some people in south Asia a better ability to detect subtle scents and helped others to survive at high altitudes.




Ancient Denisovan DNA excavated in modern Pacific Islanders   Science Daily - March 19, 2016
Archaic Denisovan and Neanderthal DNA that persists in modern Pacific islanders of Melanesia, far from the Siberian cave where Denisovan fossils have been found, is a source of information about early human history. Equally informative are genome regions where DNA from extinct, human-like species has vanished and been replaced with sequences unique to people. These large regions have genes for brain development, language and brain cell signalling. Retained archaic DNA in human genomes may confer infection-fighting advantages.




DNA analysis of Denisovan molars offers more clues about ancient human relative   PhysOrg - November 18, 2015
A team of researchers with members from Germany, Canada and Russia has conducted a DNA analysis of two molars found in the Denisova caves in Siberia shedding more light on the origins of the Denisovans - a hominin species that lived or at least visited Siberia approximately a hundred thousand years ago.




Researchers create methylation maps of Neanderthals and Denisovans, compare them to modern humans   PhysOrg - April 18, 2014
A team of Israeli, Spanish and German researchers has for the first time created a map of gene expression in Neanderthals and Denisovans and has compared them with modern humans. In their paper published in the journal Science, the team describes how they applied epigentics to the study of our two closest known ancestors and discovered variations that might account for their differences in body shape and susceptibility to some modern neurological diseases.

Scientists know that it's not just our DNA structure that determines how we look and what we're capable of doing, there's another factor involved the expression of our genes they can be turned on or off at some point, allowing or preventing certain traits from developing. This process is known as DNA methylation where methyl group chemicals attach to DNA and prevent them from behaving as they would otherwise. In this new effort, the researchers looked at methylation in Neanderthals and Denisovans to learn more about how they might have been different from us. Studying methylation in preserved fossils involves noting the way the methyl chemical cytosine decays over long periods of time. Unmethylated cytosines decay to one type of chemical while unmethylated cyctones decay to another. By measuring the amounts of the two resultant chemicals found in fossilized bone fragments, the researchers were able to create methylation maps of Neanderthals and Denisovans, which they then compared with similar maps for modern humans.




Neanderthal Genome Shows Early Human Interbreeding   Science Daily - December 19, 2013

The most complete sequence to date of the Neanderthal genome, using DNA extracted from a woman's toe bone that dates back 50,000 years, reveals a long history of interbreeding among at least four different types of early humans living in Europe and Asia at that time




Leg bone gives up oldest human DNA   BBC - December 4, 2013
The discovery of DNA in a 400,000-year-old human thigh bone will open up a new frontier in the study of our ancestors. That's the verdict cast by human evolution experts on an analysis in Nature journal of the oldest human genetic material ever sequenced. The femur comes from the famed "Pit of Bones" site in Spain, which gave up the remains of at least 28 ancient people.




Oldest Hominin DNA Sequenced: Mitochondrial Genome of a 400,000-Year-Old Hominin from Spain Decoded   Science Daily - December 4, 2013

Using novel techniques to extract and study ancient DNA researchers at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, have determined an almost complete mitochondrial genome sequence of a 400,000-year-old representative of the genus Homo from Sima de los Huesos, a unique cave site in Northern Spain, and found that it is related to the mitochondrial genome of Denisovans, extinct relatives of Neandertals in Asia. DNA this old has until recently been retrieved only from the permafrost.




Oldest Human DNA Reveals Mysterious Branch of Humanity   Live Science - December 4, 2013
The oldest known human DNA found yet reveals human evolution was even more confusing than thought, researchers say. The DNA, which dates back some 400,000 years, may belong to an unknown human ancestor, say scientists. These new findings could shed light on a mysterious extinct branch of humanity known as Denisovans, who were close relatives of Neanderthals, scientists added. Although modern humans are the only surviving human lineage, others once strode the Earth. These included Neanderthals, the closest extinct relatives of modern humans, and the relatively newfound Denisovans, who are thought to have lived in a vast expanse from Siberia to Southeast Asia. Research shows that the Denisovans shared a common origin with Neanderthals but were genetically distinct, with both apparently descending from a common ancestral group that had diverged earlier from the forerunners of modern humans.




Asians, Too, Mated With Archaic Humans, DNA Hints   National Geographic - November 1, 2011
First it was the earliest Europeans and Neanderthals. Now it appears that modern humans in Southeast Asia also became intimate with their prehistoric relatives. About one percent of the genetic makeup of people from southern China and the surrounding region comes from an extinct group of humans dubbed the Denisovans, a new study says. Considered by some to be a sort of sister species to the Neanderthals, the Denisovans - who may have represented an entirely separate human species - are largely a mystery, though they're thought to have had big teeth. The latest find suggests that the two human types mated and bore offspring, and their descendants are still alive today in mainland Asia.




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