Recovered DNA Suggests a New Type of Hominid: Stone Age Relative May Have Lived among Modern Humans

Article excerpt

A new member of the human evolutionary family has been proposed for the first time based on an ancient genetic sequence, not fossil bones. Even more surprising, this mysterious hominid, if confirmed, would have lived near Stone Age Neandertals and Homo sapiens.

"It was a shock to find DNA from a new type of ancestor that has not been on our radar screens," says geneticist Svante Paabo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

Researchers led by Paabo and Max Planck graduate student Johannes Krause propose the new hominid based on DNA from a finger bone, found in the Altai Mountains of central Asia, that dates to between about 48,000 and 30,000 years ago. The DNA suggests the hominid left Africa in a previously unsuspected migration around 1 million years ago, the team reports online March 24 in Nature.

Paleoanthropologists have generally assumed that hominids left Africa in a few discrete waves, starting with Homo erectus about 1.9 million years ago. (In the April 9 Science, researchers introduce a new hominid species, Australopithecus sediba, that they argue was ancestral to Homo erectus. But other scientists are highly skeptical of the new species' relationship to the Homo genus.)

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General consensus holds that Neandertal ancestors, known as H. heidelbergensis, left Africa between 500,000 and 300,000 years ago, followed by humans around 50,000 years ago. But the new genetic sequence supports a scenario in which at least one additional and possibly many more African hominid lineages trekked to Asia and Europe in the wake of rt. erectus, Paabo says.

The finger bone that provided the curious sequence was unearthed in 2008 at Denisova Cave in southern Siberia's Altai Mountains. Previous excavations of stone and bone artifacts in the cave have indicated that Neandertals and modern humans lived there periodically beginning at least 125,000 years ago, but few fossils have turned up at the site.

While analyzing DNA from the presumed Neandertal fossils in November 2009, Krause noticed an unusual mitochondrial sequence. Mitochondrial DNA is located outside the cell nucleus and is inherited from the mother.

Krause conducted tests to confirm that the recovered mitochondrial DNA came from an ancient hominid rather than from bacteria or researchers who had handled the fossil. Using advanced techniques for fishing DNA fragments out of fossils, the team then assembled a complete mitochondrial genome for the Denisova individual. The same approach has yielded DNA sequences for Neandertals (SN.. 3/14/09, p. 5) and a Greenlander who lived 4,000 years ago (SN: 3/13/10, p. 5).

The researchers compared Denisova mitochondrial DNA with complete mitochondrial sequences from 54 people who are living today as well as a human who lived in Siberia about 30,000 years ago, six Neandertals that lived more than 40,000 years ago, a modern pygmy chimpanzee and a modern common chimp. …