Science-- there's something for everyone

Friday, March 26, 2010

The X-woman: a new species?

Johannes Krause and Svante Pääbo from the Max Planck Institute in Germany and their colleagues have sequenced the mitochondrial DNA (mtDNA) from a 40,000 year-old hominid finger bone. To their surprise, the DNA did not match either modern human or Neanderthal mtDNA, the only two hominid species known to be in the area at that time, but may be from a third species, representing a hitherto unknown migration out of Africa. The scientists dubbed this specimen the ‘X-woman’.

The bone fragment was originally found in the Siberian Denisova Cave in 2008 by Russian paleontologists, but only recently subjected to mtDNA sequencing. The researchers were extremely thorough, rereading the sequences well over a 100 times. That mtDNA was compared with mtDNA from 54 modern humans, from a 30,000 year-old modern human found in Russia and from 6 Neanderthals. Neanderthal mtDNA usually differs from that of Homo sapiens in about 200 spots, but the mtDNA from the X-woman differed from modern humans in about 400 positions.

The implication is that the bone fragment is from a new species of hominid, one which diverged from modern humans and Neanderthals about a million years ago.

simple xwoman tree

Credit: Nature

An alternate possibility will require a brief primer on mtDNA. Mitochondria are the energy factories within our cells. Although like all parts of the cell, they rely on information stored in the cell’s nucleus (nuclear DNA), mitochondria also have DNA of their own. Upon fertilization, the much larger egg cell contains maternal nuclear DNA as well as a cell’s worth of mitochondria, each containing its own DNA. The sperm contributes only nuclear DNA to the fusion. As a result, mtDNA is passed exclusively down the female line, whereas the nuclear DNA is a mixture from both parents.

Therefore, the X- woman might represent a new species, or she might be a fully modern human or Neanderthal who had a female ancestor who was a different species, say Homo erectus. That tryst might have occurred so long ago that the X-woman’s nuclear DNA, having been diluted with human (or Neanderthal) DNA over many subsequent generations, shows no more signs of that ancestor. The mtDNA was not diluted out, and thus does differ considerably from either humans or Neanderthals.

The scientists intend to sequence the X-woman's nuclear DNA in order to put this question to rest. No matter what they find, this will be the first time DNA sequencing was used to identify an extinct species.