Mammoth hair offers new style of research
Study reveals valuable store of ancient DNA in museum samples.
Geneticists have pieced together gene sequences from ten Siberian mammoths, using tiny samples of their hair found preserved in the Russian tundra. The result uses some of the oldest DNA ever pieced together — one of the mammoths had lain in the frozen ground for some 50,000 years.
The achievement demonstrates the unexpected usefulness of hair for recovering ancient DNA sequences, and hair's amazing resistance to deterioration. One of the mammoth specimens, called the Adams mammoth, was first uncovered in 1799 and has sat in a museum in St Petersburg for two centuries.
It isn't the first time that the Siberian woolly mammoth (Mammuthus primigenius) has been sequenced — that was first done in 20051. And genome results have previously been reported for a 50,000-130,000-year-old mastodon too (see 'Mastodon DNA sequenced').
But the sequencing of ten individuals massively bumps up the genetic database for extinct animals. In fact, it more than doubles the number of extinct creatures that have had their mitochondrial genomes — which contain genes for energy production in the cell — sequenced.
Such genetic sequences have previously only been extracted from bones, including those of a 40,000-year-old cave bear (Ursus spelaeus), and those of Neanderthals (Homo neanderthalensis), who died out some 30,000 years ago.
Researchers led by Tom Gilbert of the University of Copenhagen's Centre for Ancient Genetics, Denmark, and Webb Miller and Stephan Schuster of Pennsylvania State University in University Park, extracted the DNA from hair strands belonging to the now-extinct Siberian mammoth.
They were able to piece together the mammoths' complete mitochondrial DNA sequences, using the 'shotgun' method, in which a DNA sample is broken into fragments, which are then multiplied and reassembled using a computer program. The technique used as little as 0.2 grams of DNA extracted from the hair strands.
Hair is not often usually as well preserved as ancient bone samples. But where hair is found preserved, such as in the Siberian tundra, the DNA in it seems to degrade much more slowly, perhaps because hair has a lower water content than bone, they say. As the researchers report in this week's Science2, the DNA samples from mammoth hair had damage rates as low as 0.24%, compared with around 1.7% for ancient bone samples.
It's difficult to put a maximum theoretical time limit on how long DNA might survive in hair, says Miller. "Theoretical models say we shouldn't see any DNA at all," adds Miller. "They say that after 50,000 years, and then sitting in a museum for 200 years, there should be nothing left. But they're wrong."
If the technique can be replicated for other samples of fur, or even for feathers, it could help zoologists create more accurate family trees of extinct species, using the samples housed in museums across the world. The fact that the DNA seems to survive even at room temperature opens up many historical collections for this type of study, Miller and his colleagues point out.
- Krause, J. et al. Nature 439, 724-727 (2005).
- Gilbert, M. T. P. et al. Science 317, 1927-1930 (2007).