A study of a woolly mammoth that died in Siberia several millennia ago has yielded the complete DNA sequence of the creature’s mitochondria, the energy factories of the animal’s cells. Comparison with the mitochondrial genomes of living elephants indicates that the mammoth is slightly more closely related to the Asian elephant than to the African elephant.
Fossil evidence had suggested that woolly mammoths and the living species of elephants descended from a common ancestor that lived in Africa about 6 million years ago, but the relationship among the three species remained unclear, says Michael Hofreiter, a paleontologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Most anatomical studies suggest that the mammoth is more closely related to the Asian elephant, but analyses of the small amounts of fragmented mammoth genetic material available had hinted at a closer connection with African elephants.
Now, Hofreiter and his colleagues have developed a way to reconstruct a genome from extremely small samples of tattered DNA.
The researchers began with 200 milligrams of bone from a 12,000-year-old mammoth unearthed in northeastern Siberia. They then applied a biochemical reaction that simultaneously made multiple copies of the diverse fragments of mitochondrial DNA from the tiny sample rather than just one fragment at a time. Because the fragments overlapped in many places, the scientists succeeded in sequencing the entire ring-shaped molecule of the mammoth’s mitochondrial DNA, which contains about 16,770 base pairs.
While about 95.5 percent of the mammoth’s mitochondrial DNA matches that of the African elephant, more than 95.8 percent corresponds to that of the Asian elephant. The tiny difference suggests that the last common ancestor of mammoths and Asian elephants lived about 440,000 years after that creature’s ancestors split from the lineage that led to African elephants, Hofreiter and his colleagues report in the Feb. 9, 2006 Nature.
The new findings are “technically outstanding . . . and very impressive for ancient DNA,” says geneticist Alfred L. Roca of the Laboratory of Genomic Diversity in Frederick, Md.
Nevertheless, speculations about an animal’s family tree that are based on mitochondrial DNA can be misleading, especially when bouts of evolutionary branching took place rapidly in geologic time, says Stephen J. O’Brien, also of the genomics lab in Frederick. He suspects that a comparison of DNA from cell nuclei would provide a more credible account of mammoth and elephant kinship.
Indeed, scientists are now turning their attention to that mammoth trove of DNA information. Using a new generation of high-speed genetic-sequencing machines, evolutionary geneticist Hendrik N. Poinar of McMaster University in Hamilton, Ontario, and his colleagues have assembled partial sequences of one mammoth’s nuclear DNA that add up to about 13 million base pairs.
That’s less than 1 percent of the creature’s full genome, he notes, but at the rate at which the researchers are going, they could put together its entire genome in the next couple of years.
Of the DNA they’ve sequenced so far, about 98.5 percent matches the corresponding stretches of DNA in the African elephant, Poinar and his team report in the Jan. 20, 2006 Science. The nuclear genome of the Asian elephant hasn’t yet been fully compiled.