Ancient tooth reveals elephants' family tree.
The mastodon, an extinct relative of modern elephants, has become the latest prehistoric animal to have its DNA sequenced.
Using a fossilized tooth, Michael Hofreiter of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues sequenced all the DNA of the mastodon's mitochondrion, an energy-generating structure in the cell with its own small genome.
The tooth is believed to be 50,000-130,000 years old, making the mastodon's the oldest complete mitochondrial genome decoded so far.
"This extremely old and complete sequence is of interest in its own right," says Hofreiter. But it can also help to resolve debates about the ancestry of modern elephants.
Scientists have been unable to agree how the Asian elephants, African elephants and woolly mammoths are related. The problem is that elephants have no living close kin — their nearest relatives are the ocean-going dugong and the rodent-like hyrax.
But a family tree based on the DNA of elephants, mammoth and mastodon shows that Asian elephants are more closely related to mammoths than they are to African elephants. The results are reported in PloS Biology1.
A change in climate
The mastodon sequence also enabled the team to estimate that the African elephants split from Asian elephants and mammoths about 7.6 million years ago. Mammoths and Asian elephants parted ways less than a million years after this.
These dates — and the African location of the split — are strikingly similar to the divergence of humans, chimpanzees and gorillas, says Hofreiter. "It's thought that the climate became drier, grassland expanded and forests became more fragmented," he notes. "We need to look in more detail at what happened in other mammals."
Hofreiter's team found the mastodon tooth in a riverbed in northern Alaska, in sediment dated at 130,000 years old; the rest of the skeleton was missing.
The DNA sequence shows that mastodons split from the ancestor of the elephants about 25 million years ago.
Mastodons stood about three metres tall and looked a lot like mammoths, with thick fur and a stocky build. Unlike mammoths, however, they were mostly forest animals, browsing on leaves and breaking branches and twigs with their tusks.
The animals appear in the fossil record about 28 million years ago, and survived in North America until about 10,000 years ago.
The cause of their demise is controversial. "Some blame environmental change and others blame human hunters," says Hofreiter.
The oldest previous mitochondrial DNA genome sequenced belonged to a 33,000-year-old woolly mammoth. Moas, giant flightless birds from New Zealand that are thought to have died out by about 1500 AD, are the only other extinct species to have their complete mitochondrial genome decoded by scientists.
Eske Willerslev of the University of Copenhagen and his colleagues recently sequenced mitochondrial DNA from beneath Greenland's ice sheet dated at between 450,000 and 800,000 years old (see 'DNA reveals a green Greenland').
Some researchers have claimed finding 100-million-year-old bacterial DNA. But such claims do not convince Willerslev. "These haven't received independent verification and the DNA is 99.9% similar to modern bacteria so it's very difficult to tell," he says.
"Conceivably, DNA could survive for several million years at very low temperatures," says Willerslev. "But no one really knows — there are no reliable estimates of the maximum DNA survival time."
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Rohland, N., et al. PLoS Biol. 5, e207 (2007).
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Buckley, L. Mastodon DNA sequenced. Nature (2007). https://doi.org/10.1038/news070723-3