johac wrote:

This must be Mastodon Week in the scientific literature. Here's another
article on the big critters.

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Mastodon DNA sequenced

Ancient tooth reveals elephants' family tree.

Louis Buckley


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 Ð 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 Ð and the African location of the split Ð 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.

Ancient DNA

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 Ð there are no
reliable estimates of the maximum DNA survival time."



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http://www.nature.com/news/2007/070723/full/070723-3.html

Creationists will find 50K+ year-old teeth a little hard to swallow.

They'll just have to chew it over for a while.