Analiza DNA ssaków...
: 26 kwietnia 2007, o 14:40
Analiza DNA ssak??w potwierdze hipotezĂ?, i?? niekt??re z nich to ??ywe skamienia??o??ci:
Because of a combination of interest and improved technology, new species have been described at a fairly high rate in recent years. But new mammals remain fairly hard to come by; perhaps one new mammalian species a year is typical, and many of these are simply variants of well-described species. That's why the discovery of the Laotian rock rat was exceptional. Although it was clearly a rodent, it was visually distinct from the rodent species we're familiar with. When video of the rat appeared, its strange gait and comfort with humans enhanced the otherworldly impression it made. What exactly was this creature?
Its discoverers named it a new species. But a later publication suggested that the creature's strangeness wasn't because it was new, but rather because it might be old: it was proposed that the Laotian rock rat (Laonastes aenigmamus) was the last surviving member of a once-large group of rodents that was known only by fossils. Although the group had vanished from the fossil record 11 million years ago, the morphological similarities were striking. The rock rat, it was proposed, is a living fossil.
DNA sequence analysis has now joined the argument and comes down strongly in favor of the living fossil contention. Not only is the rock rat like nothing we've ever seen before, it's not much like anything we've ever sequenced before.
The authors of the new report sequenced a small set of genes (six genes totaling 5.5 kilobases) in species from every major group of rodents. They also examined a number of repetitive sequence elements from the same groups. The data suggested that the rock rat split from the rest of rodents about 44 million years ago. For context, all existing primates derive from a speciation event about 50 million years ago.
Given this sequence data, it appears that Laonastes is the only living member of an entire family of mammals, the otherwise extinct Diatomyidae. When I first reported on the rock rat, I suggested that it might provide a unique opportunity to test our ability to accurately resolve evolutionary trees based on little more than the appearance of fossils. It is a pleasure to report that the paleontologists got it almost exactly right: the relationships they proposed, as well as the dates of separation, are strongly supported by the new molecular data. This suggests that we can view relationships proposed solely due to fossil evidence with a bit more confidence.