Many people think that the catastrophe that killed off the dinosaurs 65 million years ago--probably a giant asteroid--also took out most archaic birds and mammals, too. If so, modern birds evolved only later, from the survivors. But a report in tomorrow's Science* comes to a different conclusion: that a diverse flock of birds flew through the cataclysm unscathed.
At least 21 different avian lineages--including parrots, wrens, and penguins--survived the devastation, according to molecular evolutionist Alan Cooper of Oxford University and theoretical biologist David Penny of Massey University in New Zealand, who used differences in the DNA of modern birds as a "molecular clock" to determine the age of each lineage. Cooper and Penny looked at 16 orders of birds, using two small fragments of genes--600 base pairs of a protooncogene in the nucleus, and a 390-base pair region from the mitochondrion, a cellular organelle that has its own DNA.
First, the scientists measured the differences in the same snippet of gene between a pair of closely related birds, such as rheas and ostriches. They dated the split between these two using the earliest known fossil of each pair. For example, the oldest rhea dates to more than 60 million years ago, so the researchers reason that it took at least 60 million years to accumulate the 57 mutations that distinguish the rhea and ostrich genes. They analyzed another pair of closely related birds, such as loons and shearwaters, the same way, then averaged the mutation rates of the two pairs. By counting the additional mutations between the two pairs--38 in this case--and using their average mutation rate of about one per million years, the researchers concluded that the last common ancestor of all four birds must have lived more than 98 million years ago. That's some 30 million years before the catastrophe.
But because the molecular clock has to be calibrated with fossil dates, there's plenty of room for disagreement. Ornithologist Alan Feduccia of the University of North Carolina, Chapel Hill, for one, flatly dismisses the study as "a gross misrepresentation of the fossil record." For example, Feduccia says that a Cretaceous fossil identified as a 70-million-year-old loon, used to date the loon-shearwater split, may not be a loon at all. And the dates used for other bird fossils, such as the oldest loons and ducks, are too old, he says.
Other experts have more faith in the data used: "The fossil record is accurate," says vertebrate paleontologist Luis Chiappe of the American Museum of Natural History. "The dates are good." And if the findings hold up, says molecular evolutionist Svante PŠŠbo of the University of Munich in Germany, they "could change the textbook view that almost all the groups of birds died out."
* For more details, Science Online subscribers can link to the full text of the Report.
