Origin of Birds Debated
By Clara Moskowitz - LiveScience
Birds are living dinosaurs, nearly all scientists agree, but a debate still continues about when that first early bird glided or flew into the Mesozoic scene.
Paleontologists who study fossils think the first modern birds evolved from dinosaurs about 60 million to 65 million years ago, right about the time most dinosaurs went extinct. But biologists who investigate DNA measure the origin of birds at about 100 million years ago.
Scientists hoped that a new study analyzing all of the available genetic data with new statistical models might narrow the gap, but instead it has reinforced it and definitively put the DNA-dating estimate at 100 million years ago.
“It’s a robust estimate now,” said Joseph Brown, a biology graduate student at the University of Michigan who led the study. “We know that this gap between the fossil record and the molecular data is a real gap. In the past people in both camps would just assume that the other side had gotten it wrong. But it seems now that the discrepancy is really genuine.”
The research was published online Jan. 28 in the journal BMC Biology.
Inherent errors
There are problems associated with both methods of dating.
The fossil record is never complete — just because diggers have yet to find fossils of birds from earlier than roughly 65 million years ago doesn’t mean there were none.
It’s also possible there are errors in the way scientists analyze DNA to discover when species emerged. The technique involves comparing genetic differences among birds to estimate how long ago they diverged as new species. Although genetic mutations occur at random, if studied over large periods of time, they seem to occur periodically.
“If we know, for example, that DNA sequences diverge by an average of two percent every million years, and we determine that two species differ genetically by ten percent, we can figure out that they last shared a common ancestor five million years ago,” Brown said.
The problem is, the periodic rate of mutation may vary among lineages, so if scientists apply a single rate to an entire genetic tree, they may miscalculate.
The new study aimed to compensate for the different rates by using five different statistical models, each built around slightly different assumptions. When each model independently arrived at the same date, the scientists believed it was a definitive measurement of the time birds genetically diverged from dinosaurs.
“This paper puts a pretty solid timescale at 100 million years,” Brown told LiveScience. “We can say the old dates that we were generating are not the results of mistreatment of data. Now we can interpret the fossil record differently.”
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