Birds may have originated 40 million years earlier than previously believed

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Washington, Feb 6 : A new analysis by researchers has ffered the strongest molecular evidence yet for an ancient rigin of modern birds, suggesting that they arose more than 100 illion years ago, not 60 million years ago, as fossils suggest.

Researchers at the University of Michigan, the University of hicago, the Centre for Biodiversity Conservation Mexico and entral America, and Boston University, did the analysis.

According to Joseph Brown, who is first author on the paper egarding the findings of the analysis, "Scientists typically use wo sources of information to date biological events: the fossil ecord, which contains physical remains of ancient organisms, and olecular genetic data."

In the case of modern birds, however, the two approaches have ielded conflicting results, at times leading to heated debates etween paleontologists and molecular biologists.

While molecular biologists have asserted that the fossil record ust be incomplete, paleontologists have countered that the enetic data must be suspect.

"In fact, both approaches have their weaknesses," said Brown. ccording to him, fossils tend to underestimate how much time has assed since lineages diverged.

As for genetic data, the so-called molecular clock isn't quite as recise as once thought. The approach relies on the observation hat although mutations occur at random in the genome, when ooked at over long stretches of time they occur at a fairly onstant rate. Molecular biologists use that rate to reconstruct volutionary history.

"The problem is, different lineages can 'tick' at different ates, so applying a single rate to an entire tree could lead to ery suspect results," said Brown.

Fortunately, new methods exist for compensating for differing ates.

"What my colleagues and I did was apply all of these new methods o the problem of the origin of modern birds, with each method aking different assumptions about how mutation rate changes cross the tree," said Brown.

He hoped the analysis would narrow the gap between fossil and olecular data, but in fact it led to the finding that modern irds arose more than 100 million years ago.

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