The Descent of Birds

By Carl Zimmer|Sunday, October 01, 1995
RELATED TAGS: GENETICS
Paleontologists have been arguing about where birds come from for more than a century. Most of them think birds are descended from small carnivorous dinosaurs, but even that is in dispute. And now a lively controversy has arisen about what happened to birds in the 145 million years after Archaeopteryx, the earliest incontestable bird, appeared. Were most birds wiped out 65 million years ago, along with the dinosaurs themselves, by the mass extinction that ended the Cretaceous Period--such that living birds are descended primarily from a small group of shorebirds that managed to survive the catastrophe? Or, on the contrary, did the major bird groups originate long before the extinction, through which they passed relatively unharmed? Bird fossils and bird genes are telling different researchers different things.

Although only a few modern-looking bird fossils have been found from the Cretaceous, the conventional wisdom has been that modern bird groups did originate before the mass extinction. The current distribution of some bird orders has provided the strongest support for this view. For example, ostriches in Africa, rheas in South America, and emus in Australia all belong to an order called the ratites. Since none of these birds can fly, the simplest explanation for their scattered distribution is that their common ancestor, the earliest ratite, was already living on the southern supercontinent Gondwanaland before it broke up 80 million years ago. Similar arguments suggest that other bird orders, such as galliforms-- chickens and their ilk--also started on Gondwanaland roughly 100 million years ago.

But according to Alan Feduccia, a University of North Carolina ornithologist, the fossil evidence tells a different story. The only Cretaceous fossils that look decidedly modern, he says, belong to an obscure group of shorebirds that were similar to today’s stone curlews. On the other hand, since 1980, Feduccia and other researchers have found quite a few Cretaceous fossils of a different kind of bird, called an enantiornithine. The name means opposite bird: whereas modern birds have a pair of elongated foot bones that are partially fused from the bottom up- -the fusion may make the foot stronger--enantiornithines had foot bones that were partially fused from the top down. (They also had fat, fleshy tails and in most cases teeth.) Opposite birds originated just after Archaeopteryx and spread around the world. But they all died out in the Cretaceous extinction.

What all this suggests to Feduccia is a new picture of bird evolution. Between 145 and 65 million years ago, he now argues, virtually the only modern birds alive were the shorebirds; for those 80 million years a diverse batch of opposite birds dominated the avian world. Then the asteroid hit, darkening the sky with dust, filling it with acid rain, killing off most plants, and wiping out the opposite birds. Only the shorebirds survived, feeding on crabs and other marine life that weathered the asteroid winter--much as shrewlike mammals survived while the great dinosaurs starved.

For both birds and mammals, says Feduccia, the aftermath of the catastrophe was the same: the survivors evolved explosively, filling the ecological niches vacated by the dead. Within 10 million years, the shorebirds had given rise to the major water-bird and land-bird lineages-- to the ancestors of owls, hawks, penguins, ducks, flamingos, and parrots. None of these forms have been found in the Cretaceous fossil record, Feduccia says, because none of them existed in the Cretaceous. All of a sudden this theory makes sense of what was an incomprehensible mush, he says.

Yet other researchers see the mush very differently. Just because no Cretaceous fossils of most modern bird orders have been found, says molecular biologist Alan Cooper of the National Zoo, doesn’t mean that none exist; paleontologists have yet to really explore some of the most promising Cretaceous outcrops, in places like New Zealand and Antarctica. Cooper also finds it unlikely that new bird orders could have emerged as fast as Feduccia claims. One of the most rapid examples of bird evolution, he points out, has been observed on the Hawaiian Islands, where in the 5 million years since birds first arrived, new species have evolved to feed on various flowers and seeds. But nearly all the anatomic changes, says Cooper, have occurred in the beak and the head. The body has done nothing for 5 million years, he says. So if this is the extent of what you can get in 5 million years, it seems difficult to say that in 10 million years you can get everything from penguins to eagles to parrots.

Most of all, though, it is Cooper’s own work with bird genes that has made him doubt Feduccia. As species branch apart, their genes gradually accumulate different mutations. By measuring how many differences there are among modern birds, researchers can create a family tree that shows how those species diverged from common ancestors--and even when they diverged, if the researchers have some way of estimating how fast the genetic changes occurred.

Cooper and David Penny, a theoretical biologist at Massey University of New Zealand, collected feathers and tissue samples from birds belonging to 24 of the 30 modern orders. From each sample they extracted one particular, well-studied bit of DNA that is common to nearly every organism on Earth. (It codes for a component of the ribosomes, the cellular protein factories.) By analyzing the base sequence of the DNA chain from each feather or tissue sample, Cooper and Penny determined how many base changes separated one bird order from another, and one species from another within the same order.

What they found leads them to support the conventional wisdom on bird evolution, and to doubt Feduccia’s contention that shorebirds are the only ones with Cretaceous roots. Within the galliform order, for example, Cooper and Penny found that on average the DNA sequence of any two modern species differed by 20 to 25 bases. They found the same degree of difference among ratite species. Since the oldest known galliform and ratite fossils are around 55 million years old, that tells Cooper and Penny that it takes at least 55 million years for 20 to 25 mutations to occur in bird DNA.

But when they compared galliform DNA with ratite DNA, they found an average difference of 20 bases as well. If the DNA clock was ticking steadily, that tells them the common ancestor of galliforms and ratites must be about 50 million years older than the oldest known fossils of the two groups. No one has found fossils of this common ancestor, but Cooper and Penny are confident it lived in the Cretaceous. Their analysis of DNA from other bird orders yielded similar results. At a meeting of evolutionary biologists in July, Cooper reported that most modern bird groups are 120 million years old--twice as old as Feduccia claims.

The genetic work, of course, is far from unassailable. For one thing, the DNA sequence Cooper and Penny studied was only 400 bases long-- too short, perhaps, to serve as a thoroughly reliable genetic clock. Cooper’s entitled to his opinion, but in my opinion he’s probably incorrect, Feduccia says. Here’s the point: there are almost no fossils of modern bird orders that have been identified before 65 million years ago. He’s the one speculating, not me.

One man’s speculation is another man’s data--and there, for the moment, the great debate on bird evolution rests. There’s definitely a schism between the paleontological work and the molecular work--there’s no two ways around that, says Cooper. The thing is, the more work we do in both fields, the bigger the schism gets.
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