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Reverse Engineering Birds’ Beaks Into Dinosaur Bones
New York Times, May 12, 2015

Birds evolved from dinosaurs 150 million years ago, a slow but thorough transformation. Their bodies gained aerodynamic feathers, their digits fused into wings, and they acquired a beak used to gather food.

We can see some details of this evolutionary marvel in the fossil record. Yet even the most exquisitely preserved fossil can’t tell us which pieces of DNA had to change in order to turn ground-running dinosaurs into modern birds.

Some researchers are now trying to pinpoint those genetic changes with experiments on chicken embryos. If the scientists succeed, they should eventually be able to reverse the evolution of birds — and then they may be able to engineer animals more at home in “Jurassic Park” than in a henhouse.

One group of these scientists, led by Bhart-Anjan Bhullar of Yale University and Arhat Abzhanov of Harvard University, has spent the past eight years investigating one piece of bird anatomy in particular: the beak. Now, in a study published in the journal Evolution, they report that they have found a way to turn the beaks of chicken embryos back into dinosaur-like snouts.

In interviews, some experts hailed the new research for providing insights into the evolution of birds. But others were skeptical, arguing that the real genetic changes behind the bird beak have yet to be discovered.

The beak evolved fairly late in bird evolution, after early birds had already evolved feathers and powered flight. It originated from a pair of small, separate plates of bone sitting at the front of the upper jaw. In our own skulls, these bones — called premaxillae — anchor some front teeth.

During the evolution of early birds, the premaxillae stretched out and fused together to form a strong, lightweight beak. Muscles that anchored the new beak to the back of the head allowed birds to control this sophisticated tool.

Since then, birds have evolved many uses for beaks. Woodpeckers hammer into trees to find insects. Pelicans use their beaks like fishnets. Hummingbirds evolved slender sipping straws.

Dr. Bhullar and Dr. Abzhanov set out to find some of the genetic changes that turned the dinosaur premaxillae into a beak. To find clues, they looked at earlier experiments on chicken embryos. These studies have documented how embryonic cells make certain proteins at certain times.

The scientists were struck by the fact that even before the embryo has a developed, recognizable face, a large patch of cells in the middle of what will become the bird’s face makes a protein called Fgf8. Later, the region produces different proteins, called Lef1.

Mice, other studies have shown, also make Fgf8 and Lef1. But mice produce them in a pair of small, separate cell patches, not a single large patch.

Like the embryos of chickens, those of emus produce the proteins in a single patch of cells, the scientists learned. But in animals other than birds — such as turtles, lizards and crocodiles — the proteins are usually made in a pair of small cell patches.

Was it possible, the scientists wondered, that a key step in the evolution of beaks was a shift from small protein-producing patches to a single large one? That change might have allowed birds to develop big, fused premaxillae — the precursors of beaks.

If the hypothesis was correct, the researchers figured, they might be able to turn back the clock on evolution. If they caused a chicken embryo to use Fgf8 and Lef1 the way other animals do, it should turn out to be a bird without a beak.

“It shouldn’t produce some kind of monster,” said Dr. Bhullar. Instead, he and Dr. Abzhanov predicted, the chickens should develop skulls more like those of their dinosaur ancestors.

To reverse evolution, the scientists gently wedged a microscopic bead into the middle of what would become the faces of chicken embryos. The bead released chemicals into the surrounding tissue that interfere with Fgf8 and Lef1.

As they had predicted, the chicken embryos failed to develop beaks. Instead, the embryos gained a pair of rounded, unfused bones — more like what you might have found on a dinosaur’s head.

“I think it’s fantastic,” John R. Horner, a paleontologist at Montana State University, said of the finding.

In 2009, Dr. Horner predicted that scientists would someday be able to turn chickens into dinosaur-like forms in a book entitled “How to Build a Dinosaur: The New Science of Reverse Evolution” (co-authored with James Gorman, a science reporter at The New York Times).

Now researchers are using experiments on embryos not just to understand the origin of birds, he said, but also a number of major evolutionary transitions. “It’s an exciting time, and I envy people in the beginnings of their careers,” Dr. Horner said.

Ralph S. Marcucio, a developmental biologist at the University of California, San Francisco, agreed with Dr. Horner that these experiments held promise, but said he was not persuaded by the new study.

Dr. Marcucio noted that the scientists used chemicals to block Fgf8 and Lef1 proteins that have toxic side effects and can kill cells. The altered anatomy of the chicken skulls might not be an example of reverse evolution, he said, just dying tissue.

Dr. Marcucio also doubted that Fgf8 and Lef1 could have such a big impact on the beak. Fgf8, for example, disappears from the region that will become the face long before the premaxillae develop.

“It really makes me suspicious that it’s not involved in some kind of switch,” he said.

Dr. Marcucio predicted that the true story of the origin of beaks would turn out to be much more complicated than the new experiment suggests, involving other genes.

“It’s a simple kind of thing, but when you look at the actual pieces of data, it tends to fall apart,” he said. “It takes away from the complexity that’s the reality.”

More sophisticated experiments one day may settle this disagreement. If Dr. Bhullar and Dr. Abzhanov are right, birds must have some special sequence of DNA that regulates Fgf8 and Lef1 in a peculiar way. Ultimately, it should be possible to find that sequence.

By looking at alligators and other close living relatives of birds, scientists may be able to infer the original sequence in the ancestor of birds. If the ancient DNA can be restored, chicken embryos could once again develop dinosaur-like snouts instead of beaks.

“I’m enormously eager to find the regulatory changes themselves,” Dr. Bhullar said. “These techniques were in their infancy when I started the project, which shows you how fast the field is changing.”

Copyright 2015 The New York Times Company. Reproduced with permission.
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