The interrupted gene expression pathways are involved in transforming birds’ premaxillae—facial bones that for part of the upper jaw in most vertebrates—into myriad beak forms in about 10,000 species of modern birds. As dinosaurs evolved into birds some 150 million years ago, the premaxillae elongated and fused together to form beaks. “Instead of two little bones on the sides of snout, like all other vertebrates, it was fused into a single structure,” study coauthor and Yale paleontologist Bhart-Anjan Bhullar—now at the University of Chicago—told Nature. Abzhanov, Bhullar, and their colleagues reasoned that by altering gene expression in facial regions that eventually give rise to the premaxillae, they could affect beak formation and, in essence, turn back the evolutionary clock to encourage the formation of a more primitive facial structure.
But the altered chick embryos didn’t look exactly like dinosaurs, according to Bhullar. “Looking at these animals externally, you would still think it’s a beak. But if you saw the skeleton, you’d just be very confused,” he told Nature. “I would not say we gave birds snouts.”
Nevertheless, researchers in the paleontology community shared their fascination with the work. “I think it’s fantastic,” Montana State University paleontologist Jack Horner told The New York Times. “Very cool,” agreed Clifford Tabin, a developmental biologist at Harvard Medical School, in an interview with Nature.
But at least one developmental biologist wasn’t as excited about the newly published research. The University of California, San Francisco’s Ralph Marcucio told The New York Times the work held promise, but he wasn’t convinced that the authors had zeroed in on the evolutionary key to beak formation. Because Abzhanov, Bhullar, and their colleagues used chemicals with potentially toxic side effects to block the expression pathways in the chicks, the resultant morphology could have simply been due to damaged tissue, said Marcucio. And at least one of the proteins targeted in the experiment disappears long before premaxillae begin to develop, making it unlikely that it has a direct role in beak formation. “It’s a simple kind of thing, but when you look at the actual pieces of data, it tends to fall apart,” Marcucio said. “It takes away from the complexity that’s the reality.”
Bhullar told Nature his team plans to investigate the expansion of the mammalian skull and the unique structure of crocodiles’ lower limbs by taking similar reverse-evolution approaches. “I think it will open as big a window as you could possibly get into the deep past without having a time machine,” he said.