Carl Zimmer in the New York Times reports some provocative-sounding news under an attention-grabbing headline: “Reverse Engineering Birds’ Beaks into Dinosaur Bones.” Zimmer asserts: “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.” That would be significant, if true.
But commenting on the same journal article, Nature tells a different story. Bhart-Anjan Bhullar, one of the main scientists involved in the experiments, specifically denies that they produced snouts in these chickens:
“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 says. “I would not say we gave birds snouts.” (emphasis added)
Nature confirms that some of the altered embryos still looked like normal chicks:
In some embryos, the premaxillae were partly fused, whereas in others the two bones were distinct and much shorter; some of the altered embryos did not look all that different from those of regular chickens.
While Zimmer fails to point out that the experimenters themselves acknowledged they didn’t produce snouts, and that the results were inconsistent, he does quote other scientists who are skeptical of the research:
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.”
Those are very reasonable criticisms. The experiments in question may reveal nothing more than evolution by intelligently designed subtraction. The researchers know what bird beaks look like and what dinosaur snouts look like. They know that in bird beaks the premaxillae are enlarged and fused into a single bone, while in dinosaur snouts they’re not, so there’s a spatial gap between the two. With this in mind, the researchers put a microscopic bead in the middle of the developing chick embryo’s face, specifically designed to release chemicals to inhibit the activity of two proteins — Fgf8 and Lef1 — that are involved in facial growth.
These chemicals prevented the genes from doing their normal jobs, and probably wreaked all kinds of havoc on the surrounding tissue. In some cases, they apparently prevented fusion of the premaxillae. This might cause the chick embryos to look a little more like a dino, but in reality, the inhibiting chemicals may simply have killed the tissue that, under normal conditions, fuses the bones. But the genetic scenario they’re trying to mimic may be completely removed from the supposed dinosaurian condition. The lesson? If you put harmful chemicals where birds have bones and dinosaurs did not, don’t be too surprised if in some birds the developing bone tissue dies and the result looks a little more like a dinosaur.
What’s more, as Science reports, the genes that these experiments were designed to inhibit probably are not even involved in beak development and fusion of the premaxillae:
In 2014, UCSF’s Nathan Young and Ralph Marcucio, working with Schneider, carried out extensive skull measurements on a variety of embryonic vertebrates and determined the point during development at which the bird face begins to diverge from those of other vertebrates. The work and later experiments supported a 2009 idea proposed by Marcucio that the activity of another gene, SHH (called sonic hedgehog), was critical for forming the beak. Unlike Fgf8, he says, it’s active in the right place and right time in bird embryos.
Marcucio, a developmental biologist, also worries that the changes in facial structure observed by the Harvard team may stem from unintended cell death caused by the inhibitors they used. “Adding the fossil record to this work is really an important step, but I think they are just looking at the wrong pathway,” he says. Abzhanov and Bhullar counter that Fgf8 and SHH are often coexpressed and may work together; also, they saw no excess of cell death.
Likewise, The Scientist puts it this way:
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.”
If Marcucio is correct then the experiments weren’t even turning off the correct genes involved in building a bird beak. Any similarities between the embryos and dinosaurs could then merely reflect how investigators interfered with normal beak development and killed off tissues with chemicals.
In that case, the fact that some of the chickens resemble dinosaurs might just reflect the fact that they placed tissue-killing chemicals in the bird embryos right where dinosaurs had a gap between bones. Perhaps the inconsistent results are explained by the degree to which the chemicals killed the tissue in different chicks.
If so, then these results would be of little significance, certainly of none in the cause of demonstrating that modern birds may trace their descent from dinosaurs.