Some say it's the last holdout for a handful of scientists still not convinced that birds evolved from dinosaurs. The fingers of the two groups of animals, they say, just don't match up. As embryos, birds seem to develop the equivalent of our middle three fingers, but theropods—two-legged, primarily carnivorous dinosaurs from which birds are thought to have evolved—sport the equivalent of our thumb, index, and middle fingers. Now, a study of chick embryos shows that birds do indeed have thumb, index, and middle "fingers" in their wings.
"This very important paper is the final nail in the coffin for the remaining supporters of the nondinosurian origin of birds," says Kasper Lykke Hansen, a vertebrate zoologist at the University of Copenhagen.
The flap over bird fingers has been going on for more than a century. Given how evolution proceeds, birds should have the same fingers as their dino ancestors, but that's not what some researchers have found. In 1997, Ann Burke, now an evolutionary morphologist at Wesleyan University in Middletown, Connecticut, and Alan Feduccia, an ornithologist at the University of North Carolina, Chapel Hill, pointed out that in all five-fingered animals they looked at, the fourth digit—the equivalent of our ring finger—was the first to form. This was true of the chick leg, which has four toes, and the chick wing. Based on their similarity to the toes, the other two fingers in the bird wing were the index and middle fingers, the duo concluded. For some, the paper called into question the theropod-bird link.
Two years later, evolutionary developmental biologist Günter Wagner and paleontologist Jacques Gauthier, both of Yale University, analyzed birds and dinos and concluded that even if the fingers didn't match, birds still evolved from dinosaurs. They share many skeletal features, such as hollow bones, as well as feathers and nest-building behaviors. To explain the discrepancy in the fingers, they proposed a compromise. Early in development, the avian hand looked to have the middle three digits. But by the end of development, the cells in the middle three finger positions changed their plans, growing into thumb, index, and middle digits instead.
In a paper published online today in Science, Koji Tamura, a developmental biologist at Tohoku University in Sendai, Japan, his graduate student Naoki Nomura, and their colleagues looked into this possibility. They transplanted embryonic tissue from the legs to the wings of 3½-day-old developing chicks and vice versa and watched how the transplanted tissue developed. Then they labeled cells in even earlier embryos to track where they wound up.
The biologists focused on a group of cells along the outer edge of the limb, called a developmental "organizer." These cells make a protein called sonic hedgehog, which diffuses outward to other cells. Cells respond to this protein by growing differently depending on how much of it they're exposed to, and for how long. This allows the organizer to control the number and shape of the bones that ultimately make up a digit.
Tamura and Nomura discovered that the organizers from the wing and leg didn't work the same way. Instead, the leg organizer also carried a few extra cells that gave rise to the ring-finger digit, while the wing organizer had no such cells. This meant the wing did not have a ring finger. The outermost digits in the two limbs "must be different digits," says Tamura.
Next, the researchers labeled cells in 3-day-old embryos and followed where they wound up in the chick. They discovered that cells destined to become the middle-finger digit in the wing started in the organizer. If they had stayed put, they might have given rise to the ring-finger digit. Instead, they migrated beyond the organizer's boundaries to a position where, later on, high concentrations of sonic hedgehog turned them into the middle finger. This fits with Wagner and Gauthier's scenario, notes Tamura.
Feduccia disagrees with Tamura's conclusions. "Something very complicated is happening, but renaming digit identity based on these findings would be extremely premature," he says. Frietson Galis, an evolutionary developmental biologist at Leiden University in the Netherlands, is likewise unconvinced. "Their conclusions should be considered with some caution," she says. She thinks birds have the middle three digits and would like to see the fossil record reexamined, as she suspects dinosaurs might really have the middle three digits as well.
But others are satisfied. This "compelling evidence ... resolves this long-standing controversy" between the fossil record and developmental studies, says Sankar Chatterjee, a paleontologist at the Museum of Texas Tech University in Lubbock.