how many fingers does a bird have

In most tetrapods (land vertebrates) the fourth (ring) finger is the first to develop in the embryo. And in birds, the finger on the outside of the hand (posterior, the pinky side) appears first, which suggests that this is the ring finger. However, it can be shown that on the thumb side (anterior) an embryonic finger begins to develop, but quickly disappears—this would have to be digit I. These data argue for an identification of the fully-formed fingers as the index, middle, and ring fingers (II, III, IV).

However, the three fingers of the earliest known bird—Archaeopteryx—resemble those of the dinosaur Deinonychus, with whom Archaeopteryx was probably closely related (as was the famous Velociraptor). Successive fossils show the reduction of two fingers on the posterior side of the hand in the ancestor of Deinonychus, and thus support the thumb, index, and middle finger identification (I, II, III) of the bird fingers. Also, the genes active in first bird finger correspond with those of the developing thumb in other animals, and not those of the index finger.

To resolve this contradiction, three approaches had predominated: 1) Birds do not come from dinosaurs after all; 2) the dinosaur ancestors of birds had also the three middle fingers (II, III, IV); or 3) the three anterior fingers (I, II, III) of the birds were somehow moved to the middle three embryonic positions. In fact, none of these theories can explain all the existing data.

For the birds: Thumb, index, and middle fingers are actually index, middle, and ring fingers. “The appearance—the so-called phenotype—of the fingers is determined during embryonic development by the signalling protein Sonic Hedgehog, which emantes from the posterior side of the developing limb before any fingers appear. This simply means that the concentration of this protein on the little-finger side is highest and decreases toward the future thumb. Therefore each finger precursor (the cells that will develop into the finger) adjusts its gene expression—and in consequence, its phenotype—according to the Sonic Hedgehog concentration in its immediate environment. We have devised a hypothesis based on molecular and biomechanical mechanisms that is able to explain all the available data,” said lead author Daniel Capek, who conducted research in the group of Dr. Brian Metscher and Prof. Gerd Müller in the Department for Theoretical Biology at the University of Vienna for this project and is currently a Ph.D. student at IST Austria.

According to this hypothesis, a posterior reduction in dinosaur evolution actually proceeded with the little finger being reduced and then lost, and the ring finger was partially reduced. However, it is generally much easier to reduce the outer fingers than the more central ones, as they appear later in development. Thus the first finger is reduced (“thumbs down”) instead of the fourth finger, leaving an anterior area open for the early precursors of the other fingers to grow into. This would lead those developing fingers to encounter an anterior-like Sonic Hedgehog concentration and then to develop accordingly—more like I, II, and III.

“This mechanism explains why the fingers of Archaeopteryx and modern birds have the shapes of the anterior fingers (I, II, III), even though they are actually the central fingers (II, III, IV). At the same time, this hypothesis is consistent with the fossil findings and matches the current developmental genetics results,” says Brian Metscher of the Department of Theoretical Biology at the University of Vienna.

More information: Publication in Journal of Experimental Zoology: Čapek, D., Metscher, B. D., and Müller, G. B.: Thumbs Down: A Molecular-Morphogenetic Approach to Avian Digit Homology. Journal of Experimental Zoology Part B (Molecular and Developmental Evolution), December 2013. DOI: 10.1002/jez.b.22545

Study of animal digits helps shore up evidence that birds evolved from dinosaurs

Some claim it’s the final straw for the few scientists who aren’t ready to give up on the theory that birds descended from dinosaurs. They claim that the two groups of animals’ fingers simply don’t match up. Theropods, the two-legged, mostly carnivorous dinosaurs from which birds are believed to have descended, have the thumb, index, and middle fingers, whereas birds appear to develop the equivalent of our middle three fingers as embryos. Currently, research on chick embryos demonstrates that birds’ wings do, in fact, have thumb, index, and middle “fingers.”

“This significant study is the last straw for those who still believe that birds originated outside of the Dinosaur,” says University of Copenhagen vertebrate zoologist Kasper Lykke Hansen.

For over a century, there has been a flap over the fingers of birds. According to the theory of evolution, birds should have fingers similar to those of their dinosaur ancestors, but certain researchers have discovered otherwise. The fourth digit, or our ring finger, was the first to form in all five-fingered animals that Ann Burke, an evolutionary morphologist now employed at Wesleyan University in Middletown, Connecticut, and Alan Feduccia, an ornithologist at the University of North Carolina, Chapel Hill, observed in 1997. This applied to both the chick wing and leg, which have four toes. The pair deduced that the other two fingers in the bird wing were the index and middle fingers because of their resemblance to the toes. For some, the paper called into question the theropod-bird link.

Two years later, paleontologist Jacques Gauthier and evolutionary developmental biologist Günter Wagner of Yale University examined birds and dinosaurs and came to the conclusion that birds still descended from dinosaurs despite their dissimilar finger anatomy. They are similar in many skeletal traits, like hollow bones, feathers, and nest-building habits. They suggested making a concession in order to clarify the difference in the fingers. The middle three fingers of the avian hand appeared to be present early in development. However, by the time development was complete, the cells in the middle three finger positions had altered their original plans and were developing into the thumb, index, and middle digits.

Koji Tamura, a developmental biologist at Tohoku University in Sendai, Japan, his graduate student Naoki Nomura, and their colleagues investigated this possibility in a paper that was published online in Science today. They observed how the transplanted tissue developed after transferring embryonic tissue from the legs to the wings of developing chicks that were three and a half days old. After that, they labeled cells in even earlier embryos to follow their eventual fate.

The biologists concentrated on a collection of cells known as a developmental “organizer” that is located near the limb’s outer edge. These cells produce a protein known as “sonic hedgehog,” which spreads to neighboring cells. Depending on the amount and duration of exposure to this protein, cells grow in different ways. This gives the organizer the ability to decide on the quantity and form of the bones that eventually comprise a digit.

Tamura and Nomura found that the leg and wing organizers didn’t function in the same manner. As opposed to the wing organizer, which lacked any such cells, the leg organizer also contained a few additional cells that gave rise to the ring-finger digit. This meant the wing did not have a ring finger. According to Tamura, the two limbs’ outermost digits “must be different digits.”

Subsequently, the scientists annotated cells in embryos that were three days old and tracked their movement within the chick. They found that the cells that would eventually form the wing’s middle finger digit originated in the organizer. Had they remained in that location, they could have produced the ring-finger digit. Rather, they moved outside the boundaries set by the organizers to a spot where, subsequently, a lot of Sonic Hedgehog made them become the middle finger. This fits with Wagner and Gauthiers scenario, notes Tamura.

Feduccia disagrees with Tamuras conclusions. “Renaming digit identity based on these findings would be extremely premature, but something very complicated is happening,” he says. Similarly skeptical is Frietson Galis, an evolutionary developmental biologist at Leiden University in the Netherlands. “Their conclusions should be considered with some caution,” she says. She believes that birds possess the middle three digits and would like to see the fossil record reexamined because she believes dinosaurs may also possess these digits.

But others are satisfied. This “compelling evidence . “resolves this long-standing controversy” between developmental studies and the fossil record, according to paleontologist Sankar Chatterjee of Texas Tech University’s Museum in Lubbock.

Liz Pennisi is a senior correspondent for Science who covers a wide range of biological topics.

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The fourth (ring) finger is the first to develop in the embryo in the majority of tetrapods (land vertebrates). Additionally, the ring finger of birds is suggested to be the finger that emerges first on the outside of the hand (posterior, or pinky side). Nonetheless, it can be seen that an embryonic finger rapidly vanishes from the thumb side (anterior)—this would have to be digit I. These data support the designation of the index, middle, and ring fingers as the fully formed fingers (II, III, IV).

In the case of the birds: The thumb, middle, and index fingers are actually the middle, ring, and index fingers. Before any fingers appear, the developing limb’s posterior side emits the signaling protein Sonic Hedgehog, which controls the appearance of the fingers during embryonic development. This process is known as phenotyping. This simply indicates that the little finger side has the highest concentration of this protein, whereas the thumb side has a lower concentration. Therefore, in response to the concentration of Sonic Hedgehog in its immediate environment, each finger precursor (the cells that will develop into the finger) modifies its gene expression and, consequently, its phenotype. We have developed a theory based on molecular and biomechanical mechanisms that can account for all of the information that is currently available,” lead author Daniel Capek, a researcher in Dr. Brian Metscher and Prof. For this project, Gerd Müller, who is a Ph.D. candidate in the Department of Theoretical Biology at the University of Vienna D. student at IST Austria.

More information: Publication in Journal of Experimental Zoology: Čapek, D., Metscher, B. D., and Müller, G. B.: Thumbs Down: A Molecular-Morphogenetic Approach to Avian Digit Homology. Journal of Experimental Zoology Part B (Molecular and Developmental Evolution), December 2013. DOI: 10.1002/jez.b.22545

This theory suggests that during the evolution of dinosaurs, a posterior reduction actually took place, resulting in the partial reduction of the ring finger and the reduction and eventual loss of the little finger. But because the outer fingers develop later than the central ones, it is usually much easier to reduce the outer fingers. As a result, the first finger is shortened (“thumbs down”) rather than the fourth, creating an opening in the anterior region for the early forerunners of the other fingers to develop. This would cause those growing fingers to come into contact with an anterior Sonic Hedgehog concentration, at which point they would develop more in line with I, II, and III.

“This mechanism explains why, despite being the central fingers (II, III, IV), the fingers of Archaeopteryx and contemporary birds have the shapes of the anterior fingers (I, II, III). In addition, this theory aligns with the results of contemporary developmental genetics and the findings from fossil research, according to Brian Metscher of the University of Vienna’s Department of Theoretical Biology.


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