The Bird Wing Dilemma

For years, scientists have attempted to answer the question: which the five-fingered hand’s digits correspond to the bird wing’s three digits? Based on evolutionary and embryological evidence, two reasonable and convincing conclusions on the identity of these digits exist. However, Günter Wagner, Yale Professor of Ecology and Evolutionary Biology, and his team have provided new insight into this controversial subject.

From a paleontological standpoint, it appears as though the digits of the bird’s “hand” are digits I, II, and III. Archaeopteryx, a therapod dinosaur that was a close ancestor of the modern-day bird, had only three digits in the forelimb. The forelimb of Archaeopteryx’s close relative, Herrerasaurus, contained all five digits, with clearly reduced digits IV and V. From this observation, it seems that the three digits of Archaeopteryx were I, II, and III, leading many to conclude that the forelimb of the modern bird is comprised of these digits as well.

Contradicting this theory is the observation that the embryonic precursors of avian wing digits are characteristic of those that usually develop into digits II, III, and IV. Evolutionarily, too, the first digit is consistently the first to go, and the fifth is usually the second to be lost.

In order to explain this conclusion, those in favor of this hypothesis have argued that birds are not, in fact, related to Archaeopteryx or any other therapod dinosaurs. However, this does not help explain the hypothesis because scientists know with high certainty that birds are in fact descendents of therapod dinosaurs.
In order to resolve this conflict, Wagner and Jacques Gauthier, Yale Professor of Geology and Geophysics, have proposed that the birds’ digits have changed place. Although their new idea was not well-received initially, they have produced compelling evidence supporting it.

Forelimb (left) and hindlimb (right) digits in chickens. Photo Courtesy of Professor Günter Wagner.

It is clear that the digits of a bird’s foot are derived from digits I, II, III, and IV. Based on the idea that digit identity is related to genetic expression, Wagner compared the genes in the digits of the foot, or hindlimb, to the digits in the wing, or forelimb, in order to determine which digit in the hindlimb (I, II, III, or IV) corresponds to that in the forelimb.

Dr. Gunter Wagner, Yale Department of Ecology and Evolutionary Biology. Photo Courtesy of Professor Günter Wagner.

Wagner found that there is a clear similarity in genetic expression of digit I in both the hindlimb and forelimb. Surprisingly, what appears to be digit I of the forelimb is actually in the developmental position of digit II. His research suggests that in the evolution of birds, the first digit experienced a translocation from the original position of digit I to the position of digit II. Essentially, the expression of the regulatory genes for digit I simply occurs in the wrong place. Wagner’s work confirmed the idea that digits can change place and that position does not rigidly determine the identity of a digit.

The investigation of the identity of the other two digits in the avian forelimb proved much more difficult, as no clear genetic signal identifying them with specific digits of the hindlimb was present. It seems possible that the avian forelimb may be comprised of digits I, III, and IV. However, it is also possible that the last two digits have completely novel identities, only expressed in bird wings, and that they do not correspond to the digits of any other animal. There is no clear answer at the moment regarding these last two digits.

Wagner continues to research the nature of the genetic changes that resulted in the shift in position of digit I and hopes to better understand the identities of digits IV and V.