Evolution adds and subtracts, and nowhere is this math more evident than in vertebrates, which are programmed to have five digits on each limb. Many species do not; birds have three but snakes have none at all.
Yale scientists now have a good handle on how these developmental changes are orchestrated in the embryo, but there is still one outstanding debate on birds: Which digits are they: a thumb with index and middle fingers, or the index, middle and ring fingers?
In five-digit vertebrates, the thumb comes from the precursor stem cells labeled pa. While birds have a digit that looks like a thumb, pa precursor cells die off during development and never produce a digit in adults. As a result, scientists have wondered whether precursor cells in pb can make a thumb.
Yale scientists have completed a genomic analysis of birds that reveals the answer (shown in orange on illustration). It is a hands down "yes" — even though the first bird digit develops where the index finger on a five-finger vertebrae should be.
A genomic analysis shows that precursor cells pb that form index finger in five-fingered vertebrates can form the "thumb" (in orange) or first digit in three-digit bird wing
(Photo Credit: courtesy Yale University)
Source: Yale University