Almost all land vertebrates have five fingers, but moles flout this rule. On top of their five digits, the creatures have co-opted a wrist bone to evolve a pseudo-thumb that increases hand-surface area for digging.
Marcelo Sánchez-Villagra at the University of Zürich in Switzerland and his colleagues tracked key molecular markers in embryos of the Iberian mole (Talpa occidentalis; pictured) and the North American least shrew (Cryptotis parva), a close relative that lacks the long, sickle-shaped bone. They found increased expression of Msx2, a gene that promotes digit development, in the area of the developing mole paw in which a wrist bone becomes elongated. The gene product was absent from this region in the shrew.
The pseudo-thumb is not technically a sixth digit, because it comes from a wrist bone, and develops later than the five true digits.
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How the mole got its 'thumb'. Nature 475, 142 (2011). https://doi.org/10.1038/475142c