In “How the Mole Got Its Twelve Fingers” (ScienceDaily, Sep. 7, 2011) , we learn:
Polydactyly is a hereditary anomaly that is relatively common in both humans and animals.
That depends on what you mean by “relatively common.” Most humans would regard anyone with six fingers as abnormal.
Try putting a ring on the hand of a six-fingered person. Especially if it is destined for a particular finger.
As it happens, most additional digits are not functional. Still,
Moles also have additional fingers. In their case, however, the irregularity compared to the five-finger formula of land vertebrates is the norm. An international team of researchers headed by paleontologists from the University of Zurich has now uncovered the background to the development of the mole’s extra “thumb”: A bone develops in the wrist that stretches along the real thumb, giving the paw a bigger surface area for digging.
So it is a power assist for the thumb. For example,
Unlike the other fingers on the mole’s hand, the extra thumb does not have moving joints. Instead, it consists of a single, sickle-shaped bone. Using molecular markers, the researchers can now show for the first time that it develops later than the real fingers from a transformed sesamoid bone in the wrist. In shrews, however, the mole’s closest relative, the extra thumb is lacking, which confirms the researchers’ discovery.
So the extension isn’t a “real finger” exactly.
The five-fingered limb is much easier to subtract from than add to. That’s the fundamental problem with Darwinian evolution. There is an original law, probably based on design.
See also: Evolution is smart enough to do subtraction, not addition
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