I received my National Geographic magazine (January, 2012 issue) in the mail today and just read a brief the one page write-up they did on the mole titled Digit No. 6.
It outlines the findings of recent studies examining the mole‘s ‘second thumb’ or 6th ‘digit’. However, it differs from what can often lead to the addition of an extra digit in animals like humans, cats and dogs, known as polydactyly, meaning ‘many fingers’. Instead, the moles have been found to have an elongated wrist bone, or modified sesamoid bone, that works as a pseudo-thump to effectively give them a wider shovel to move more soil when they dig.
Unlike polydactyly in humans, which is a congenital physical anomaly caused by an autosomal dominant gene. Furthermore, all moles exhibit this phenotype, that is that every mole expresses this trait of having a 6th ‘digit’. Although these two conditions are caused by different genes, they have now been found to share a single genetic marker in common. What’s also interesting, and almost definitely an example of convergent evolution, two different groups of animals evolving the same trait separately, are the both species of panda (Giant panda and Red panda).
However, what’s interesting is that for these species of panda the modification of their sesamoid bone to form a 6th ‘digit’ occurred under different selection pressures. It evolved to allow them to more easily grasp and manipulate their staple food bamboo.
If suddenly it became incredibly beneficial for humans to possess polydactyly, say if baseball became the end all of success and women would only procreate with the best catchers, we too as a species could end up with a 6th digit reaching fixation in our gene pool. And if it had happened earlier on in our evolutionary history we might have ended up with a completely different numerical system with base 12 instead of base 10.
But I think worst of all, “Hey man, High-6!” just doesn’t have the same ring to it though.