Tuesday, July 19, 2011

The mole's new thumb

Close-up of mole Giant Panda Eating
Two instances of making do with what you have at hand.
With their huge hands, pointy snouts, and apparent lack of eyes, moles certainly appear to us as very peculiar creatures. These traits, however, are adaptations to a subterranean lifestyle. Those big front paws, in particular, are used for digging tunnels into the soil. Aside from being large relative to the rest of its body, the paws also have an elongated and enlarged radial sesamoid bone just beside the thumb. In some species of mole, this false 'thumb' (although it doesn't actually protrude as a distinct digit) is actually capable of some independent movement, spreading outwards to widen the hand.

This same wrist bone is also modified in giant pandas to form their peculiar 'thumb'. The panda uses its false thumb in feeding, to strip down the bamboo shoots that comprise its diet. (A photo of this in action can be found here.) This was the subject of a well-known essay by Stephen Jay Gould that later titled one of his published collections. As an example of what he called 'Tinkertoy evolution', it illustrates how evolution often makes do with existing structures to fulfill new functions. It is examples like these, which are less than perfect and highly contingent functional solutions, that break the illusion of a perfect world as posited by natural theology, and demonstrate evolution in action, he argued. (It's also the name of a popular evolution blog.) 

The case of the mole's 'thumb' also bears out this theme. Developmental biologists have recently found that Sox9, a gene involved in limb chondrification (the formation of cartilage tissue), is expressed in the region of the enlarged radial sesamoid during the development of the front paws of a mole embryo. This gene is also expressed in the 'normal' developing digits. However, the timing is different: it is expressed in that region of the wrist after Sox9 expressed has faded away in the normal digits. They compared this pattern of gene expression in embryos of the shrew, which are the closest relatives to moles, and did not find any Sox9 expression in the wrist at the same developmental stages.

Therefore, similar developmental mechanisms (the 'toolbox') have been co-opted to form the mole's false thumb, but their timing has been changed. That seems an easier option than to invent an entirely new set of tools.

Instances of obvious 'tinkering' such as this one make the messiness of evolution more apparent to us. Even structures of of evident perfection such as the eye (or eyes, since image-forming eyes have evolved multiple times) evolved by a long process of making do with existing structures to form new ones (clip from the BBC science show 'Bang Goes the Theory'). After all, isn't this just descent with modification, the very concept of biological evolution itself? Even at the level of genes and genomes, new genes often originate by duplication of existing genes (or even entire genomes, in the phenomenon of polyploidy) followed by divergence in function of these new copies from their originals. A whole book has recently been published on this subject.

What intrigues me is that the evolution of language also seems to have a similar pattern, and not just in terms of coining new words or borrowing vocabulary from other languages, but in the very grammatical structure of a language. Apparently 'perfect' systems like the Latin noun cases are actually the intermediate products of an ancient and on-going process of decay, accretion, and modification. A readable account is found in a popular-linguistics book I recently read, The Unfolding of Language.

Perhaps I'm straying too far from the original point of this post, and making a mountain out of a molehill, but the parallels to be found throughout the natural world and between the different sorts of evolution, biological and cultural, continue to fascinate me. Nature has immense and dizzying diversity, but that doesn't mean it has to be opaque to our understanding. 


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