We have seen that prestin is a motor protein found in the outer hair cells of the inner ear of the mammalian cochlea. It vastly enhances auditory sensitivity by converting the energy from an ion gradient to force such that if you eliminate this protein in mice, there is a greater than 100-fold loss in auditory sensitivity.
Of course, what is most remarkable about this protein is its convergent evolution in certain bats and dolphins, where the prestin protein from both species share at least 14 independently derived amino acid sites. These changes apparently played key roles in the independent evolution of echolocation. The molecular design of this protein seems to have facilitated the appearance of echolocation, which takes auditory sensitivity to the next level. What’s more, there are other signs of convergent evolution at the anatomical level:
Echolocation requires exceptionally high frequency hearing and, though echolocating whales and bats generate their calls differently, their cochleae show multiple convergent anatomical features . In particular, the cochlear OHCs in both taxa are shorter and stiffer than in other mammals , and this inferred adaptation for processing ultrasound is supported by audiograms that reveal correspondingly higher frequency thresholds . (Yang Liu, James A. Cotton, Bin Shen, Xiuqun Han, Stephen J. Rossiter and Shuyi Zhang. 2009. Convergent sequence evolution between echolocating bats and dolphins. Current Biology Vol 20 No 2).
Prestin would thus appear to be a good candidate for something I describe in The Design Matrix.
In the book, I discuss the protein hemoglobin and its ability to front-load and shape subsequent evolution:
Molecular properties, which can be designed, may serve to “guide” organismal evolution more than appreciated. In this case, the properties of designed proteins in bacteria have channeled evolution such that this three-component gas exchange would eventually emerge in a mammal-like creature and function as it does. The design of a protein may have front-loaded the appearance of a particular type of organ system that handled gas delivery and exchange.
So if the molecular design of prestin, needed for enhanced auditory sensitivity, can nudge organisms, in the right context, toward similar functional, biochemical, and anatomical states, this may be just the tip of the iceberg. Recall that the nudging, or front-loading, of evolution is about using a particular “choice architecture” to stack the deck so a particular objective can be reached. I think prestin, and the convergent evolution of echolocation, adds to the plausibility of this design strategy by demonstrating it could work.
If prestin fits so well within the teleological framework of front-loading, then we might turn around and use this hypothesis to predict that it would also demonstrate a key feature of front-loading – deep homology. Let’s consider that next.