Since prestin represents a splendid example of convergent evolution at the molecular level, which in turn, supports the position that the blind watchmaker can be guided, one has to wonder about the origin of prestin itself.
To these ends, I have run across the following paper:
Proc Natl Acad Sci U S A. 2003 Jun 24;100(13):7690-5.
Expression of prestin-homologous solute carrier (SLC26) in auditory organs of nonmammalian vertebrates and insects.
Weber T, Gopfert MC, Winter H, Zimmermann U, Kohler H, Meier A, Hendrich O, Rohbock K, Robert D, Knipper M.
Prestin, the fifth member of the anion transporter family SLC26, is the outer hair cell molecular motor thought to be responsible for active mechanical amplification in the mammalian cochlea. Active amplification is present in a variety of other auditory systems, yet the prevailing view is that prestin is a motor molecule unique to mammalian ears. Here we identify prestin-related SLC26 proteins that are expressed in the auditory organs of nonmammalian vertebrates and insects. Sequence comparisons revealed the presence of SLC26 proteins in fish (Danio, GenBank accession no. AY278118, and Anguilla, GenBank accession no. BAC16761), mosquitoes (Anopheles, GenBank accession nos. EAA07232 and EAA07052), and flies (Drosophila, GenBank accession no. AAF49285). The fly and zebrafish homologues were cloned and, by using in situ hybridization, shown to be expressed in the auditory organs. In mosquitoes, in turn, the expression of prestin homologues was demonstrated for the auditory organ by using highly specific riboprobes against rat prestin. We conclude that prestin-related SLC26 proteins are widespread, possibly ancestral, constituents of auditory organs and are likely to serve salient roles in mammals and across taxa.
Fascinating. So homologs of prestin have not only been identified in both other vertebrates and non-vertebrates, but are also expressed in their auditory organs. Okay, so we can say that a prestin-like protein was already in place to carry out auditory function in the last common ancestor of mammals and insects. That’s some pretty deep homology. But does it go further?
This time, I decided to use the amino acid sequence of mouse prestin to BLAST the sequence data bases. What did I find?
There are clear homologs of prestin in jelly fish and placozoa with 30-40% identical positions and 50-60% positions with similar amino acids. But I could not find any homologs among the sponges. So did prestin co-evolve into existence with the appearance of metazoan life?
Not so fast.
Let’s consider protozoa. No luck with Tetrahymena, Paramecia, or Trypanosomes. But wait! Various other protozoa do have clear homologs of mouse prestin, including Toxoplasma, Chlamydomonas, Monosiga, and even slimemolds. And in all cases, the identical positions lies in the 30% range, with similar positions in the 50% range.
So the deep homology of prestin extends to the first single-celled eukaryotic cell itself!
But it goes deeper. Homologs of mouse prestin are also found in archaebacteria (although restricted to methanogens) and is widespread among the eubacteria. One can thus make the case that a prestin-like protein was found in the first cells.
So we have the broad outline of an interesting hypothesis. A prestin-like protein existed among the first cells and played some role involving anion transport. While it was not essential for life itself, it was extremely useful, as seen by the fact that it has been retained by so many lineages of bacteria and protozoa. Since it was likely to be retained and propagated, it would likely find itself in that lineage where metazoan life emerged. Once metazoan life emerged, the stage was set to unfold the auditory role of this protein and it quickly found itself in this new context. Once this stage was set, it would eventually facilitate the appearance of echolocation.
Think of this as the hypothesis of recursive front-loading, where state A nudges the existence of state B, which in turn, can nudge state C into existence. Such recursive front-loading would not have to occur at a particular place and time, but would be something that was eventually going to happen if we simply stepped back and considered the larger picture. In this case, a particular anion transporter fulfilling a need for single-celled life would eventually facilitate the appearance of hearing in multicellular life, and then later, the very same protein would facilitate the appearance of echolocation. This all raises the possibility that without prestin, the blind watchmaker would not have been a stunning success in evolving hearing, and then echolocation.