In the Design Matrix, I wrote, “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.”
The basic idea here is that tissues, organs, and even organ systems can be viewed as the emergent expression of key macromolecules. Such macromolecules can serve as “bait” that would lure certain expressions of evolved complexity. All of this would speak to evolution being under some form of intrinsic guidance.
What’s more, during the summer of 2008, I wrote a series of essays for the old book blog (that was hacked) that highlighted the evidence which suggested that the vertebrate endocrine system could have been front-loaded into protozoa (for example, human insulin triggers a biological response in Tetrahymena).
Well, along comes a study that moves toward both of these points.
From Besant K. Tiwary and Wen-Hsiung Li. 2009. Parallel Evolution between Aromatase and Androgen Receptor in the Animal Kingdom. Mol. Biol. Evol. 26:123–129.
There are now many known cases of orthologous or unrelated proteins in different species that have undergone parallel evolution to satisfy a similar function. However, there are no reported cases of parallel evolution for proteins that bind a common ligand but have different functions. We focused on two proteins that have different functions in steroid hormone biosynthesis and action but bind a common ligand, androgen. The first protein, androgen receptor (AR), is a nuclear hormone receptor and the second one, aromatase (cytochrome P450 19 [CYP19]), converts androgen to estrogen. We hypothesized that binding of the androgen ligand has exerted common selective pressure on both AR and CYP19, resulting in a signature of parallel evolution between these two proteins, though they perform different functions. Consistent with this hypothesis, we found that rates of amino acid change in AR and CYP19 are strongly correlated across the metazoan phylogeny, whereas no significant correlation was found in the control set of proteins. Moreover, we inferred that genomic toolkits required for steroid biosynthesis and action were present in a basal metazoan, cnidarians. The close similarities between vertebrate and sea anemone AR and CYP19 suggest a very ancient origin of their endocrine functions at the base of metazoan evolution. Finally, we found evidence supporting the hypothesis that the androgen-to-estrogen ratio determines the gonadal sex in all metazoans.
From the discussion:
This work demonstrates for the first time a highly significant parallel evolution between two proteins having different structures and functions (supplementary figure 1, Supplementary Material online). The parallel evolution between AR and CYP19 is well reflected in terms of high correlation between their ML distances and interspecies similarity to the human sequences and similar topology of phylogenetic trees in both proteins. The common factor posing functional constraints on AR and CYP19 is their ligand, androgen. Therefore, this finding reinforces the notion that functional constraint of a protein is the principal evolutionary force in its evolution (Li 1997). We also infer that genomic toolkits required for steroid biosynthesis and action were present in a basal metazoan, cnidarians. The close similarity in vertebrate proteins and sea anemone proteins suggests a very ancient origin of their endocrine functions at the base of metazoan evolution. This suggests that the origin and evolution of the vertebrate-like endocrine system started early in the animal kingdom. Possibly, the endocrine system started its evolutionary journey along with the nervous system in the last common ancestor of bilaterians and cnidarians.
So the androgen imposes similar patterns of evolution on two different proteins with different functions. It does so merely because the two different proteins bind to the same molecule which has probably been around long before complex animals came into existence.
Furthermore, the researchers end with a comment that makes bunnies happy:
This study is the beginning of an analysis of parallel evolution among functionally related proteins in the endocrine system. It is expected that many novel parallel evolutionary trends among various proteins in different physiological systems will be unveiled in the near future.
Another glimpse of the ordered essence of evolution?