Molecular clues to evolutionary relationships
Hidden Origins of Similar Compounds?
Most biochemical molecules we often think of belonging to recent groups may have had origins much earlier than current evidence shows. The presumption is that two different groups having a unique compound must either represent descent from a common ancestor or they must be a case of convergent evolution. But compounds active in minute amounts, such as hormones, may have been present in common ancestors in such low levels they have not been detected.
One possible bit of evidence might be the presence of ecdysone, a hormone important in controlling molting of insects, has also been found in bracken ferns. If it were present at extremely low levels, in ancestral species reaching back to a common ancestor among one-celled organisms and had little use, it might not be detected until it was produced in sufficient amounts as part of a new process to benefit insect molting. The near ancestors of bracken ferns that developed increased levels of ecdysone sufficient to disrupt fern-eating insect adults and/or larvae would eventually survive better and replace those being killed by insect activity.
The long periods involved in evolution of different forms of similar compounds is likely to allow the demise of intermediate stages of the evolution once a perfected solution is reached. Until the activity of a substance is useful in larger amounts there is a selective advantage of not making larger or detectable quantities. So a substance produced in the cell and doing an activity within the cell by inducing gene action within the cell is not needed in the high levels the amounts hormones transported by the blood require. My attempt to discover "protozoan hormones" as noted in an earlier post was based on this line of reasoning.
The Genetic Code Evolution and Phylogeny
The example of cytochrome c, a compound required by all higher animals and one of the first successes of molecular phylogeny relating diverse phyla, should be revisited. The use of the polymerase chain reaction (PCR) for studying nucleotide sequences might be applicable to the regions of DNA coding the cytochrome. The data could then be used for groups where adequate amounts for comparing cystochromes were not available at the time of the original study*.
*Fitch, Walter M., and Emanuel Margoliash. 1967. Construction of phylogenetic trees. Science, 155:279-284. [