In my introduction thread the common genetic code was brought up as evidence of the common ancestry of bacteria and eukaryotes (along with the ribosome and the large numbers of proteins and RNAs in common between the two groups). The ribosome is already being discussed in the thread, but I decided to start a new thread for discussing the genetic code.
The genetic code is almost universal; a number of variants have been found, all of which are derived from the standard genetic code (Osawa et al., 1992; Knight, Freeland, & Landweber, 2001).
The near-universality of the genetic code has been cited as evidence for universal common ancestry (Crick, 1968; Hinegardner & Engelberg, 1963). According to geneticist Theodosius Dobzhansky, these biochemical universals are “the most impressive” evidence for the interrelationship of all life (1973, p. 128).
And indeed, from the non-teleological perspective this makes sense: If undirected abiogenesis had occurred several times, it would be an amazing coincidence if in every case the resulting organisms had struck upon the same genetic code. Therefore, universal common ancestry is the best explanation.
This changes the moment we throw teleology into the mix. Rather than having to choose between common descent and convergence, the investigator must now also consider the possibility of common design.
Suppose that the first life on Earth consisted of a diverse population of designed cells. Why would designers employ the same genetic code instead of giving each cell its own code?
Before answering this question, let us ask a counter-question: Why not? What would the point be, from an engineering perspective, of reinventing the wheel? Making multiple codes is extra work and increases the risk of mistakes when genes have to be designed in different languages.
Not only is there no reason for designers to adopt multiple codes, there is good reason to use the same code. If different cell types used different codes, they would be unable to tap into the power of horisontal gene transfer (HGT).
HGT plays an essential role in bacterial evolution, where genetic models indicate that substantial HGT is required for the survival of bacterial populations (Takeuchi, Kaneko, & Koonin, 2014). Though less common in eukaryotes, HGT is not restricted to bacteria. For example, a study found that ferns adapted to shade by horisontal transfer of a gene from the moss-like hornworts from which they diverged 400 million years ago (Li et al., 2014).
In other words, categorizing the standard genetic code as an example of common design is not an ad hoc rationalization; rather, there is a good engineering reason for reusing the code.
Crick F.H.C., 1968, “The Origin of the Genetic Code”, Journal of Molecular Biology 38(3):367-379
Dobzhansky T., 1973, “Nothing in Biology Makes Sense except in the Light of Evolution”, The American Biology Teacher 35(3):125-129
Hinegardner R.T. & Engelberg J., 1963, “Rationale for a Universal Genetic Code”, Science 142(3595):1083-1085
Knight R.D., Freeland S.J., & Landweber L.F., 2001, “Rewiring the Keyboard: Evolvability of the Genetic Code”, Nature Reviews Genetics 2(1):49-58
Li F. et al., 2014, “Horizontal Transfer of an Adaptive Chimeric Photoreceptor from Bryophytes to Ferns”, Proceedings of the National Academy of Sciences 111(18): 6672-6677
Osawa S., Jukes T.H., Watanabe K., & Muto A., 1992, “Recent Evidence for Evolution of the Genetic Code”, Microbiological Reviews 56(1):229-264
Takeuchi N., Kaneko K., Koonin E.V., 2014, “Horizontal Gene Transfer Can Rescue Prokaryotes from Muller’s Ratchet: Benefit of DNA from Dead Cells and Population Subdivision”, G3 (Bethesda) 4(2):325-339