On the limits of evolution.

A Limit to Evolution? Where Is Their Imagination?

Evolution News & Views 

fe? While there are a couple of lesser-known amino acids used (selenocysteine and pyrrolysine), those follow special pathways that don't use the genetic code. Hundreds of amino acid forms exist, yet life universally uses the same subset of twenty. Here's a contest for the explanatory power of design over evolution.

In Science Advances (the open-access publishing arm of the AAAS), evolutionists from Spain tackled this question with a new proposal: the "saturation of recognition elements blocks evolution of new tRNA identities." Evolution hit a wall. Once twenty transfer-RNA (tRNA) came into operation, there weren't any more binding pockets left for additional ones in the ribosome. 

Understanding the principles that led to the current complexity of the genetic code is a central question in evolution. Expansion of the genetic code required the selection of new transfer RNAs (tRNAs) with specific recognition signals that allowed them to be matured, modified, aminoacylated, and processed by the ribosome without compromising the fidelity or efficiency of protein synthesis. We show that saturationof recognition signals blocks the emergence of new tRNA identities and that the rate of nucleotide substitutions in tRNAs is higher in species with fewer tRNA genes. We propose that the growth of the genetic code stalled because a limit was reached in the number of identity elements that can be effectively used in the tRNA structure. [Emphasis added.]

Notice, right off the bat there is a genetic code that is faithful and efficient. The authors account for this with a reasonable-sounding explanation: evolution was going about expanding the code, but ran out of possible recognition signals. Adding more would have reduced the fidelity and efficiency of protein synthesis. But do we really want to put limits on evolution? Natural selection is the force that gave birth to almost a trillion species, according to new estimates (Science Daily).

News from IRB Barcelona attempts to justify the proposal in layman's terms: 

Nature is constantly evolving -- its limits determined only by variations that threaten the viability of species. Research into the origin and expansion of the genetic code are fundamental to explain the evolution of life. In Science Advances, a team of biologists specialised in this field explain a limitation that put the brakes on the further development of the genetic code, which is the universal set of rules that all organisms on Earth use to translate genetic sequences of nucleic acids (DNA and RNA) into the amino acid sequences that comprise the proteins that undertake cell functions. [Bold in original.]

The limit is imposed by shape, the article explains:

Saturation of the genetic code has its origin in transfer RNAs (tRNAs), the molecules responsible for recognising genetic information and carrying the corresponding amino acid to the ribosome, the place where chain of amino acids are made into proteins following the information encoded in a given gene. However, the cavity of the ribosome into which the tRNAs have to fit means that these molecules have to adopt an L-shape, and there is very little possibility of variation between them. "It would have been to the system's benefit to have made new amino acids because, in fact, we use more than the 20 amino acids we have, but the additional ones are incorporated through very complicated pathways that are not connected to the genetic code. And there came a point when Nature was unable to create new tRNAs that differed sufficiently from those already available without causing a problem with the identification of the correct amino acid. And this happened when 20 amino acids were reached," explains Ribas.

You could choose to accept this explanation and rely on evolution's ability to explain puzzling realities. But here are some questions that show how shallow these explanations really are. Try for starters:

1. Why didn't the ribosome evolve to accept more shapes?
2. Why didn't the tRNAs co-evolve with an evolving ribosome?
3. Why didn't the aminoacyl-tRNA synthetases co-evolve with the tRNAs?
4. Why don't some species substitute different amino acids for the same codon?
5. Since the genetic code is degenerate, isn't there ample space for evolutionary experiments with additional amino acids?
6. You say this limit was reached 3 billion years ago. Isn't that a long time for evolution to push the envelope?
7. Evolution seems perfectly capable of building giraffes from microbes in 500 million years. Where is your faith?
8. Why wouldn't a critic be justified by calling this explanation a post-hoc fallacy, i.e., "it is, therefore it evolved"?

There are more questions, of course. The point is, evolutionists have no problem invoking natural selection to overcome far more stringent limits than this. It seems highly contrived to impose a limit 3 billion years ago that just happens to coincide with the observation that all of life uses the same genetic code, ribosome and transfer-RNA system.

If observation still matters in science, we can infer a very different explanation. The crucial observations here are: universality, fidelity, and efficiency. The incredibly efficient system elegantly portrayed in Unlocking the Mystery of Life is a lean, mean machine. It uses enough amino acid species to allow for constructing millions of protein products, but not so many as to clutter the factory. And the same elegant system appears universally in three kingdoms of life, as different as archaea and aardvarks.

The system also includes a translation from one language into another. The genetic code, composed of DNA bases, translates into the protein code, composed of amino acids. The tight coupling of these systems argues against their independent emergence. Add to that a whole squadron of error-correction mechanisms, and the resemblance to human software is uncanny -- except that the biological system far surpasses anything humans have ever devised.

Whenever we find systems that are universal, faithful, and efficient, based on coded information, we rightly infer an intelligent cause. Whitewashing these observations with a narrative gloss tends to obscure understanding, not promote "scientific advances."