Scientists will often point out the simple fact that every living thing on the planet (so far discovered) uses the exact same codons to translate DNA into proteins. The same amino acids are used, the same coding system is used (CUC is always leucine, no matter if one is looking at archaebacteria or a blue whale). This is taken to be evidence of common descent. It makes sense, if everything uses the exact same system, then they must be related somehow (considering that there are no ‘decisions’ made by organisms without a mind to make decisions.)
Of course, the anti-science supporters will often say, “Of course, it’s the same, it was designed that way.” Which gets us into the whole teleology that is modern creationism. But that’s not why we’re here.
The question remains, how did this system develop?
Edward Trifonov may have answered this. Remember, this may not be The Answer, but it shows that the system is plausible without outside interference. This is all science has to show.
Previous articles here have shown that amino acids do indeed form from non-organic starting materials. The classic Miller-Urey experiment showed this almost 60 years ago. But, there was something hidden in the Miller-Urey data. Something no one even really thought about until Trifonov took a look at all the data.
He and Thomas Bettecken wondered if the GCT triplet, which extends itself rapidly in disease, has/had an evolutionary advantage. If so, then this system (GCU) could have been the first codon.
Point mutations are simple one nucleotide replacements in the genetic sequence. (How’s that for a poor segue?)
What they found was that six of the amino acids in the Miller-Urey experiment were encoded by point mutation derivatives of GCU. We know these are easy to form, just about anyone with access to some glassware and pure gases can make these amino acids in a way similar to what may have happened on the prebiotic Earth.
It’s surprising, though it shouldn’t be, that the evolutionaryily powerful GCT triplets are closely associated with the earliest and easiest to make amino acids.
Well, that’s interesting, but where do we go from here?
Trifonov decided to go all the way, all 20 amino acids were individually characterized with 40 criteria. He used everything from how many non-hydrogen atoms where in the molecule to the thermostability of the duplex RNA genes. Now we get to the interesting bit.
What he found was that using all the criteria, 9 of the Miller-Urey amino acids, would have been the first 9 in a chronological sequence. Then he took the Miller-Urey data out and checked again. The same first 9 appeared (though one moved from position 6 to position 5). Then he filtered the data for only certain things, like closely related, but different criteria. Almost no matter what he did to the data, the rankings of the amino acids came out the same.
Oh a couple of amino acids moved up or down one spot, but the correlation between all data sets combined, and groups of data, and experimental evidence was very, very high.
What’s even more interesting is that he looked at the RNA codons for each amino acid. Now that he had a temporal sequence of amino acids, he could actually look at the temporal sequence of the codons.
He found that, starting from GGC (which for other reasons is considered one of the earliest codons), every codon followed a simple step-wise path of point mutations or complimentary sequences.
Let’s go through an example, and please bear with me:
- GCC alanine
- GAC aspartate
- GAG glutamate
- CUC leucine (this is the compliment to GAG)
- CUG leucine
- CAG glutamine
Using these steps, every amino acid / codon combination (all 64 of them) can be developed through either point mutations or complimentary sequences.
The main factor he found, was thermostability we very important. The most thermostable of a codon was the first one developed. Even though a future codon might have been more stable than earlier codons, if there was no step-wise or complimentary path to the more stable one, it did not emerge until such a path was available.
There are four fundamental conclusions developed by this work.
- The first amino acids were abiotic.
- Thermostability played a major role in the development of the triplet code
- New codons appeared in complimentary pairs
- New codons were simple derivatives of chronologically earlier ones.
So, unlike what many believe, even scientists, then entire codon/anti-codon/amino acid system did not appear, fully complete. It, just like everything else, evolved.
Trifonov, E. (2000). Consensus temporal order of amino acids and evolution of the triplet code Gene, 261 (1), 139-151 DOI: 10.1016/S0378-1119(00)00476-5