The origin of the genetic cipher, the most perplexing problem in biology
The genetic code, with a few recently discovered minor variations, is common to all known forms of life. That the code is universal is extremely significant, for it suggests it was used by the common ancestor of all life, and is robust enough to have survived through billions of years of evolution. Without it, the production of proteins would be a hopelessly hit-or-miss affair. Questions abound. How did such a complicated and specific system as the genetic code arise in the first place? Why, out of the 10^84 possible codes based on triplets, has nature chosen the one in universal use? Could a different code work as well? If there is life on Mars, will it have the same genetic code as Earthlife? Can we imagine uncoded life, in which interdependent molecules deal directly with each other on the basis of their chemical affinities alone? Or is the origin of the genetic code itself (or at least a genetic code) the key to the origin of life? The British biologist John Maynard Smith has described the origin of the code as the most perplexing problem in evolutionary biology. With collaborator Eörs Szathmáry he writes: “The existing translational machinery is at the same time so complex, so universal, and so essential that it is hard to see how it could have come into existence, or how life could have existed without it.” To get some idea of why the code is such an enigma, consider whether there is anything special about the numbers involved. Why does life use twenty amino acids and four nucleotide bases? It would be far simpler to employ, say, sixteen amino acids and package the four bases into doublets rather than triplets. Easier still would be to have just two bases and use a binary code, like a computer. If a simpler system had evolved, it is hard to see how the more complicated triplet code would ever take over. The answer could be a case of “It was a good idea at the time.” A good idea of whom ? If the code evolved at a very early stage in the history of life, perhaps even during its prebiotic phase, the numbers four and twenty may have been the best way to go for chemical reasons relevant at that stage. Life simply got stuck with these numbers thereafter, their original purpose lost. Or perhaps the use of four and twenty is the optimum way to do it. There is an advantage in life’s employing many varieties of amino acid, because they can be strung together in more ways to offer a wider selection of proteins. But there is also a price: with increasing numbers of amino acids, the risk of translation errors grows. With too many amino acids around, there would be a greater likelihood that the wrong one would be hooked onto the protein chain. So maybe twenty is a good compromise. Do random chemical reactions have knowledge to arrive at a optimal conclusion, or a " good compromise" ?
An even tougher problem concerns the coding assignments—i.e., which triplets code for which amino acids. How did these designations come about? Because nucleic-acid bases and amino acids don’t recognize each other directly, but have to deal via chemical intermediaries, there is no obvious reason why particular triplets should go with particular amino acids. Other translations are conceivable. Coded instructions are a good idea, but the actual code seems to be pretty arbitrary. Perhaps it is simply a frozen accident, a random choice that just locked itself in, with no deeper significance.
That frozen accident means, that good old luck would have hit the jackpot trough trial and error amongst 1.5 × 1084 possible genetic codes . That is the number of atoms in the whole universe. That puts any real possibility of chance providing the feat out of question. Its , using Borel’s law, in the realm of impossibility. The maximum time available for it to originate was estimated at 6.3 x 10^15 seconds. Natural selection would have to evaluate roughly 10^55 codes per second to find the one that’s universal. Put simply, natural selection lacks the time necessary to find the universal genetic code.