Provided one realises that Nucleic acids have to be a product of life not its origin it is not difficult to imagine how life may have originated.
Given the formation of protocells in a rich medium with a source of energy, such as is provided in a submarine geothermal spring, there is a logical sequence starting with the sequestration of inorganic catalysts within the protocell by complexing with polypeptides, and leading to a high concentration of more or less random sequences of proteins and nucleic acids, which could lead to the origin of a primitive system for controlling amino acid sequences.
The origin of life would be the formation of entities with some form of membrane separating the outside from the inside world. Such protocells form in hot springs. They have osmotic properties and might therefore grow by intussusception and reproduce, usually by budding, sometimes by fission. They would have to be sessile or they would soon be washed away from their founding and sustaining environment Many propagules would be lost to the unfavourable colder distant environment but some would survive.
Among the molecules included, or entering through the membrane, there would be inorganic catalysts, which would promote growth of the protocell and the consequent production of propagules. Protocells containing such catalysts would be favoured by selection. Simple organic catalysts would, however, readily be washed out of the protocell and lost, but the catalysts would necessarily promote the polymerisation of molecules washed into the protocell. Those polymers which remain attached to the inorganic catalyst would be effective in sequestering the catalysts within the protocell. Such polymers might further capture new inorganic catalysts washed into the cell from the outside environment, so that there would be a high concentration of catalysts within the parent protocells and in the propagules as well.
For this sequestering of catalysts within protocells almost any sort of molecule might do, so long as the complex retained catalytic activity, but perhaps the most effective sequestering would be by polypeptides. These would vary in sequence as well as size, as well as differing in their inorganic catalyst, and such differences might have slight effects on the activity and specificity of the catalysts. Some might increase the affinity of the catalysts for the growing regions of the boundary membrane, thus increasing growth rate, and propagation rate. Others might be more effective at promoting polymerisation of the solute molecules entering the protocell, thus making possible increasingly effective sequestration of catalysts in increasing number and variety.
It should be stressed that at this stage that the catalysts have only two functions in the economy of the protocell, the primary function being promotion of growth of the membrane, the secondary function the making of catalysts that do this. However the catalysts must also break down some of the molecules entering the cell, thus adding to the energy provided by the geothermal environment
Thus far the evolution of enzymes only involves polymerisation of and catenation with molecules synthesised in more or less random sequences. No hereditary information or translation has been required. Further developments would require some control of amino acid sequence.
As is generally recognised, this is where fundamental difficulties arise. In the DNA RNA protein system that we know today, mRNA seems to have no possible function without a variety of tRNAs, and vice versa, and DNA none without both, not to mention the role of ribosomes. Any selective explanation would have to assume a role for each component separately, which seems unlikely.
The alternative is to suppose that a diversity of nucleic acids evolved without significant contribution to the economy of the protocells in which they occurred. This seems a logical consequence of the low specificity of the protoenzymes, which must necessarily polymerise any polymerisable molecules entering from the rich external medium.
Some polymers would be broken down or escape through the membrane, but the more stable polymers would survive even if they made no contribution to growth. Some of them might even be transcribed, or even replicate and pass into the propagules.
Thus we may envisage protocells containing a variety of random sequence nucleic acids making no contribution to protocell growth. But, potentially, they might have the properties of primitive tRNAs, mRNAs or DNA.
It seems highly unlikely that any single protocell could in this way come to contain all the makings of a primitive replication -translation system. However the protocells and their propagules in any particular geothermal spring would not only multiply, but occasionally they would collide and fuse, or one would engulf another, thus bringing together separately evolved molecules. [This would be the origin of sex] In addition some of the nucleic acid polymers might not themselves be sequestered, and might therefore pass out of their native protocell and enter others. Likewise the polymers freed when any protocell burst might enter other protocells. In these ways some primitive translation mechanism might be brought together by accident.
The production of a primitive replication/translation system by these means may have had a very low a priori probability. But a low probability is all that is needed. Suppose that, in the initial abiotic environment, one protocell was formed per cubic metre of water per day (a figure that would have a very high variance, but all water must ultimately pass through geothermal springs), then in 1000 million years something like 1020 protocells would have been tried. This figure does not take into account the contribution of reproduction to the number of protocells tested. There seems therefore to be room for a certain amount of coincidence.
Once a replication translation system became available the way would be open to the rapid selection of enzymes of high specificity and activity. This increased activity would make it possible for the enzymes to function at lower temperatures, which, together with evolution of reactions capable of replacing the energy provided by the geothermal environment, would permit the survival of propagules that were washed away from the immediate vicinity of the geothermal spring. Then the whole marine environment would be available for rapid expansion and developing diversity.
© J M Thoday 1980