|MadSci Network: Evolution|
I don’t know that 'we' (as opposed to 'some of us') necessarily say that proteinoid microspheres are the origin of today’s cells. Nobody knows how life arose, or what chemical characteristics of life as we know it are universal and which are peculiar to our own small corner of the universe. Thus, even when we finally do create something that is unambiguously life in the laboratory, we won’t be able to say with certainty that life on earth actually began that way.
There are several different aspects of living organisms, all of which are displayed by something which is truly living, which are not found in combination in the various candidates for 'protolife'.
(1) First, a living cell needs the ability to reproduce itself. This often seems to be thought of as THE defining characteristic of life, ever since the structure of DNA was solved and captured everyone’s imagination. Coacervate droplets and proteinoid microspheres can only reproduce themselves in the most trivial way, 'budding' or 'splitting' under physical stress to make little ones. All self-replication that we know of involves the nucleic acids, DNA and RNA. The complicated behaviour of DNA- and the slightly less complicated behaviour of the 'RNA world' that may have preceded it- must have developed in a fairly secure chemical environment, which probably already showed some of the other aspects of life. Were these environments alive? It is a question of what we want the word 'alive' to mean… These environments almost certainly had:
(2) A well defined ‘inside’ and ‘outside’ with some degree of control over what molecules get into the inside, and
(3) The capacity to catalyse a variety of chemical reactions.
Coacervate droplets and proteinoid microspheres both have a well- defined inside and outside, and will tend to retain any large molecules that arise inside them. Small uncharged molecules will tend to move through both a protein and a lipid membrane. One important difference that I can see is that the inside of a coacervate droplet will always tend to equalize in composition with the outside. It will be difficult to maintain a different concentration inside and outside for any small molecule (even of charged species which can't move through the lipid membrane), because water will move freely through the membrane to try and equalize their composition. On the other hand, a membrane that is filled with a lot of polymerized amino acids will have much more capacity to hold on to various small molecules, if they interact strongly with some of the (many different) chemical environments that one can find in a proto- protein. So microspheres seem to be a slightly better candidate from the point of view of providing a controlled environment.
Proteins are the most versatile catalysts we know of; they can catalyse a vast range of chemical reactions, under gentle conditions (room temperature, atmospheric pressure) that would make a chemical engineer weep. This link mentions a wide variety of chemical reactions of biological interest that have been reported to be catalysed by proteinoid microspheres. While nucleic acids also play important catalytic roles in life as we know it, lipids don't tend to do much. So microspheres seem to be a rather better candidate from this point of view, as well.
I can’t see how microspheres can grow, except in an environment where there are copious amounts of amino acid lying around, nor how it can really undergo molecular evolution without being self-replicating. But they seem to be an interesting phenomenon for further investigation…
In conclusion, "one lipid membrane does not a cell make"… I don't think we are going to stick out our neck and call anything the origin of today's cells, but proteinoid microspheres seem to be more relevant to the study of the origins of life than lipid coacervates.
Unfortunately I don't seem to have been able to work in a link to the work of Addy Pross, as I had originally intended… :(
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