When I decide on a topic, I usually hold onto it in my mind, and maybe flesh out a few ideas mentally. I’ve mentioned this before. This topic suddenly came to mind for no good reason, and I haven’t thought of any significant lines of discussion. I wish I could snapshot the “Related Articles” that have popped up, but it looks like they will lead me to places I do not intend to go. An example is “Casual sex isn’t just for college kids”. Mmm. As if college kids a) invented it b) have a monopoly in it.
But I digress. Sex. Most living organisms have it. Amoeba, the popularly held archetype of the simple single celled organism, was believed to reproduce simply by fission. I’m unable to understand much of the scientific literature about amoeba reproduction, and there doesn’t seem to be much material about it anyway, but fission, I believe, results in each child organism having half the genetic material of the parent cell.
Maybe nuclear genetic material is doubled before the split. Maybe each ‘individual’ is half an individual and needs to find another ‘individual’ in the same state to merge with? Merging has been observed in amoeba.
What is certain is the enormous size of the genome of an amoeba species. Some of them have genomes which are more than 200 times the size of the human genome. Amoeba are presented to us in school as possibly the simplest organism that there is. Based simply on the size of genome, this isn’t true.
I can conjecture, based on little to no knowledge at all of the genetics of amoeba, that fission and fusion would enable amoeba species to mix and match their genetic material with much greater freedom than simple sexual reproduction.
So, amoeba splitting and merging could create an enormous genome, even in a simple organism. The size of a genome could be just a result of a less restrictive reproductive process than applies to more “advanced” multi-celled organisms (not to mention more “advanced” single-celled organisms.
If I’m correct or anywhere near close to correct about the amoeba genome and its reproduction, amoeba may represent an early stage of sexual reproduction. Amoeba were inventing reproduction, in a way. One can imagine that early organisms would absorb other weaker organisms, and in doing so, acquire their genetic material or proto-genetic material.
Of course in most cases they would simply digest it, but in those days, the early days of life, when the chemical processes and genetic processes of life were shaking down into the rules that we know today, things would have been more fluid and the genetic material could have been incorporated into the organism’s own genetic material. Indeed, in the beginning the genetic material would probably not be distinguishable from other material in the organism. There wouldn’t have been a nucleus, as such.
One can imagine that in the beginning, organisms just didn’t reproduce, by fission or any other method. They would have fairly quickly died out. Then organisms could have happened which just grew and grew until they split. Parts would have died off, parts would have lived.
The parts that survived would have been changed, modified by the environment, until the bits that would have earlier died, survived as new individuals. Maybe they couldn’t themselves reproduce, but eventually, the split off bits would have survived and been able to reproduce.
In retrospect, it appears that the best way to be able to reproduce is to separate the reproductive materials and functions mainly into a single location, the nucleus.
Organisms can they reproduce simply by duplication of the genetic information in the nucleus, producing a clone of themselves, which they can hive off as a new individual. Some organisms bud off a clone of themselves as a reproductive process.
This doesn’t allow for change in environment though. A self-cloning organism can’t react to environment changes. However if organisms can exchange genetic material while creating a child, it may be that the child’s genetic make up may allow it to survive where its parents would struggle.
The process used by amoebas, that is to say the process of division and merging of individual organisations could be the first step in that direction. Of course, uncontrolled merging could result in possibly viable individuals with large genomes, which is what we see in some amoeba.
There are two routes here. Either an organism would clone its nucleus including its genetic material, then split, producing two identical organisms. or it could halve its genetic material and merge with a similarly haploid organism, resulting in a diploid individual.
The advantages of the haploid/diploid cycle are obvious – genetic material is mixed so at least some individuals may survive an environmental change, because the expression of the genome in the individual (the phenotype) allows them to differ from their parents and survive the change.
What is less obvious is why organisms split into male and female sexualities. It’s possible that the difference is caused by the necessity of one set of haploid individuals to supply an environment in which the child organism can develop. The other set of haploid individuals merely supplies the other half of the necessary genetic material.
So the female supplies the support environment plus the genetic material, or egg (ovum) and the male supplies only the genetic material, the sperm. One can imagine that originally organisms would directly exchange genetic material by fusion and fission, like amoeba, but at some time it became more efficient to disseminate genetic material outside the organism.
Cells within multicellular organisms or possibly unicellular organisms developed the ability to create new haploid cells with a copy of half the genetic information leaving behind unicellular haploid organisms or haploid cells within a diploid organism. In female organisms the haploid cell would be an egg and would have the support environment to create a new diploid individual, and in male organism the haploid cell would just have half the genetic material and be a sperm.
There are some hermaphroditic animals, for example some snails and slugs, which produce both eggs and sperms and many plants have both male and female characteristics, but many, many animals have separate male and female individuals. (I’m not keen on saying the majority of animals display sexual differentiation, because I don’t know if it is true.)
So, when life began it would have been simple unprotected self-replicating molecules. Growth would have been by accretion. At some stage the molecules would have evolved to the point where they developed some structure around themselves, maybe by rejecting some unwelcome molecules. Organelles, small biological factories would have developed as the organisms became more complex, all enclosed in a membrane that allowed the necessary chemicals in and unwanted ones out. This membrane would eventually enclose the nucleus of the cell. More complexity, more biological factories, and the cell would have formed an outer membrane, that enclosed all the necessary mechanisms that modern cells contain and require. (OK, I’m no expert so some of these conjectures may be wrong).
Cells would initially have not had any reproductive mechanism. They would grow and then split when they got too big. When cells developed specialised mechanisms for reproduction they needed some way of passing on the genetic material. Some cells would have developed a method of creating haploid individuals and these would have then merged with other haploid individuals to create normal diploid individuals.
Or maybe so-called haploid individuals arose first and diploid individuals arose from the merger of two haploid individuals. When multi-cellular organism arose, they evolved special organs related to reproduction. Such organs created haploid versions of the organism and a method of delivery to the outside world of these eggs and sperm.
Once individuals have evolved to specifically create eggs or sperm, they are sexual individuals. If an individual evolved to create a support system for their haploid genetic material, for example eggs, it would find it difficult to find similar individuals to merge with since eggs are not particularly mobile. Sperm on the other hand are specialised to be mobile, so are ideal for merging with the eggs.
Once the individuals have sexual differentiating characteristics this would flow through to the phenotype (physical expression of the genetic material – the multi-cellular organism). And that is my guess, as a complete amateur in the field of genetics is where sex came from. So the above may make sense at some level, or not. Even it does make a sort of sense, I may well be wrong about the detail! But it has been fun speculating.