I have read the OP then hit the “end” button to avoid reading anyone else’s responses. Apologies for inaccuracies, and hoping for pointers where I have stuff wrong.
So, we have a planet that, several billions of years ago, was chugging along with water and geysers and volcanic activity. Due to the circumstances, various molecules are being formed and destroyed on a very regular basis. Some of these, amino acids, are robust molecules, and in certain reactions they bond with each other and form proteins. A few of these proteins have self-replicating qualities, and are in the circumstances to do just that. This is abiogenesis, but not necessarily life.
[I am very ropy about the next part…]
After a few million squillion years, these self-replicating proteins have become so complex that they have bonded together in myriad reactions and eventually some of them have become ribonucleic acid (RNA) and deoxyriobnucleic acid (DNA), which are supremely robust self-replicating molecules, that also contains complex information about that self-replication. A single unit of informational coding on the double-helix of the DNA molecule is known as a gene.
Through the process of self-replication, mutation, and “survival of the fittest”, various forms of DNA begin to appear in extremely complex forms - the DNA allows complex structures to arise around itself, and we see for the first time things that resemble cells: simple bacterium-like lifeforms that have an outer protein shell, and an inner nucleus that contains the important DNA. Other protein molecules arise within these structure to maintain it.
[At this point in my explanation, I myself am starting to believe in Intelligent Design…]
From the start these unicellular “lifeforms”, which, due to mutation, can be very different depending on their circumstances, interact and compete. Many die out, but some of them also end up in symbiotic relationships with their peers. In particular, one RNA-based structure becomes incorporated into a larger DNA-based structure, and this symbiosis proves extremely productive: the RNA-based “bacterium” becomes what we now know as the mitochondrium, a structure which coexists within the nucleus of a cell, and provides proteins that service and maintain the instructions of the DNA.
Over billions of years, thanks to environment, mutation and selection, unicellular organisms become more and more complex, for example, some growing scillia (hairs) for propulsion, light-sensitive structures, simple food-sensing parts. Meanwhile the symbiosis between the various unicellular organisms continues, and gradually colonies of these cells become indistinguishable from multicellular organisms.
At this time, all evolution was in the sea, and these organisms, with various appendages and tools, created from mutation and selection, spread throughout the seas. Each organism, over generations, adapts and changes successfully, but the majority die out. At some point [and I have no fucking idea when, or if I’m even correct], the method of replication alters: where cellular organisms replicated by splitting, some multicellular organisms developed two different kinds of the same organism, bearing different DNA codes (known as “chromosomes”). The chance for genetic alteration and mutation was greatly increased by this shift, leading to what we know as sexual reproduction, and organisms that developed this tended up adapt faster than asexual reproducers.
Meanwhile, for many organisms, the amount of information - the genes - contained on the self-replicating DNA molecules increased and increased, and the complexity of the lifeforms increased too. Gradually, these creatures developed structures that allowed them to reach to all parts of the oceans, and eventually they evolved structures that allowed them to leave the oceans.
Through the same process, multitudes of what we know as plants, and what we know as animals, flourished in the oceans and on land, continually mutating, colonising, working in symbiosis, and adapting. Almost all were still based on the DNA/cellular/genetic structure Some were cold-blooded, some warm-blooded, some had scales, some had feathers, some had leaves and bark. Millions of species continued adapting, splitting from each other based on circumstance, mutation, adaptation, extinction, until the moment we now arrive at.
Eventually, life conquered the earth.
), until members of population A can’t breed with members of population B anymore. This is called Speciation. That’s basically it.