You’re at the beginning of life on Earth… you are a bubble of fats, held in place by proteins, containing some salty water and the amazing molecule, DNA.
You have genes[1] within your tiny chromosome which express proteins which can process the simple sugars which move easily through your fatty wall and use the energy to create copies of themselves[2]. Sometimes these copies are not quite the same, and a different protein is expressed, which does something different.
Eventually, one bubble accidentally creates a gene which makes a simple cell wall form, and although it reduces the rate at which sugars can enter, it means that this particular set of genes get isolated and can create copy after copy of themselves without merging in any other set. This is the point at which we can start thinking of a single organism… it is called a prokaryote[3].
In order to create the most copies of yourself, you need to make efficiencies in the way you process the available energy.
There is another energy source which you can use: the Sun. If you find a protein that will enable you to store the Sun’s energy, you can create sugars right inside you, then process those later, when you need the energy.
Alternatively, you can be more of a generalist about which chemicals you can consume. After all those chemical pathways are close to yours, so only a small error might be enough.
[1] At this stage, reproduction is still very much at the gene level. There are so few genes in existence, with such general functions, and such a thin cell membrane, that it doesn’t really make sense to describe you as a reproducing set of genes, an organism.
[2]Slowly sugars enter your cell, and slowly you utilise the energy they contain, make copies of your dna and reproduce. It’s a relatively long-lived form of life for this era. You get to make many copies of yourself before eventually some bad luck takes you out. You still, on occasion, swap genes with others around you, and sometimes there are errors in copying, both of which create new organisms.
Sometimes your bubble splits and a selection of those copies gets separated, creating a new ‘you’. Sometimes your bubble bounces into another and you merge. Sometimes the copy process is disturbed, and a new gene is created.
[3] This isn’t entirely true. There’s another family called Archea which has a different type of cell wall. However, it’s only a few years since we realised they aren’t bacteria, and so their evolution is incredibly poorly understood. For a good discussion of the current state of Archea, try Wikipedia. If some research comes up that changes what’s been written here, then I’ll bring it up to date.
