”Yes, but things like how DNA and ribosomes work, and the basic molecular machinery would have already been set in stone even in bacteria that old.”
Abiogenesis is thought to have taken place somewhere between 3.9 and 3.5 billion years ago and these traces of life (textures on the surface of sandstone that have altered C12/C13 ratios suggestive of life) are dated 3.49 billion years ago. Calling them bacteria, or even saying that they had DNA and/or ribosomes, may be presumptuous. They’re old enough that conditions of the RNA world hypothesis might still apply. They might not have DNA at all but use RNA (or something else) as genetic material. They might use RNA instead of proteins for catalysis, which could obviate the need for protein-building ribosomes. This life might not be cellular, could just be primitive liposomes that chaotically break and reform, briefly shielding some set of catalytic molecules that when you average them out over a large area—say a cubic millimeter—the whole system is able to keep functioning and making more of itself.
But let’s ignore all of that and say that this stuff, whatever it is, has DNA. Does it only use the four canonical bases or does it use them and/or something else? How good is it at keeping deoxyribonucleic acids from being used alongside ribonucleic acids, or is a mix important in some function(s) at this very early stage? Suppose it does use just the four canonical bases, and just the four (five) bases for RNA, and has ribosomes, and has the central dogma in place of DNA->RNA->protein. What’s the protein like? Is the universal* genetic code in place at this point? Are there just 20 amino acids, the same 20 currently in use, and are they encoded by triplet codons? After all valine, leucine, and isoleucine are pretty much the same as far as protein biochemistry is concerned and usually can substitute for each other with little or no impact, so why have all three? Could there be a different set of amino acids, one that is potentially encoded by pairs of codons or mixed pairs and triplets?
Let’s ignore all of that and say we’ve got life, actual cellular life, that uses DNA with just the four bases, with negligible confusion with RNA, that the mRNA and tRNA and ribosomes are all worked out (and ignoring ongoing evolution), with just the 20 amino acids using the universal* genetic code. Does this organism make its own cellular membrane? There’s a whole bunch of synthesis involved with making the components of a membrane. Does it use cholesterol or other steroids? A modern cell membrane has more than phospholipids. Does it have a cell wall? That’s a completely different set of questions as there are many different cell wall structures and components in modern prokaryotes. What is the energy source for these organisms? Are they heterotrophic? How? Are they photosynthetic? How? Are they sulfur-reducing prokaryotes? How? Are they predatory? Do they secrete chemical compounds that lyse their neighboring prokaryotes? How?
It’s trivially easy to ask questions about basic chemistry, biochemistry, and genetics when it comes to these organisms, assuming of course we would grant them as being alive, when we’re dealing with something from 3.49 billion years ago. I do not necessarily agree with “fine tuning” either since there are geologically short periods of time that witness tremendous changes in life forms. The emergence of aerobic life about 2.5 billion years ago is one such point; oxygen would likely have been poisonous to the life forms in TFA. The emergence of eukaryotes about 1.6-2.1 billion years ago would be another, as would multicellular life appearing shortly afterwards. Throughout all of this the archaea, bacteria, and/or their ancestors would be present, and would be evolving in response to their changing environment.
I happen to have done some work in entomology so I have to mention insect evolution. The Paleozoic period is 541 to about 250 million years ago. The oldest definitive insect fossil is Rhyniognatha hirsti and dates to about 400 million years ago, about the same time that the first terrestrial ecosystems were being formed. Beetles didn’t emerge until 300 million years ago and there are now nearly 400,000 known species. 220 million years ago—well after the Paleozoic—the mosquito and the house fly diverged. The diet, ecology, reproductive strategies, and mouth parts of modern mosquitoes and house flies are radically different. I’ve done work on mosquitoes and their evolution is (unfortunately for half the world’s population) very interesting. They are arguably the worst spreaders of disease, being vectors of malaria, dengue fever, yellow fever, chikungunya, West Nile, several different encephalitis-causing viruses, and heartworm. Insecticide resistance is exploding and speciations are being observed in disease vector mosquitoes. Climate change is permitting formerly tropical mosquitoes to move into new territories and new speciation events are inevitable. Sure someone could protest that they’re still bugs with six legs, but then humans and turtles are still just tetrapods and those have been around for almost as long as insects.