I don't agree with so much in your comment, that I have to reply, sorry. First of all, we don't start with the same gene set. Ok, maybe we did 4 billions years ago, but it's a pure speculation and we don't have any instruments to infer anything valuable from the fact that 'there was a single organism at some point of time from which everything evolved'. I'm pretty sure the whole picture is much-much more complicated. As for humans, I can say with a high confidence that there was not a single human being evolved that become the genesis of all humans. Specification is a complex thing and there's always a period of hybridisations with closely related species. I won't delve into it deeper, but some fishes even rely on other species for their reproduction. This fish is actually really amazing and there's a lot to unpack with its reproduction. Secondly, there's no such thing as random distribution that does something for organism fitness. The species that got an appropriate set of genes (and maybe even more than genes, like some epigenetic markers) survived. The ones who didn't, didn't. That's it. In many organisms there's not a unique set of genes ("formula") that lets it survive in a given environment. Gene regulation is incredibly intricate and has a ton of feedback loops. For example some genes, that are very important are often duplicated, like ribosomal or histone genes, and mutations in them doesn't do much.