Summations Only

But, of course this was the great insight of Darwin - it doesn't matter. Darwin was observing a breeder of pigeons - watching the breeder select the best-suited individuals from his flock to be mated with each other, and the worst-suited individuals become the sunday roast - when he realized it works the same way in the natural world.In the natural world, those that are possessed of the adaptations crucial for survival in their environment survive and prosper, and most importantly mate, and those that do not quickly become someone's lunch. It's no different than the selection by the pigeon breeder, except that it's the natural world doing it all on its own, so he called it "natural selection."Humans do have an effect on the species around us. Frequently a detrimental one. But to the species themselves, it doesn't matter. They don't and can't distinguish between a "natural" change in their environment and a human one. We haven't. And even if we have it doesn't matter. To DNA there's no difference at all between a change in the environment due to natural causes (say, a forest fire) and a change in the environment due to human causes (say, logging.)To DNA there is only random mutation and natural selection. The same phenomenon used by the pigeon breeder to alter pigeon populations is at work in nature, altering species by natural, unguided means. That mechanism is natural selection and random mutation.

We have access to it because they passed it along to their descendants when they reproduced. And then the ancestor of that ancient fish would be something that was not a fish, some finless marine chordate perhaps.

If you think DNA can determine parenthood and lineage, why don't you think it can determine when species are related?Since those are the same thing, how could DNA be used to do one and not the other?