I have no idea what you mean by "all the possible DNA you'd ever need." That's not something that makes sense to me. <_< There was never a cell that had every possible type of every gene ever, if that's what you're asking. That's just impossible.
But if you're starting with a cell, obviously it has to have the basic instructions in place to keep it alive. How to take up food, get rid of waste, and most importantly, divide and copy its DNA for the next generation. Over time, mutations are going to happen, though, and that's how things change. The ones that change the DNA and the protein it makes without killing the organism are the ones that survive. Plus you can get more experimentation with stuff like chromosome duplication events. For example, you've got a cell that has one chromosome that contains all the stuff it needs to survive. Imagine if that entire thing were to duplicate. Now the cell has two working copies of every gene. That gives mutation a LOT of room to work, since as long as between the two chromosomes, there is a full set of working copies of the "base" genome, there's another copy that can mutate and make an entirely different product with a different function. So with that example, maybe it's easier for you to see how life became more complex.
Also a common misconception: Evolution =/= "improvement." Evolution is heritable change.
edit: oh ok
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