DNA isn't quite exactly like a program that you compile into a living organism. The conditions for its reproduction and expression are partially determined by i

DNA isn't quite exactly like a program that you compile into a living organism. The conditions for its reproduction and expression are partially determined by its environment- so a shift from single celled from multi-celled life might not be able to be pinned on any particular gene or set of genes as a direct cause. You might end up with what is essentially the same sequence, with different parts now triggered by a slightly different proportion of a particular protein present within the cellular membrane.