Nancy, it's not just time you have to take into consideration. At any point in history, you have millions or billions of organisms, each reproducing with a probability of producing a mutation. You have hundreds of thousands if not millions of different mutations going through each population at any given time (after all, just look at hair color, eye color, etc...). Evolution didn't create a kidney and then create a bladder, and then mesh it all together, they developed together very, very slowly.
You also have to take into account lifespans. The shorter the reproductive cycle of an animal, the faster it will evolve. That is how scientists have managed to selectively breed a completely different species of bacteria in, like, 20 years. If you only take bacteria into consideration (assuming they reproduce every 20 minutes), 4.5 billion years is about equal to 2.4 * 10^24 generations. That's a LOT of generations.
And then you also have to consider that different species of animals are remarkably similar to each other genetically. For example, Humans and Mice supposedly only have a difference in 2.5% of their DNA. (Sauce on that one). Very little change in genotype, as was said earlier, can create very large changes in phenotype.