Segmentation, the replication of anatomical structures throughout the body, is found in many animal species. It's also a huge reason why all those species succeeded, and it comes from a single common ancestor 600 million years ago.
Specifically, segmentation refers to instances where identical anatomical units are repeated on the axis running from the top to a bottom of an animal. (So the fact that we have eyes, ears, arms, and legs that are all identical doesn't count as segmentation.) Obviously segmented species include centipedes and millipedes, in which a single structure is repeated dozens, even hundreds, of times over, but they're hardly the only examples. Anything from earthworms to humans can possess segmented features.
Three of the most basic groups of animals - arthropods (insects, arachnids, and crustaceans), vertebrates (most animals that we're familiar with), and annelid worms (sea and earthworms, basically) - all heavily make use of segmentation throughout their individual species, and yet they're very distantly related groups. Recent evidence indicates that the genes controlling segmentation are essentially the same in anthropod and worm species, indicating that there was indeed a single common ancestor, probably a worm-like creature, some 600 million years ago that proved phenomenally successful because of its ability to segment.
So why does segmentation provide such a huge evolutionary advantage, and why has it helped bring about such fantastic animal diversity? The answer might be quite simple: segmentation creates ready-made spare parts, producing duplicated anatomical units that can be repurposed as needed. If a species is under heavy pressure to fit into a changing environmental niche, it may need to develop new structures that can deal with the altered conditions. In that instance, it would be much easier to modify an existing organ than build a whole new one. Segmentation would give species a better shot at quickly adapting to new environments, which would create more pronounced changes in the species and, thus, greater diversity.
In that case, segmentation is the ultimate example of what Stephen Jay Gould dubbed an exaptation, in which a trait becomes extremely useful for reasons unrelated to its initial development. If nothing else, there's a certain evolutionary irony in that the exact duplication of body parts is responsible for why animal species all look so wildly different.