Millions of different kinds of organisms have evolved on earth and each can be placed in one of these five kingdoms. More importantly though, each interacts with many other species and plays a part in a highly interconnected web of living things on earth. Underlying unity at lower levels There is an underlying unity between all these very diverse forms of living things. Again, an evolutionary perspective tells us that that is due primarily to the fact that they all share a common ancestor. It would be broader and more complex still if all living things on earth did not share a common ancestor at some distant point in time that allows us to study all living things together.
Evolution ends up being an important and central theme.
Evolution tells us that all living things share many complex structural, strategical and chemical features because they once all shared a common ancestor that possessed those very same traits. The differences we see between them today are due simply to changes that have accumulated in them over the course of time. Without evolution we would not be able to understand nearly as much as we do about what makes things alive and causes them to stop being alive. If every organism or major group of organisms arose on earth independent of the others each would be expected to have arrived at different ways to accomplish the countless tasks that living things must perform to be able to survive and reproduce. This is a science course offered by a science department.