ABSTRACT
In the last chapter we learned that microorganisms vary considerably in how they respond to environmental conditions. Some, for example, require oxygen, while others are killed by it. The reason for this, as we saw, was that some organisms have enzymes to detoxify dangerous oxygen compounds while others do not. But why would organisms develop in such different ways? When we ask about the origin of such a difference, we are going straight to the heart of one of science’s most fundamental questions: what accounts for the remarkable variety of living things that exists today?
