From the perspective of a biologist, the history of life on the Earth is a recurring story. Although details may be different, what happens today has precedence in the occurrences of the past. Two underlying ideas are vital for students to understand a modern conception of the theory of evolution. First, over long periods of time, and due to natural events, the environment of the Earth has changed, sometimes dramatically. Second, each environmental change has created more favorable conditions for some species to live and reproduce, while producing less favorable conditions for others. This first lesson in our unit on biological evolution addresses these two conditions explicitly.