Chapter 4 Evolution and Biodiversity Summary 1. Life emerged on the earth through two phases of development: a chemical evolution of the organic molecules, biopolymers, and systems of chemical reactions to form the first cells and the biological evolution from single-celled prokaryotic bacteria to single-celled eukaryotic creatures, and then to multicellular organisms. 2. Evolution is the change in a population?s genetic makeup over time. Evolution forces adaptations to changes in environmental conditions in a population. The diversity of life on earth reflects the wide variety of adaptations necessary and suggests that environmental conditions have varied widely over the life of the earth.