What theory explains both the diversity of life on Earth and the similarities of all organisms at the chemical, cellular, and molecular level?

The theory that explains both the diversity of life on Earth and the similarities among all organisms at multiple biological levels, including chemical, cellular, and molecular, is the theory of evolution. This theory posits that all species of organisms have developed over time through processes such as natural selection and genetic variation. As organisms adapt to their environments, they evolve, leading to the vast diversity we observe today.

At the same time, the theory of evolution highlights the common ancestry of all life forms, which accounts for the similarities in fundamental biological processes and structures. For instance, the basic building blocks of DNA, cellular mechanisms, and metabolic pathways are remarkably conserved across different organisms, indicating that these features evolved early in the history of life and have been maintained due to their effectiveness.

In contrast, other theories such as those focusing solely on genetics or adaptation do not encompass the full breadth of biological diversity and the shared characteristics that arise from evolutionary processes. The theory of genetics primarily deals with heredity and the variations in traits passed from one generation to the next, while the theory of adaptation specifically addresses how organisms change in response to their environment, but does not explain the overarching connections and commonalities among all life forms.