While it's not technically a "theory" in the sense of a single, specific idea, the theory of evolution by natural selection, primarily attributed to Charles Darwin and Alfred Russel Wallace, is the unifying principle of all biology.

Here's why:

* Explains diversity: Evolution explains the incredible diversity of life on Earth, from the smallest bacteria to the largest whales.

* Connects all organisms: It shows how all living things are related through a common ancestor and have evolved over millions of years.

* Underpins biological processes: It provides a framework for understanding biological processes such as adaptation, speciation, and extinction.

* Guides research: The theory of evolution directs scientific research in areas like medicine, agriculture, and conservation.

So, while many scientists contributed to our understanding of evolution, Darwin and Wallace's theory of natural selection provides the core explanation for how life has evolved and continues to evolve.