Here's why:

* Explains the diversity of life: Evolution explains how all living things on Earth are related through a common ancestor and how they have diversified over time.

* Provides a framework for understanding: Evolution provides a framework for understanding how organisms function, interact, and adapt to their environments.

* Connects all levels of biological organization: Evolution operates at all levels of biological organization, from the molecular level to the ecosystem level.

* Guides research and discovery: The understanding of evolution drives scientific research and helps us understand the origins of diseases, develop new medicines, and conserve biodiversity.

While other principles like the cell theory and the principle of homeostasis are important in biology, evolution is the overarching principle that unites all of biology.