The credit for first stating that all animals are composed of cells goes to Theodor Schwann in 1839.

He, along with Matthias Schleiden, formulated the cell theory, which stated that:

1. All living organisms are composed of one or more cells.

2. The cell is the basic unit of structure and organization in organisms.

3. All cells arise from pre-existing cells.

While Schleiden focused on plants, Schwann extended the theory to include animals, after observing that animal tissues were also composed of cells.

This discovery revolutionized our understanding of life and laid the foundation for modern biology.