The concept that all animals are composed of cells was established by the German zoologist Theodor Schwann in the 19th century. Schwann, along with his contemporary Matthias Schleiden, contributed significantly to the formulation of the Cell Theory.
Schwann’s work in the 1830s demonstrated that not only plants but also animals are made up of cells, which was a revolutionary idea at the time. He meticulously studied various animal tissues and concluded that the cell was the fundamental unit of all living organisms.
This was a critical advancement in biology, leading to a better understanding of life itself. The implications of Schwann’s findings laid the groundwork for modern biology and helped shape how we view the organization of living organisms.