Final answer:
The microscope is the tool that led to the development of cell theory by enabling scientists like Robert Hooke and Anton van Leeuwenhoek to observe cells, which was later developed further by Matthias Schleiden, Theodor Schwann, and Rudolf Virchow.
Step-by-step explanation:
The tool that led to the understanding that cells are the basic unit of life was the microscope. In 1665, Robert Hooke used a microscope to observe cork tissue and coined the term "cell". Following this, Anton van Leeuwenhoek furthered this work by discovering bacteria and protozoa. Progress in microscope technology eventually enabled scientists like Matthias Schleiden, Theodor Schwann, and Rudolf Virchow to establish the unified cell theory. This theory holds that all living things consist of one or more cells, the cell is the building block of life, and new cells come from existing cells.
These discoveries, bolstered by advances in microscope design and staining techniques, laid the groundwork for the modern understanding of cell biology. In particular, the invention of the electron microscope in the 1950s allowed for even further insight into the complex structures within cells.