Final answer:
The microscope's first contribution to the cell theory was in demonstrating that all organisms are composed of cells, thereby establishing them as the basic units of life.
Step-by-step explanation:
The invention of the microscope significantly contributed to the development of one of the fundamental theories of biology, the Cell Theory. To the question of which aspect of the cell theory did the microscope first contribute, the answer is that microscopes enabled scientists to observe cells and confirm that all organisms are composed of cells.
This was crucial in establishing that cells are the basic units of life, as postulated by early scientists such as Matthias Schleiden, Theodor Schwann, and later Rudolf Virchow, who also added that new cells arise from preexisting cells. These discoveries formed the cornerstone of the Cell Theory, which ultimately states that all living things are made up of one or more cells, cells are alive and the basic living units of organization in all organisms, and all cells originate from other cells.