In the history of cell biology, what was Antonie van Leeuwenhoek’s key contribution that influenced the development of cell theory?

• A. He proposed that all living organisms are composed of cells and that cells arise only from preexisting cells.
• B. He discovered the cell nucleus and argued it controls cell division and heredity.
• C. He used improved single-lens microscopes to observe and describe living microorganisms (“animalcules”), providing early evidence that microscopic life exists.
• D. He developed the first electron microscope, allowing visualization of viruses and cell ultrastructure.

Answer: (C)