In the history of cell biology, how did Antonie van Leeuwenhoek contribute to the development of cell theory?

• A. He proposed that all living organisms are composed of cells and that the cell is the basic unit of life.
• B. He demonstrated that new cells arise only from pre-existing cells by observing cell division in animal tissues.
• C. He discovered the cell nucleus and argued it controls heredity and cell division.
• D. He used handcrafted microscopes to make some of the first detailed observations of living microorganisms ("animalcules"), providing evidence that cells and microscopic life exist.

Answer: (D)