In the history of cell biology, what was Antoine van Leeuwenhoek’s most important contribution that helped shape ideas later incorporated into cell theory?

• A. He identified the nucleus as a distinct cellular structure and suggested it controls cell function.
• B. He concluded that all cells arise from pre-existing cells through cell division, rejecting spontaneous generation at the cellular level.
• C. He proposed that all living organisms are composed of one or more cells and that the cell is the basic unit of life.
• D. He used improved single-lens microscopes to make detailed observations of living microorganisms ("animalcules"), providing early evidence that microscopic life exists.

Answer: (D)