Final answer:
In the 17th century, scientists observed microscopic life using microscopes, leading to the development of the cell theory. Robert Hooke coined the term 'cell' for the structures he observed, and van Leeuwenhoek discovered bacteria and protozoa. Electron microscopes enable even closer examination of cell structures today.
Step-by-step explanation:
In the 17th century, scientists made significant observations of microscopic life using microscopes, leading to the development of the cell theory. The cell theory states that all living organisms are composed of cells, that cells are the fundamental unit of life, and that new cells arise from pre-existing cells.
Robert Hooke, an English natural scientist, used a microscope to observe cork tissue and coined the term 'cell' to describe the box-like structures he observed. Anton van Leeuwenhoek later discovered bacteria and protozoa, and other scientists used advances in lenses and microscope construction to observe components inside cells.
Today, electron microscopes allow even closer examination of the internal structures of cells, providing more detailed insights into their organization and functions.