Antoni van Leeuwenhoek and Robert Hooke first discovered the presence of the microorganisms in the environment during 1665-1683. Using the microscopes, the discovered that there are many tiny life forms that are not visible to the naked eye. This discovery led to the shattering of the belief that the life forms can appear out of the ruined non-livings forms, and gave rise to the cell theory.
The cell theory states that the cells are the basic unit of the life, and every living organism is composed of these cell(unicellular) or cells(multicellular). The new cells arise from the already existing cells.
Hence, the answer is cell theory.