The cell was first recognized by Robert Hooke, but all the cells he observed were dead.
After advancement in microscopy, in year 1838 Schleiden and Schwann proposed cell theory.
The cell theory proposed by Schleiden and Schwann suggested that all the living organism are made up of cell. These cells are the smallest unit, which can perform all the function, which is required for a life, they can also divide, hence, it is the functional unit in all forms of life.
Later, Rudolf Virchow said, all cell arise from the existing cell, which further supported the idea of cell as a universal unit of life.
