According to the cell theory:
- The cell is the smallest functional unit of life.
- All organisms are made up of cells.
- Cells come from pre-existing cells.
There is a large body of evidence to support these claims. Microscopic observations have shown that the cell is the smallest functional unit of life. We now know the various organelle (or organs) of an individual cell and how they work. For example, a bacteria is a single-cell organism and is capable of carrying out all its life process (growth, division, metabolism, etc.) and is thus a unit of life. It has also been seen that nothing smaller than a cell can survive on its own. All the organisms, including human beings, are made up of cells. We can check that out by using microscope, for example to study our skin cells, blood cells, plant cells, etc. All these cells work together to keep us alive and well. It has also been found that cells cannot come out of nothingness. We can sterilize something (by say, autoclaving or heating) and see that nothing will grow on it. One can try this at home, by sterilizing food and leaving one container open to the atmosphere and other one sealed. The sealed container will have no growth, while the other one will show growth.
Thus, there is evidence for all the tenets of cell theory.
Hope this helps.