How did scientists discover that all living things are made of cells?

The study of the cell is called cell biology.

The correct answer to the question is by cell theory.

The scientist Rudolf Virchow states the cell theory after the study of plant and animal cells.

After 1900, when the electron microscope was discovered, the study of the cell can be more efficient and discover that all the species is made up of cells.

The first organism is made up of cells.

Hence, the correct answer is mentioned above.

For more information about the cell, refer to the link:-

link

Explanation:

All  living things are made out of cells. But how did scientists figure that out? Well it dates back all the way to 1839. Two scientists, Matthias Schleiden and Theodore Schwann, were viewing some animal and plant tissues when they found that there were little cella meaning "little rooms" in the tissues. Robert Hooke, a scientist as well, had given this name to cells. He had discovered cells in a cork. He name these little holes in the cork cells because they looked like the cells that monks lived in, in monasteries.