The original cell theory was first purposed by Schleiden and Schwann in 1838. Originally, it stated three main characteristics of all cells. The first characteristic in traditional cell theory states that all forms of life are made from at least one or more cells. The second characteristic in traditional cell theory is that cells only come from already pre-existing cells. Lastly, in traditional cell theory, the cell is known as the smallest living thing to exist.
More recently, scientists have expanded the original cell theory. This expansion, known as modern cell theory, still follows all of old cell theory but adds three further requirements. The first is that energy to sustain life is generated within the cell. The second states that cells contain DNA which contains the instructions for life, and it is passed from cell to cell during cellular division. The final new addition in modern cell theory states that all cells in closely related species must function similarly to one another. Hope this helps!