Final answer:
The key aspect of the cell theory in biology is that all living things are made of cells, the cell is the smallest unit of life, and new cells come from pre-existing cells.
Step-by-step explanation:
The key aspect of the cell theory in biology is that all living things are made of cells, the cell is the smallest unit of life, and new cells come from pre-existing cells.
This theory was proposed by Matthias Schleiden and Theodor Schwann in the 1830s after they observed cells in both plants and animals. Rudolf Virchow later added to this theory by discovering that cells divide to produce more cells.