Cell Theory
Cell theory states that all living things are made of cells, the cell is the basic unit of life, and all cells come from existing cells.
Definition
Cell theory is a set of three big ideas about life: all living things are made of cells, the cell is the smallest unit of life, and new cells come only from cells that already exist. Stated more formally, all organisms are composed of one or more cells, the cell is the basic unit of structure and function, and all cells arise from pre-existing cells by division. The theory was developed in the 1800s by scientists including Schleiden, Schwann, and Virchow using improved microscopes, and Virchow's phrase omnis cellula e cellula captures the third principle. Modern extensions add that cells carry hereditary information as DNA, that energy flow (metabolism) occurs within cells, and that cells of similar species share a basic chemical composition. Viruses, which lack cells and cannot reproduce on their own, sit outside the theory.
Example
When you scrape your knee, the new skin that grows back comes from skin cells dividing to make more skin cells; the new cells did not appear from nowhere. A bacterium dividing every $20$ minutes under ideal conditions gives $2$ cells after $20$ minutes, $4$ after $40$, and $8$ after $60$, and after $6$ hours ($18$ divisions) one cell has become $2^{18} = 262{,}144$ cells, every one descended from the original. Pasteur's swan-neck flask experiments made the same point in the lab: a sterilized broth stayed clear as long as airborne cells could not reach it and turned cloudy with microbial growth only after the neck was broken, supporting the continuity of cells rather than their spontaneous origin.
Key Insight
Before microscopes, people believed tiny living things could spring up from mud or old meat, and cell theory replaced that guess with evidence. The third principle rules out spontaneous generation, so every cell in your body traces an unbroken chain of divisions back to a single fertilized egg. Because all cells descend from earlier cells, the features they share, such as the genetic code, ribosomes, and ATP, are evidence of common ancestry. Cell theory is therefore a bridge between cell biology and evolutionary theory.