1. Cell Theory Cell theory states:
All living things are composed of one or more cells.
The cell is the smallest unit that retains the properties of life.
New cells arise from pre-existing cells.

2. Abiogenesis vs. biogenesis
Abiogenesis (aka spontaneous generation): the theory that living things can come from non-living things.
Ex. Aristotle said that flies could be produced by rotting food.
Biogenesis: the theory that living things must come from other, pre-existing living things.
Ex. Flies come from the reproduction of other flies.
 Accepted