Cell theory
1. all living things are made of one or more cells - microscopes allow us to observe that all living things are either unicellular (one cell) or multi-cellular (many cells)
2. the cell is the smallest unit of life - nothing smaller than a cell can survive independently, subcellular structures or organelles cannot survive independently (examples of organelles: nuclei, ER, golgi, chloroplasts, mitochondria)
3. all cells come from pre-existing cells - this seems to imply that life has always existed, which is incompatible with geological evidence about the age of Earth therefore, an exception is made for the origin of life, when cells must have arisen from non-living substance since the conditions of early Earth were anaerobic (without oxygen), they allowed for cells to form from non-living substances. The conditions of present Earth are aerobic (with oxygen), precluding the formation of cells from non-living substance.
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