The third statement of the cell theory holds true now but on early Earth, the first cells must have come from inorganic substances present at the time in the Earth's atmosphere.
All living things contain at least one cell. Cells are the functional units of living things. All cells come from pre-existing cells. The work of Stanley Miller, a chemist who along with Harold Urey performed an experiment in 1952 which showed that organic compounds found in living things could be synthesized from inorganic substances under the simulated conditions of early Earth. It is hypothesized that this ancient atmosphere contained hydrogen gas, methane, ammonia and water vapor. There was heat from volcanic eruptions and solar energy(there was no oxygen, thus no ozone to absorb harmful rays). There was also electrical energy due to lightning. In his experimental set-up, he allowed the hydrogen gas, methane, water vapor and ammonia react for several days under these conditions. At the end of the experiment, he collected the chemical building blocks of life--amino acids. Many other scientists repeated this experiment and were able to get similar results.
The conclusion--although he never was able to create a living cell from inorganic compounds, the chemicals of life did form under the conditions of early earth's atmosphere. Therefore, it is further surmised that once these chemicals evolved and became more complex, eventually a cell arose.
It is hypothesized that single stranded RNA must have evolved before DNA, and it can self replicate. Once RNA and its protein product was enclosed in a lipoprotein "bubble" and could replicate itself, this would have been the first "cell". This would only happen in the early atmosphere previously described. On today's earth, these chemicals would be destroyed due to the presence of oxygen.
The use of better technology has allowed a modern man(or woman) of science to not only take the cell theory that was presented many years ago but also to see this in action. Technology has allowed us to view the act of bio-genesis and record the findings. The recordings allow for deeper investigation of the theory of cell reproduction.