All cells are derived from a single PRIMORDIAL cell

This hypothesis is based upon circumstantial evidence  and
                                                 quasi experimental data of the commonality seen
                                                 in all current living organisms (life forms),
      such as...

  1.  all living things are composed of very similar organic molecules:
            the same proteins, lipids, carbohydrates, etc...

  2.  all proteins, the biological catalysts responsible for life's
            chemical reactions, are made from one set of 20
            standard amino acids all of one of two types of optical isomers

  3.  all contemporary organisms carry their genetic information
            in nucleic acids [DNA/RNA] and use the same genetic CODE.