Stage 2 Hypothesis: Organic polymers may have formed on the surface of clay
In 1951, John Bernal proposed that organic polymers may be formed on clay surface.
Scientists showed that divalent cations, such as Mg2+, within clay bind nucleotides to clay surface,
thereby positioning the nucleotides in a way that promote bond formation.
Stage 3 Hypothesis: Cell-like structures originated when polymers were trapped in an enclosed
boundary.
When certain types of lipids were dissolved in water, they spontaneously form liposomes
(figure 3). The lipid bilayer surrounding liposome was permeable to certain substances. If RNA was
present on clay surface, it would be trapped inside liposomes. Some liposomes could discharge
electrical signal, similar to nerve cells.
Stage 4 Hypothesis: A chemical selection gave rise to cellular characteristics, starting with an
RNA world.
Most scientists believed that RNA was the first macromolecule within liposomes that
acquired cellular characteristics, because RNA had the following properties.
(1) RNA could store genetic information.
(2) RNA could self-replicate.
(3) RNA could carry out enzymatic functions as ribozymes, catalyzing the synthesis of
macromolecules.
How did RNA evolve into a complex molecule with cellular functions? Scientists proposed
that a chemical selection process was responsible. Figure 4 shows two steps of chemical selections.
First step of chemical selection
1a) Mutation: The first mutation causes RNA to have an enzymatic ability to self-replicate.
1b) Chemical selection: The amount to mutated RNA increases.
Second step of chemical selection
2a) Mutation: The second mutation causes the RNA to have ability to synthesis nucleotides.
2b) Chemical selection: Liposomes containing two types of RNA are favorable