Protobionts: the Simple Cell that is the Precursor to Life
Protobionts were the precursor to early life, and resemble very simple cells. They are microspheres composed of inorganic and organic molecules trapped inside of a lipid bilayer membrane. The lipids that form the membranes (which resemble cellular membranes), called liposomes, automatically form a bilayer in the shape of a sphere when put in water. Also like cell membranes, the membranes can be selectively permeable and undergo osmotic swelling and shrinking. Some liposomes are also able to maintain a voltage across the membrane, just like the membrane potential that is vital in many in cell processes. Protobionts are also able to go through simple "reproduction" by giving birth to smaller liposome microspheres. If amino acid polymers became trapped in the liposome membrane during it's formation, the protobiont can become selectively permeable to organic molecules. When a self-replicating molecule is trapped inside or is formed inside of the protobiont, this structure has many characteristics of a prokaryote. Scientists speculate that a series of events similar to this did occur, and through random variation and "natural selection" of these structures, the first living prokaryotes began to arise (AP BIOLOGY TEXTBOOK). Scientists speculate that RNA was the first genetic material to develop, because unlike DNA, RNA is able to replicate itself. Ribozymes, RNA molecules folded into an particular structure (often associated with other molecules) are able to facilitate the replication of other short RNA molecules (TALK ORIGINS). The self-replicating RNA molecules would produce multiple copies and when it's protobiont "reproduced" the new micropsheres would contain the same genetic material as the parent, with possibly some variation due to mistakes. The RNA molecules that replicated most effectively would produce the most copies, and just like in natural selection, this would cause RNA to "evolve" over generations of the self-replicating molecules. These protobionts containing replicating RNA molecules would be very similar to early prokaryotes, and as they continued to evolve they eventually became them (AP BIOLOGY TEXTBOOK).