The Origins of Life

Various theories for the origin of life seem to agree that in order to build the most primitive living organism, you need three components:

1. a metabolism, that is, a dissipative structure or autocatalytic cycle of chemical reactions that uses an external source of neguentropy to maintain itself and grow;
2. a membrane, to separate and protect the living system from too strong perturbations from the outside;
3. an RNA or DNA-like "replicator" molecule, that stores information about the organism, and can transmit it, possibly with variation, to copies or "offspring".

Each of the components on their own could have emerged relatively easily. It has been shown by numerous experiments that simple lipid molecules in water spontaneously produce "bilipid" layers, which tend to close in on themselves, thus forming a cell-like compartment. Prigogine and his collaborators have shown that systems far from equilibrium, that is, systems with a continuous input of energy or material, tend to self-organize into dissipative structures, characterized by a cyclical flow of matter and energy. On theoretical grounds, Kauffman has argued that if you put a large enough variety of interacting chemicals together, it becomes extremely likely that at least one autocatalytic cycle of reactions will emerge. For self-replicating molecules capable to store large amounts of information, the mechanism is as yet not so clear. The basic ingredients of RNA are simple molecules that are easy to produce, though, and there are different scenarios for how these molecules might have assembled themselves into self-replicating RNA molecules via a number of intermediate steps (see e.g. de Duve's scenario based on thioesters).

Once the three components would have been united, all necessary functions would be available for life not only to maintain but to develop and evolve quickly. The mixture of chemicals in the primitive oceans, produced by energy from the sun, lightning, heat of the earth and impacts from meteorites, would have provided a rich broth containing enough "food" to let simple cells grow and survive. The bilipid membranes allow small food molecules to be absorbed by the cell and waste products to be excreted. The autocatalytic cycle would use any available "food" to grow and develop, through positive feedback, but once it reaches maximum capacity, negative feedback would have stabilized the metabolic activity at that given level.

What is still missing in this scenario is exactly how the information storing replicator molecule would control this cycle, that is, fix the goal or "reference level" of the negative feedback cycle, so that it would become independent of the environmental situation. Only then could we really call the resulting system "living".

See also: Christian de Duve "The Beginnings of Life on Earth"
For a control-theoretic scenario for the origin of life, see Powers's paper "The Origins of Purpose"