[Thor]

Talkorigins response to this common abiogenesis question.

The most primitive cells are too complex to have come together by chance. (See also Probability of abiogenesis.)

Response:

1. Biochemistry is not chance. It inevitably produces complex products. Amino acids and other complex molecules are even known to form in space.
2. Nobody knows what the most primitive cells looked like. All the cells around today are the product of billions of years of evolution. The earliest self-replicator was likely very much simpler than anything alive today; self-replicating molecules need not be all that complex (Lee et al. 1996), and protein-building systems can also be simple (Ball 2001; Tamura and Schimmel 2001).
3. This claim is an example of the argument from incredulity. Nobody denies that the origin of life is an extremely difficult problem. That it has not been solved, though, does not mean it is impossible. In fact, there has been much work in this area, leading to several possible origins for life on earth:
   • Panspermia, which says life came from someplace other than earth. This theory, however, still does not answer how the first life arose.
   • Proteinoid microspheres (Fox 1960, 1984; Fox and Dose 1977; Fox et al. 1995; Pappelis and Fox 1995): This theory gives a plausible account of how some replicating structures, which might well be called alive, could have arisen. Its main difficulty is explaining how modern cells arose from the microspheres.
   • Clay crystals (Cairn-Smith 1985): This says that the first replicators were crystals in clay. Though they do not have a metabolism or respond to the environment, these crystals carry information and reproduce. Again, there is no known mechanism for moving from clay to DNA.
   • Emerging hypercycles: This proposes a gradual origin of the first life, roughly in the following stages: (1) a primordial soup of simple organic compounds. This seems to be almost inevitable; (2) nucleoproteins, somewhat like modern tRNA (de Duve 1995a) or peptide nucleic acid (Nelson et al. 2000), and semicatalytic; (3) hypercycles, or pockets of primitive biochemical pathways that include some approximate self-replication; (4) cellular hypercycles, in which more complex hypercycles are enclosed in a primitive membrane; (5) first simple cell. Complexity theory suggests that the self-organization is not improbable. This view of abiogenesis is the current front-runner.
   • The iron-sulfur world (Russell and Hall 1997; Wächtershäuser 2000): It has been found that all the steps for the conversion of carbon monoxide into peptides can occur at high temperature and pressure, catalyzed by iron and nickel sulfides. Such conditions exist around submarine hydrothermal vents. Iron sulfide precipitates could have served as precursors of cell walls as well as catalysts (Martin and Russell 2003). A peptide cycle, from peptides to amino acids and back, is a prerequisite to metabolism, and such a cycle could have arisen in the iron-sulfur world (Huber et al. 2003).
   • Polymerization on sheltered organophilic surfaces (Smith et al. 1999): The first self-replicating molecules may have formed within tiny indentations of silica-rich surfaces so that the surrounding rock was its first cell wall.
   • Something that no one has thought of yet.

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