The same problems which beset origin-of-life research also bedevil efforts to show how virtually any complex biochemical system came about. Biochem- istry has revealed a molecular world which stoutly resists explanation by the same theory that has long been applied at the level of the whole organism. Nei- ther of Darwin’s black boxes—the origin of life or the origin of vision (or other complex biochemical systems)—has been accounted for by his theory.
In The Origin of Species Darwin stated:
If it could be demonstrated that any complex organ existed which could not pos- sibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.
A system which meets Darwin’s criterion is one which exhibits irre- ducible complexity. By irreducible complexity I mean a single system which is composed of several interacting parts that contribute to the basic function, and where the removal of any one of the parts causes the system to effec- tively cease functioning. An irreducibly complex system cannot be produced directly by slight, successive modification of a precursor system, since any precursor to an irreducibly complex system is by definition nonfunctional.
Since natural selection requires a function to select, an irreducibly com- plex biological system, if there is such a thing, would have to arise as an in- tegrated unit for natural selection to have anything to act on. It is almost universally conceded that such a sudden event would be irreconcilable with the gradualism Darwin envisioned. At this point, however, “irreducibly complex” is just a term, whose power resides mostly in its definition. We must now ask if any real thing is in fact irreducibly complex, and, if so, then are any irreducibly complex things also biological systems?