Fig. 1.

Ratchet-like processes build-up complexity by increasing the number of gears that underlie clock function. In a hypothetical clock, three gears (Gear H, Gear M, and Gear S) control the dials (hr, min, and sec, respectively). They are put in motion by the central gear (Gear C), which is powered by the battery (indicated by the green light at its center). From panel a to b the clock increases in complexity. Step a to b indicates the addition of a previously non-interacting gear (Gear N), which does not affect the function of the clock. Step b to c introduces a defect in the central gear (Gear C), making the new gear (Gear N), which has its own power source, essential for the clock to function. Step c to d separates the new gear (Gear N) from the rest of the machinery. This change renders the clock non-functional due to lack of access to power. In sum, the presence of a new powered gear (Gear N) introduces excess capacity, which provides the opportunity for another gear (Gear C) to lose power and results in an increase in complexity that is difficult to revert