In an efficient and highly-evolved system, many different things are each responsible for a few percent of the final result. No single factor dominates. If one element were so important, then work (human, or via natural selection) could be devoted to improving that element --- optimizing it --- and thereby it would become less critical, less of a bottleneck. Every piece is in turn subject to that focused effort, until no piece stands out from the crowd. The system moves toward balance.
(correlates: BarryLawsAndPrecepts, TheoreticallyKnown, BuSab, ...)