[dragonwriter]

> Each step along the way must produce a net benefit or it gets discarded.

More precisely, each step must not impose a net fitness cost or it gets selected against to a degree positively linked to the degree to which it imposes a net cost. But that's, more importantly, each genetic variation taken in total, not actually each individual phenotypic effect of the genetic variation (e.g., sickle cell trait was preserved not because sickle cell anemia is not a net negative, but because the malaria resistance that comes even when an individual is heterozygous for the trait is a strong benefit that manifests more often than sickle cell disease which occurs when an individual is homozygous for it.)