Those scientists and institutions--the ones sitting at the experienced far end of the continuum--are exceptional. Indeed, second-tier research institutions tend to expect the most experience; Harvard and Johns Hopkins do not expect you to have your own research grant. Most hiring committees aren't looking for completely independent work; they're looking for original, creative ideas, together with a record of accomplishment. Few people applying for tenure-track jobs have had the opportunity to start their own research programs. After all, traditionally that's what assistant professorships are all about, and most institutions still think that way. It helps to be somewhere in the middle of that continuum, but most committees are still looking more for promise than for guarantees.