A ladder is not the right metaphor for evolution. A bush is.
New species arise in very small populations that become isolated from their parental group at the periphery of the ancestral range, the "bush". Speciation in these branched groups is very fast by evolutionary standards - hundreds or thousands of years (a geological microsecond).
The steps of this speciation might not be captured in fossils since it happens very fast. We encounter the new species when it reinvades the ancestral range and becomes a major population by itself. The process repeats and we find "sudden" changes in fossil records.
There were several co-existing brances of the human bush. We are merely the surviving branch.
Ever Since Darwin: Reflections in Natural History by Stephen Jay Gould
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