Not a ladder. A bush of species, several of which overlapped. Brain volumes are typical adult means; dates are first-to-last fossil appearances as currently argued. The corridor will not tell you who is your grandmother.
Dates Ma (million years ago). Brain cc after common textbook means (Aiello & Dean / later reviews). Species concepts in palaeoanthropology split and lump; Homo habilis in particular is a drawer some curators would empty. Neanderthal admixture in non-African living humans is genetic fact; the cultural stories are still being rewritten.