It cannot be Cenozoic because that is an era. Not a period. Paleogene, Neogene, and Quaternary (our current period) are all periods within the Cenozoic Era (our current era). On the geologic time scale, there are Eras, Periods, and Epochs (and a few more classifications). Humans first appeared in the Miocene epoch which is an epoch in the Neogene period (in the Cenozoic era).
