In the lists of "once's", we have: 1) proto-life, 2) single-cellular life, 3) multi-cellular life, 4) neurons (which eventually form brains), 5) enough intellig

In the lists of "once's", we have: 1) proto-life, 2) single-cellular life, 3) multi-cellular life, 4) neurons (which eventually form brains), 5) enough intelligence to launch rockets. Now it is possible (maybe even likely) that other life on earth will develop human-level intelligence (not only other primate, but thinking of crows and dolphins too). But intelligence is rare enough that it takes a long time to achieve.