“So the statement ‘all beings started out as humans’ could be very misleading.”
No. That is a correct statement FOR THAT TIME at the beginning of a new maha kappa. By the end of the previous maha kappa all beings in all apayas, human realm, deva realms, and a few lower brahma realms moved up to that brahma realm, which was not affected by the destruction of the other realms. Not a single living being could survive in those realms.
ALL BEINGS gradually move up to that brahma realm, NOT because their gati had changed. All pleasing things the world were gradually being destroyed. So, gradually less and less things the world were appealing to the senses. We could say, they were FORCED to give up cravings.
– Of course, seeing the devastation around (this happened over a long time, over hundreds of millions of years), it is possible that more and more people could have become more “religious” too.
– Since those gati had not changed significantly, those old gati started to take hold gradually in those “early brhama-like humans” (P.S. even better to say that anusaya or “hidden defilements” start to re-surface). That is how those early humans lost their supernormal powers. Of course, those with worse gati were the first to become animals, hell beings, etc. That happened over billions of years.
On the issue of human birth being rare: When the Buddha stated how rare the human birth is, that is regarding “normal times” on Earth, like the present time.
It is good to contemplate on these issues. We also need to think about the time scales involved (millions and billions of years).