The way I understand this ‘descent into matter’ of those first ‘humans’ is like this:
I must first regress a bit. When the destruction of the planet took place, all living beings found themselves in the Abhassara realm. Being in that realm, they assumed the qualities of those brahmas, ‘physical’ and mental ; or, to put it another way, they ‘levelled out’ to brahmas, all of them: those who had been human, animals, fishes, insects, ALL living beings. Now, when the descent back to the newly-formed Earth took place, they all started out as humans. The human realm is the one that lies in between the ‘good’ (deva and brahma) and the ‘bad’ (apayas) realms. So it makes absolute sense that beings ‘start all over’ from a mid-way point, with a human MINDSET. (There may be the question here of how all those trillions of beings could fit physically AS HUMANS on the surface of the planet – but remember at this earliest of stages they are still brahma-like in ‘form’, i,e. suddhashtakas, practically formless).
This state of affairs continued until the planet evolved air, vegetation, and so on, and could support living physical forms. Depending of the natural conditions, those beings then started to assume physical forms in accordance with their predominant gathi up until that time. And as we know that beings spend most of their time in sansara in the apayas, the gathi of most of them would consign them to the apayas. From the geological standpoint ,the various hells would have formed first. Now if we also keep in mind that a human birth is very rare (certainly the Buddha was not referring to all those ‘humans’ who started out on the newly-formed Earth) there would be only a very few, relatively, who actually take on a human form.
We talk about this event happening ‘in the deep past’. But in fact it is only the one that has happened most recently, because this has been going on from a time without beginning.
So the statement ‘all beings started out as humans’ could be very misleading.