Regarding Rishi’s comments: I may have left out some important facts about those “first humans” on this Earth. This sutta requires many posts to discuss various aspects.
1. Those “first humans” are called “brahma kayika” because their bodies were very fine. But they had the ability travel through air, see long distances, hear things far away, etc (basically like brahmas).
2. So, it is not possible to compare them to single-cell organisms. They had more capabilities than present-day humans.
3. Things started going “downhill” when old “bad gati” started to come back in those early humans. With them came more and more hardships.
4. After long times (probably hundreds of millions of years), they lost “free food” and had to grow food, for example.
5. Furthermore, their desire for sexual relationships also cam back, and that is when the bodies got denser and sex organs started to appear. That probably took a few billions of years, but that is just a guess. No precise times are given in the sutta. Always says, “after long times”.
6. Also, those with “very bad gati” started being born as animals gradually, again “after long times”. Same for beings in the other lower realms. They were all humans at the beginning.
7. Therefore, any animal, peta, asura, hell being today was a human in the beginning. Thus any single cell organism living today has gone through a “downward path”.
8. But of course, it is very likely that we had been in those lower realms too before moving up to born human again. Scientists estimate the age of the Earth to be about 4.5 billion years. That is a very long time! We are likely to have had births in most of the 31 realms many times over, especially in the lowest four realms.
These are things to think about carefully.