This question requires a lengthy explanation. I will try to provide a summary.
1. A critical factor in rebirth is gati. One’s gati is determined by sankhara (kamma) they engage in.
2. Paticca Samuppada determines not only future rebirths but also the environments where sentient beings with the majority of gati can be reborn.
- For example, at the beginning of a “new Earth,” all sentient beings on Earth have fine bodies like those of Brahmas. Initially, there is no vegetation, animals, etc. and also no males/females. They have just descended from Abhassara Brahma realm and are mostly like Brahmas.
- With time, their “manussa gati” (with raga, dosa, and moha) slowly emerge. Their bodies become dense, and initially, there is “free food” in terms of natural rice-like vegetation that can be eaten without any preparation.
- Then sexes emerge and more incentive to engage in defiled activities.
- With time, those with worse gati are reborn as animals. By that time, there is more vegetation, rivers, etc.
- With time, the nirayas inside the Earth take shape, and some humans/animals are born there.
- That is an example of how the environment changes as the dominant gati of sentient beings change.
3. Toward the end of the Earth (and the Sun and surrounding stars), the environment changes drastically again.
- The Sun (and the Earth) start getting hot, and gradually, rivers dry over millions of years. Mind-pleasing things, as well as food start to become scarce, and people naturally become “more moral.” They start cultivating (anariya) jhana and migrating back to the Abhassara Brahma realm over millions of years. By that time, apayas would have been destroyed, and beings there also gradually moved to the Abhassara Brahma realm (via the human realm).
4. That is the “life cycle” on a grand scale. That cycle has been repeating from a “beginning-less time.” More details in “Buddhism and Evolution – Aggañña Sutta (DN 27)“
5. Another point is the following: On that time scale, the overall gati of sentient beings (and thus the environment or the “structure of Earth”) remains roughly the same. For example, the main continents, oceans, etc., are re-formed similarly, not the same, but roughly the same.
- People with similar gati are born in similar places/countries. All Buddhas are born in the “same” geographical area of India/Sri Lanka. They speak similar languages, have similar names, etc.
- That does not mean the same people recycle back to their original places of birth. Rather, people with similar gati AT THAT TIME are reborn in similar geographical locations. Even now, as gati changes, one’s birth location can change.
That is a brief summary, but it should be enough to get the general idea. It is not worthwhile to spend time on these issues, but it is good to have a general idea. The task at hand is to cultivate the path and be free of suffering because no one knows how long one will live.
