Tobias wrote: “Thus this “chemical base” + gandhabba (patisandhi vinnana) is the “origin of life” with a dense body. It is only that modern science does not know the gandhabba is required, right?”
Yes. That is correct.
– We see a lot of confusion these days about “when a baby can be called human”?
– In the US, there are many “theories” about when a baby becomes “fully conscious.” Then that evolves into arguments about “a baby not being human” up to a few days, a few months, or even at birth. Some even speculate that since a child cannot “think rationally” until at least a couple of years of age, they may not be “human.” How ridiculous is that?
A zygote in a womb becomes human as soon as a gandhabba merges with it!
– See “Buddhist Explanations of Conception, Abortion, and Contraception”
Tobias asked: “What is spontaneous reproduction of “certain humans” and “certain beings in the lower realms”?”
– There are a couple of accounts in the Tipitaka where a human baby was born on a flower. Here, it is a saṁsedaja birth. I think bhikkhuni Uppalavaṇṇā, who became an Arahant, was born on a flower. That means a zygote assembled on a flower by natural means (chemical composition), and the gandhabba of Uppalavaṇṇā merged with that zygote. That is an extremely rare event.
– There are some petas (or pretas) and niraya beings who are born spontaneously (in the final form), just like the Devas and Brahmas.