post on Antarābhava Discussion in Kathāvatthu – Not Relevant to Gandhabba

In MN12 that you referenced, the four types of yoni (ways of birth) are stated:
“Sāriputta, there are these four kinds of reproduction” (Catasso kho imā, sāriputta, yoniyo.)”

A better translation would be: “Sāriputta, there are these four kinds of births”

The sutta states those four kinds as, “Aṇḍajā yoni, jalābujā yoni, saṁsedajā yoni, opapātikā yoni” which can be translated as “Born from an egg, from a womb, from moisture, or spontaneously.”

In a way, ALL births (that take place at the cuti-patisandhi moment) are opapātika or spontaneous births.
– The “four modes of birth” apply to mainly humans and animals.
– The manomaya kaya/gandhabba created spontaneously by kammic energy can lead to “births with physical bodies” in the other three ways.

Thus, the “seed” for the other three modes comes from the manomaya kaya/gandhabba.
– For humans, a gandhabba gets into a womb and is born with a physical body. That is the “jalābujā yoni” or “birth from a womb” with “breaking out of the amniotic sac.”
– For animals, all three modes apply. Some (monkeys, deer, etc) are born like humans (jalābujā yoni“); Chicken, birds, etc. are born from an egg (aṇḍajā yoni). The last mode of saṁsedajā yoni is rare for big animals and applies mostly to smaller ones. Here the required “chemical base” for the gandhabba happens naturally in rotten meat or leaves.
– Note that in all three of those modes, a “seed” (manomaya kaya/gandhabba) must have been first produced by kammic energy.

Also, note that in all realms above the human realm, all births involve one step, i.e., spontaneous birth (opapātika) via kammic energy.

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.

I just posted a new post that makes further clarifications:
Four Types of Births in Buddhism

Please feel free to ask questions or make comments.