SuttaCentral
What are the spontaneously reborn humans mentioned in the Maha-sihanada Sutta (MN 12)? Are there any suttas that refer to them?
What is spontaneous generation? There are gods and denizens of hell and certain human beings and some beings in the lower worlds; this is called spontaneous generation. These are the four kinds of generation.
People who died as human and being born into the human realm again. Human to human basically. Not human to deva, or human to heaven, or human to asura, or human to peta, or human to Brahma. Human in human out. HIHO.
Or do you mean like beings that are being born without parents? I think all the astral and corporeal beings are born this way. Exceptions only goes for humans and animals. Cmiiw.
I wonder if cloned humans (if ever made it possible) or animals could be considered spontaneously generated beings. I heard Ambapāli and Cincā Mānavikā were born this way. But there is no direct evidence for this in early Buddhist scriptures, but Therīgāthā commentary says;
“Tena carimattabhāve vesāliyaṃ rājuyyāne ambarukkhamūle opapātikā hutvā nibbatti.” -Ambapālītherīgāthāvaṇṇanā
In this life, She was born spontaneously in the King’s graden of Vesali.
It’s said (though I forget where) that opapātika is the normal way for humans to take birth in Uttarakurū, but that it rarely happens here in Jambūdīpa. The brahmin Pokkharasāti of the Ambaṭṭhasutta is one of the exceptions, having arisen inside a lotus flower. This account, however, is from the commentary, not the sutta itself. Here’s Mingun Sayadaw’s paraphrase of it:
What confuses me is that devas are considered born spontaneously but they still have parents and siblings and such, because the Maras are devas and Moggallana was a Mara in a past life with a sister and a nephew, according to MN 50.
Can you please explain to me what Maras are? I’m a bit confused of the term too. What constitutes a mara? Or can you provide me with directions or link what sources I should read? Thank you
Humans cannot be reborn spontaneously.
Mara is basically Satan, an evil angel, who tempts people with sensual desires to keep them trapped under his power. In Buddhism Maras exist in the highest sensous heaven (paranimmita-vasavatti deva) right before the jhana level planes (aka fine material planes) this is why you need jhana to escape Mara. Mara wields a lot of powers, he is a trickster and deceiver, for example he tried to tempt Gotama from attaining enlightenment by sending his daughters to seduce him. Moggallana was a Mara and was reborn in hell for throwing a rock at an Arahant. The Buddha equates Mara to a hunter and humans as deer (prey).
You can see the 31 planes of existence here, Mara is on plane 11 The Thirty-one Planes of Existence
Yes thank you I have read it very interesting story. But why are Maras so smart? They can even deceive Brahmas into thinking that they’re the most high creator.
I don’t think that’s true, I don’t remember ever reading that in the canon. Brahmas have jhanas so they’re more powerful than Mara, and there’s a sutta that explains how Maha Brahma comes to believe he’s the creator because when the universe is expanding beings go from the formless plane to the form plane and Maha Brahma is first so he thinks he created the universe.
Maras are smart because they’re at the top of the sensous planes. It’s like being the head of the hyena rather than the tail of the lion (jhana planes), even though you’re smart you’re still a hyena.
SN 29.2 & SN 30.2 are interesting because they seem to say the same type of being can be born in four different ways:
Dragons are born from an egg, from a womb, from moisture, or spontaneously.
Aṇḍajā nāgā, jalābujā nāgā, saṁsedajā nāgā, opapātikā nāgā.
Of these, dragons born from a womb, from moisture, or spontaneously are better than those born from an egg.
Tatra, bhikkhave, aṇḍajehi nāgehi jalābujā ca saṁsedajā ca opapātikā ca nāgā paṇītatarā.
Dragons born from moisture or spontaneously are better than those born from an egg or from a womb.
Tatra, bhikkhave, aṇḍajehi ca jalābujehi ca nāgehi saṁsedajā ca opapātikā ca nāgā paṇītatarā.
Dragons born spontaneously are better than those born from an egg, from a womb, or from moisture.
Tatra, bhikkhave, aṇḍajehi ca jalābujehi ca saṁsedajehi ca nāgehi opapātikā nāgā paṇītatarā.
Phoenixes are born from eggs, from a womb, from moisture, or spontaneously.
Aṇḍajā …pe…
Of these, phoenixes born from an egg can only carry off dragons born from an egg, not those born from a womb, from moisture, or spontaneously.
Tatra, bhikkhave, aṇḍajā supaṇṇā aṇḍajeva nāge haranti, na jalābuje, na saṁsedaje, na opapātike.
Phoenixes born from a womb can carry off dragons born from an egg or from a womb, but not those born from moisture or spontaneously.
Tatra, bhikkhave, jalābujā supaṇṇā aṇḍaje ca jalābuje ca nāge haranti, na saṁsedaje, na opapātike.
Phoenixes born from moisture can carry off dragons born from an egg, from a womb, or from moisture, but not those born spontaneously.
Tatra, bhikkhave, saṁsedajā supaṇṇā aṇḍaje ca jalābuje ca saṁsedaje ca nāge haranti, na opapātike.
Phoenixes born spontaneously can carry off dragons born from an egg, from a womb, from moisture, or spontaneously.
Tatra, bhikkhave, opapātikā supaṇṇā aṇḍaje ca jalābuje ca saṁsedaje ca opapātike ca nāge haranti.
Experimenting hypothetically with speculative alternate translations, plus keeping in mind the births below are listed from inferior to superior, the following is found in the dictionaries:
Whilst being mere brainstorming, the above sounds like birth related to father, birth related to mother, birth related to physical effort/work (sweat) and birth related to intention/kamma/mind (such as Non-Returners are opapātika due to ending the five lower fetters). 
Regarding SN 29. Naga and 30. Suppanna Samyuttas, the following article by Choong Mun-keat may be useful:
“A comparison of the Pāli and Chinese versions of Nāga Saṃyutta , Supaṇṇa Saṃyutta , and Valāhaka Saṃyutta , early Buddhist discourse collections on mythical dragons, birds, and cloud devas”, Journal of the Oxford Centre for Buddhist Studies , 2020 (18), pp. 42-65. (PDF) A comparison of the Pāli and Chinese versions of Nāga Saṃyutta, Supaṇṇa Saṃyutta, and Valāhaka Saṃyutta, early Buddhist discourse collections on mythical dragons, birds, and cloud devas | Mun-keat Choong - Academia.edu
Thank you. It seems the above article may be useful if the relevant passages were quoted/posted by a person fluent in the book. 
No, this is never what is meant by spontaneously reborn.
The quote that the OP gave from MN 12 shows that they can. Can you provide a source from the texts that says they can’t?
Yes, humans can be reborn spontaneously, according to the text.
The story of Moggallana is also interesting. He was murdered by thugs sent and paid by Jains because he said negative things about Jainism. When the Jains believed strongly in ahimsa. Strange that they don’t just slit his throat or something, instead they break all of his 4 limbs and leave him to die in his hut. Why did they do this? Strange way to end someone, I even read somewhere that Jains don’t even boil water to not to kill the bacteria in the water.
The following article by Choong Mun-keat may be relevant:
“A comparison of the Chinese and Pāli Saṃyukta /Saṃyutta s on the Venerable Mahā-Maudgalyāyana (Mahā-Moggallāna)”, Buddhist Studies Review (Journal of the UK Association for Buddhist Studies), v. 34.1 (2017), pp. 67-84.
I wonder how/why do you think like that because evil kind of person can also be in jain religion. They were(not all but some) evil people who wanted to destroy mahamoggalana, so they paid money to bandits to do this task for them. Well bandits are not kind hearted people. Nor were the Jain’s who wanted mahamoggalana dead. They may be kind to animals, bacteria…but not to mahamoggalana because they saw him as their enemy.
I guess it’s like killing someone without directly killing them, like they used to do 1000 years ago in the middle east, build a wall around someone and let them starve to death, as to not violate the 10 commandments.
Now this actually makes sense. Religious people are sometimes strange like that they don’t want to kill but instead they torture which generates even worse karma 
Some petas are described to have great powers. It might be they were victims of brutal murders. So they end up avenging their death to the perpetrators.