Found this in MN12:
Four Kinds of Generation
32. "Sariputta, there are these four kinds of generation. What are the four? Egg-born generation, womb-born generation, moisture-born generation and spontaneous generation.
33. "What is egg-born generation? There are these beings born by breaking out of the shell of an egg; this is called egg-born generation. What is womb-born generation? There are these beings born by breaking out from the caul; this is called womb-born generation. What is moisture-born generation? There are these beings born in a rotten fish, in a rotten corpse, in rotten dough, in a cesspit, or in a sewer; this is called moisture-born generation. 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.
What does the part about spontaneous generation mean or what is it talking about.....especially with regard to human beings.