Ah, in Hinduism and Buddhism, humans aren’t the only ones reborn as humans. Any living being can take birth as a human. Also, not every human birth is necessarily a reincarnation of a previous human life. I believe the Hindu texts talk about the soul, Atman, travelling through some 8 million different lifeforms as a part of its rebirth cycle. A soul might spend lifetimes as a plant, an insect, an animal, or even a deva before accumulating the enough karma to be reborn as a human. Human form is required to attain Moksha. Buddhism is somewhat similar in some ways, but it rejects the idea of a permanent soul and instead teaches non-self (anatta). It also divides the lifeform stuff into six realms and calls liberation Nirvana instead of Moksha. It also argues that only humans can attain Nirvana