There is no other person who saw the existence of birth and death (the Sansara) like the Supreme Buddha. The Great One saw each and everyone’s births as animals like pigs, chickens, snakes, dogs, and so on and so forth. He also revealed the existence of low-level (Niraya, Preta, Asura) and high-level (deity) worlds, and how the beings are born in those worlds. Accordingly, not everyone is reborn as humans after their deaths.