Gotch long queue below
According to buddhism, souls doesnt only consist of human ones. Animals, insects, living organism all got souls and take turns to reincarnate.
Got different ranking of lifeforms and even each lifeforms got different ranking in social status.
Highest form before attaining enlightenment = human. Then animals, then insects, then other smaller living organism.
Among humans, got born poor, born rich, born in powerful families, born as slave.
Among animals, born as predators (tiger, lion), born as preys (chicken, rats).
Among insects, born as spiders, caterpillar, ants. Etc etc
After they die (can be very short lifespan), they go down to get judged and queue to reincarnate again in their next lifeform. Yes can be downgraded