Answer by Jim Ashby:
It's not just humans that get reincarnated: all animals do, even gnats and dung beetles. When it comes to which animal host a soul-in-waiting chooses, certain species can be in vogue for eons at a time. Dinosaurs of all kinds were a popular choice of host for reincarnated souls until, after many millions of years, they fell out of favor. Souls-in-waiting can turn fickle.
Humans hosts have been gaining in popularity for many thousands of years now. Souls that used to choose hosts like the bandicoot, the Bali tiger and the Zanzibar leopard have all jumped on the human host bandwagon.
Other species currently enjoying popularity among reincarnated souls include: cats, dogs, cows, chickens, pigs, rats and cockroaches.
The increase in souls that choose popular hosts, like humans, is precisely offset by a decrease in souls that choose other species.