Reincarnation, as I'm aware of it, bases itself upon a (in the most basic form) a rewards scheme, start off on a lowest form of life, live a good life, and you move up a level, bad deeds means you're going down or repeating your current level
But you can look at it this way, back in "bible times" (used for your own reference) there were a lot more trees and wildlife around, these days, lots more humans (this is assumeing that humans are infact a higher form of existance) so you could make a logical connection betweent the number of "souls" as trees has decreased leading to more humans, "souls" were living much better lives to get to this point, and now there are many people either looping as humans and some have acsended to a higher plane of existance? some have decended to a lower form.
Of course, this is all speculation and complete BS even if I do say so myself

but in a way, the logic is impressive