The theory that I have arrived at, after years...nay, decades...of extensive Biblical research is this:
They were all hunted down and subdued by Adam, as commanded by God in Genesis 1:26-28. People lived a lot longer back then, Adam being 930 years old when God finally slew him (Genesis 5:5)...more than enough time to dispatch a bunch of dumb pest-reptiles, amen?
Of course, Adam had only begun the good work that we Christians must finish. Do you realize how many untamed beasts on this planet mock God and His children with their very continued existence, on a daily basis? I was homeschooled by my mother, so math is not my strong point...but probably thousands!
If it is not being barbecued, experimented on to make sure my shampoo doesn't melt my face off, or wallowing in its own feces in a zoo for my personal amusement, what good is it? No good at all, I tell you... just another drain on the natural resources put here exclusively for the benefit of True Christians.