“The Amazon is possibly the most diverse ecosystem on earth, with animals ranging from snakes to apes,”
Apes? Far as I know the only ape in North or South America is Sasquatch.
So, is this an argument from Credulity?
“from all varieties of insects to birds and spiders.”
ALL insects? Really?
Um… Sand Fleas?
“How can all these have a common ancestor?”
Evolution. It’s, like, in the introductory chapter of every textbook.
“I would make sense if there were simply diverse arrays of birds and such, and perhaps diverse insects, but if these creatures evolved, then they really shouldn't share the same niche.”
THey do not. The spiders you mention? They don’t eat the plants, they eat the insects that eat the plants. So, they chare the same geographical area, but not the same niche.
“Unless we're to believe birds and spiders have a common ancestor,”
Yes.
“ or snakes and monkeys in the Amazon branched from the same ancestors.”
If you go far enough back, yes. They all have vertebrae, so that’s a clue about when they split off from the spiders and sand fleas and rubber tree plants and jellyfish….