Would it be possible for a planet to evolve only one form of animal life?

If a planet requires, or is likely to have, a diverse biosphere, does that mean there have to be a lot of animals on the planet? Could Earth have evolved with lush forests, lofty mountains, forbidding deserts, and absolutely no creatures except for people?

Would it have been possible for humans to appear on the planet, either after or instead of the dinosaurs, with no other creatures? That would make us herbivores, obviously, and it seems that most advanced species are predators. Would we still be living in caves if that were the case? Would all our primate cousins still be coexisting with us?

Maybe “no animals” is too restrictive. How about fish in the sea and insects to pollinate flowers, but no birds or mammals?

Anything’s possible, but this seems very implausible. Evolution involves competition, genetic drift, and lots of other concepts which require the existence (to my knowledge) of different species.

Define “one form” of animal life. One can make the case that all life on Earth, from the tiniest archaebacteria to the blue whale and the giant sequoia, are all “one form” of life, since we all trace back to the same origin and all use the same genetic code.

Species in the kingdom Animalia? Vertebrates?

An Earth where there are trees and plants, seaweed, and humans. Insects, fish, and birds are optional. No mammals.

Highly unlikely. In order for only one species of mammal to develop, you would need to somehow prevent any speciation to have occurred during the entire development path from fish to human. To do this, you’re going to have to keep the entire population very small and prevent them from ever splitting into multiple reproductively isolated groups. I don’t think that’s even possible on an earth-like planet, you’re inevitably going to get a situation where a successful species has spread enough for multiple reproductively isolated populations to occur. Even if your pre-human mammal species is limited to a tiny island with no chance to become physically isolated, you still have the chance of isolation via different populations on that island becoming adapted for different ecological niches. Consider, for example, the finches of the Galapagos - started out from one initial population and proceeded to differentiate into multiple species adapted for different food sources.

Secondly, even if you did somehow prevent any speciation during the evolutionary period, I doubt you’d end up with something like a human. Evolution is driven by competition and change. An isolated population in an unchanging environment and no opportunity to diversify into multiple competing populations is going to reach a form optimized for the local environment, and then not change.

I think the only way you could get a world with humans and no other complex animals would be with outside intervention, some deliberate force weeding out side branches and only permitting development towards modern humans. A sure sign of Intelligent Design, or alien manipulation or something.

Extremely unlikely, barring divine magic.

This doesn’t even make sense. Insects, fish and birds are all “forms of animal life”.

He’s saying the one form of animal life that he’s “choosing” is vertebrates.

But he forgot that both fish and birds are vertebrates. Wait, what the hell… Humans are OK but not mammals? Good luck with that one. :wink:

And what directly preceded humans in our evolution . . . seaweed?

Somebody’s been watching South Park.

Let’s assume that you can arrive at a situation where only humans exist. Then what? How are you going to stop humans from speciating?

In this world there are huge untapped resources in the form of garbage, vegetation, seafood etc. and vast uninhabited areas with no predators or disease. How are you going to stop humans from occupying those niches?

IOW even if you did have a planet with only humans, it wouldn’t exist for more than a million years.

I can’t imagine only one form of animal life evolving. I think it’s marginally possible that some tremendous environmental upheaval might result in only one form of animal life surviving.

Without insects, how would flowers bloom ?

Or, more to the point, without insects, why would plants evolve flowers?

Umm, you did notice that the OP specified “insects to pollinate flowers”.

Well it would have to be a cannibal life form.

Are you thinking of a zombie planet, perhaps?

Elephants are highly intelligent herbivores.

And you could go back in time to a period when there were “fish in the sea and insects to pollinate flowers, but no birds or mammals”, so the answer is yes.