Could aliens see Earth before humans existed?

Yes—if aliens were far enough away and had powerful enough telescopes, they could be seeing Earth as it was before humans existed.

That’s because looking across space is also looking back in time: light from Earth takes time to travel. An alien civilization 1 million light-years away would see Earth as it was 1 million years ago. One 100 million light-years away would see Earth during the age of dinosaurs. One 4.5 billion light-years away, in principle, could see the early Earth forming—though actually resolving Earth at that distance would be far beyond any realistic telescope we know of.

So the answer is: physically yes, because Earth’s old light is still traveling outward. Practically, seeing details like oceans, continents, life, or pre-human animals would require extraordinarily advanced observing technology.