When people say "save the earth", I think they mean "don't make all other large land animals go extinct".

People don't like to think this, but if humans go extinct, no other intelligent species will emerge for perhaps millions of years. Lots of millions.

Unless larger animals like bears, wolves, large cats, and other large animals like that stick around.

Animals that are already intelligent and large will likely evolve into more human-like beings, and then they will probably utilize some of our existing machines, and rapidly develop primitive societies, leading to a scientific and technological civilization.

But this can't happen if all the large land animals are dead or mutated into non-intelligent beasts, such as cows.

If humans all die off, every single one, and so do all land animals, there might never be a truly intelligent species on earth ever again.