The only mammals alive at that time lived in holes in the ground.

The existence of civilization--cities--is the issue. It depends on farming. Farming has only existed for a few thousand years, while the planet as a whole has had a stable climate of a certain kind. Destabilizing that climate will assuredly destroy the agricultural network that allows cities to exist as they do today.

Cities--civilization--will collapse. The collapse of civilization will be literal hell on Earth: billions of people abandoning the cities and doing whatever it takes to try to survive in the ensuing hellscape that the most idyllic countryside will become.

If you can find it, watch the movie No Blade of Grass: in it the cause of the collapse isn't global warming (though warming makes that cause more likely), but as a depiction of what life will be like it can't be bested.