What makes you think they have any need to overpower us? What would be the evolutionary advantage for ants in not having humans around? And, as far as I can work out, there is only one organism on earth that actively tries to wipe out its competitors - and that's us. The rest of the animal and plant and bacterial kingdoms seem to do pretty well taking it as it comes.

From a purely biological point-of-view (mainly for the reason above), humans have all the hallmarks of a fairly short-lived species. I'd give us 10 Million years tops (bearing in mind we've used up the first 5 million of those just getting ourselves off the bloody savannah and into some decent clothes and coming up with the ipod.) Bacteria, ferns, crocodiles and turtles have it all over us.