Rats

"I have a post-Armageddon vision.

We and all other large animals are gone.

Rodents emerge as the ultimate post-human scavengers. They gnaw their way through New York, London and Tokyo, digesting spilled larders, ghost supermarkets and human corpses and turning them into new generations or rats and mice, whose racing populations explode out into the cities and into the countryside. When all the relics of human profligacy are eaten, populations crash again, and the rodents turn on each other, and on the cockroaches scavenging with them. In a period of intense competition, short generations perhaps with radioactively enhanced mutation-rates boost rapid evolution.

With human ships and planes gone,
Islands become islands again,
With local populations isolated
Save for occasional lucky raftings:
Ideal conditions
For evolutionary divergence.

Within 5 million years, a whole range of new species replace the ones we know. Herds of giant grazing rats are stalked by sabretoothed predatory rats.

Given enough time, will a species of intelligent rats emerge? Will rodent historians and scientists eventually organize careful archaeological digs (gnaws?) through the strata of our long-compacted cities, and reconstruct the peculiar and temporarily tragic circumstances that gave ratkind its big break?"

Richard Dawkins, from "The Ancestor's Tale"