However, radio signals had been detected at a planet 200 light years away, indicating intelligent life had arisen.
While the aliens explore the old cities, it is revealed that it is typical for species to take thousands of years to go from radio transmission to space travel.
During their exploration of the Earth, the aliens find a communication tower transmitting what is unfolding into what looks like empty space.
Humans do not possess faster-than-light ships, but have built generational starships in the hope that their descendants will one day find a new home.
I don't believe I've reread it since its original appearance, and I refuse to do so now — for fear of discovering how little I have improved in almost four decades.