IRC there is a short story of Asimov that can be considered part of the foundation series; here an humanity that's come very lately in the building of faster than light engine is basically limited to the solar system, but humans had a big tech advantage...time travel tech.
Basically one group alter the timeline so will be the most favorable to humanity...aka wiping out all the other species before they existed.
i take it you mean asimov's "end of eternity" ?
http://en.wikipedia.org/wiki/The_End_of_Eternity
indeed in the end of that novel a change is made in time so mankind would discover spacetravel early, but would not discover time travel.
as for the lack of aliens, wasn't there some hint that they were killed by an external source?