"Our Children's Children" (1974) by Clifford Simak is a story that attempts to fuse several over used science fiction tropes into something new and different. What results is a meandering tale with the distinct feel that it had been hastily contrived, with a truncated ending.
Alien invaders have overcome our children's children some 500 years into the future. These aliens are very nasty. How nasty you ask, well, they love to kill, are super fast, breed like rats and eat people...nasty. Our peace loving progeny decide to evacuate their entire population of 2 billion, via time tunnels back to our time. ("How do you do, we are from your future escaping man eating aliens, do mind if we move in? Here is a bag of diamonds for your trouble.") Well, some aliens slip back along with the humans to our time. This unfortunate event keeps the story line lumbering along.
The narrative is told from the perspective of the White House press office. This gives the author several opportunities to introduce us to various newspaper types. Actually these are the most interesting characters in the book. It's no surprise considering Simak's press background. There are several conferences with the President and his advisers that attempt to comprehend 'the big picture' and what to do about it...ho-hum. There are a few chapters with plot threads that read as it they could have been edited out the story with none the wiser. The "love story" plot line is so sappy I had trouble reading it with a straight face. The whole concept of time-travel, multiple "time tracks" going back and forward in time and the discussion of alternative universes is so unconvincing it reads like ragtime.
It is noteworthy for Simak devotees that this story does not take place in the bucolic environs of southwest Wisconsin or near the Twins Cities of Minneapolis-St. Paul but in Washington D.C.
This 1974 story is still modestly entertaining and thankfully a quick read. It was first published in IF science fiction magazine (1973). It was last published in the U.S. in 1983 as a DAW paperback with a nifty Frank Kelly Freas cover.
The author used the identical time travel concept [resettling Earth's population into the far past] in his 1978 novel "Mastodonia".