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I read the subject book in 1981 and it wasn't new then. The book probably would be described today as a space opera.
From what I can remember there is an intersolar system war between two solar system races NOTE humanoid, not human. One planet is eventually destroyed (the asteroid belt) and the other is laid waste...
There is a colony on the dark side of the moon that is destroyed in a massive meteor storm with the survivors walking to rescue on the moons surface gradually dying off until only # one makes it through sheer bloody minded determination to the rescue site where a small number of survivors revamp to earth Now the story swaps back and forwards with many reverses and twists but in the end archaeologists find the one walkers equipment buried for over 2000 yrs...aka our forefathers..
And that's what I can remember, the story is intense and the revelations during the book are stunning I seem to remember.