Giants #3: Giants’ Star by James P. Hogan

ONE:
Eons ago, a gentle race of giant aliens fled the planet Minerva, leaving the ancestors of Man to fend for themselves.

TWO:
50 thousand years ago, Minerva exploded, hurling its moon into an orbit about the Earth.

THREE:
In the 21st century, scientists Victor Hunt and Chris Danchekker, doing research on Ganymede, attract a small band of friendly aliens lost in time, who begin to reveal something of the origin of Mankind.

Finally, Man thought he comprehended his place in the Universe…until he learned of the Watchers in the stars!