All Tomorrows by Olaf Stapledon

The story begins in the near future, as burgeoning population pressures force humanity to terraform and colonize Mars. After a brief but violent civil war between the two planets, the genetically engineered survivors begin a new wave of colonization, spreading across the galaxy. Everything is looking up for the human race... until the colonies encounter the Qu, technologically advanced aliens on a religious mission to remake the universe. Although humans fight valiantly, the Qu easily overpower humanity; as punishment, the aliens decide to genetically modify the survivors, turning most of them into mindless, animalistic creatures before departing.