Alien Genesis by Gary Beene

In 1954, Ensign James Cortell wakes up from a coma with memories of an alien scientist who had visited Earth some fifty thousand years ago. Years later, he realizes this knowledge should not be taken to his grave.

Earth’s extraterrestrial visitors altered Homo sapien DNA to produce modern humans. Soon the industrialist overlords among the alien race realize that genetically altered humans offer a source of cheap labor. A galactic slave trade is born. When an unplanned mutation begins causing gigantism among some of the sapiens, Dr. Kadeya and her grandson, Ramuell, travel to Earth and plan to rectify the situation.

As the scientific and industrialist factions of Earth's rulers clash, Kadeya and Ramuell are caught in the middle. But behind the scenes, an even greater power is at play, and very soon their future - and the future of both races now inhabiting the planet - will change forever.