The notion that Earth has been visited by intelligent beings in our past, who tinkered with human genetics has been explored by a number of authors, such as Zecharia Sitchin(1). In his book, The Gods of Eden(2), William Bramley coined the term "Custodians" to describe the ancient astronauts that were documented by the Sumerians. Bramley chooses not to speculate on the Custodians' origins and believes that "it is conceivable that the alleged ownership of the Earth may have changed hands over the millennia," in the same way a corporation can change owners without the public being generally aware.
Mesopotamian tablets recount a creation story where one "god" is put to death, and "the body and blood are then mixed into clay, and out of this concoction a human being is made," Bramley writes. Sitchin has concluded that this process was actually a reference to genetic engineering, where humans were developed to be a slave race to the Custodians.
Could this have had something to do with the demise of the Neanderthals? "It might be conjectured that Neanderthal man was either exterminated or hauled off to make room for the new slave race, and perhaps to prevent breeding between the two subspecies," wrote Bramley.