According to one theory put forward by David Icke, humanity is actually under the control of shape-shifting alien reptiles, who require periodic ingestion of human blood to maintain their human appearance.
Reportedly the Bush family and the British Royal Family are actually such creatures, and Diana, Princess of Wales was aware of this, presumably relating to her death. The proponents claim a number of evidence ranging from Sumerian tablets describing the “Anunnaki” (which he translates as “those who from heaven to earth came”), to the serpent in the Biblical Garden of Eden, to child abuse and water fluoridation.
A variant tells that particular technologies, notably the transistor—were given to American industry in exchange for alien dominance. The enforcers of the clandestine association of human leaders and aliens are the Men in Black, who silence those who speak out on UFO sightings. This conspiracy theory has been the basis of numerous books, as well as the popular television show The X-Files and the movies Men in Black and Men in Black II.
In the far distant past around 240 millions BC, an intelligent technological species of theropod did evolved on earth. This species developed space travel and established colonies in nearby star systems. The asteroid impact that apparently occurred 65 million years ago devastated this civilization as well as nearly destroying the entire ecosystem. With their home civilization on earth devastated, the colonies were left to evolved on their own.
It would be likely that the earth would be monitored and as the ecosystem recovered these reptoids would resume operations on earth. Given the distances and effort involved to move resources back to earth, it is likely that a need to augment the local work force with a native labor force that could accomplish the more menial and laborious tasks.
A simple means of acquiring this labor would be to utilize local terrestrial stock and genetically alter this stock as necessary to accomplish these goals. This idea is not without precedent, since mankind itself has done this with horses and other domesticated livestock. It is likely that this is how modern man was developed.