One idea of the Greys is that they are not extraterrestrials at all but are actually humans from the distant future.
The theory suggests that in our future we have meddled with our genes so badly that the human race as changed to the point that only the most basic of features still remain. What if a virus was developed that altered our very DNA causing us to have the appearance of a Pokemon? Many suggest that the cartoon was developed by occultists in an attempt to prepare us for the shocking visage of the greys. What if a mutation was introduced which suddenly enabled those infected to reproduce at superhuman rates? What if this same virus gave us long life at the cost of losing reproduction? Greys are invariably described as having no visible genitalia. The "future humanoid" theory suggests that they have come back in time so that they may abduct us to recover lost genetic information, or to cross breed in hopes of saving the human race. Such genetic mining operations are not limited to merely humans. Some creatures posses qualities which are highly appealing should they ever be adaptable to a humanoid form. Superior eyesight from night dwelling creatures and the regenerative abilities of some invertebrates.