The Alien Theory

Given the current status of humanity, it is likely that problems arose almost immediately with this race, simply put, they could not overcome the instinctive aggressive and independent nature of the base stock. It was because of these problems that another subservient race was developed. This race however, was not derived from local stock but was engineered from scratch to fill the need and to overcome and prevent the problems encountered earlier. The result of this effort was the grays. It was discovered that for certain tasks, the independent nature of humans was a desirable trait, and also these humans proved to be a ideal host for the gestation of the gray's fetus. Therefore, although the grays replaced humans for most tasks, human were continued to be utilized within the society on a limited basis. It is from this human group that the Adamski/Meier humans might be derived.

After a time, the interest in earth declined, and only a small observation post was left in operation. Mankind was therefore left to develop on earth with only occasional interaction with its former inhabitants. In the meantime, the reptoids were continually modifying the grays, and developing specialized grays for specific tasks.
At some point, the grays evolved enough cognitive abilities to realize there status, and it is this faction of grays that is active on earth attempting to overcome their built in limitations. It is the remoteness of earth that allows this group of grays the required freedom of action.

To gain sustenance these grays must obtain certain nutrients and the simplest and most direct means is to mine them from local fauna.
To reproduce, the grays must obtain host females (called a "container"). Since utilizing the native population of humans currently carries little risk of interdiction, and no requirements for maintaining the host, this method is preferred. Specialized grays, (these grays are taller and more slender that the basic stock) developed specifically to work in genetic engineering labs, oversee these operations as well as the effort to cross-breed their race with humans in an effort to overcome the built in limitations. Females are taken, the DNA evaluated, and if acceptable, the female is tagged for future recovery. Males are also evaluated, and if suitable, tagged for future recovery, some of these males may be used to impregnate current stock of host females in order to assure the transfer of desirable traits to future hosts females.
Other activities involve evaluating the nervous and other systems and tissues in order to optimize the ongoing development their cross-breed's capacities.

The greys have several ongoing problems:

1.The need to increase their number as quickly as possible.
2.As their population increases, more support facilities are required and must be developed. This diverts needed resources from the effort of obtaining self sufficiency. Also as more excursions are required to obtain the suitable containers, the likelihood that their purpose will be discovered and coordinated native interdiction initiated increases. Also the sheer numbers required to continue the breeding operation mandates that they seek hosts in more densely populated areas.
3.As the local military capabilities become more sophisticated it becomes increasing difficult to maintain covert activities.
4.The master race may arrive and intervene at any time.
5.The governments that are aware of their presence may discover the true purpose of their activity and attempt active intervention.