The following is hypothetical. . .
The year is 2023. During a testing of a new fusion reactor design, an accident occurs. The accident could not have been predicted according to the theories the scientists were using.
The initial result appears to be that all sexual life forms, as well as most asexual life, within a 2.5 light-year radius from Earth have been sterilized. The mysterious accident destroyed, among other things, all currently produced sex cells, whether in cold storage or naturally occurring, in addition to damaging all sex organs. No adequate shielding existed on Earth and therefore nothing was protected.
Faced with what is considered a worldwide crisis, the nations of Earth form a counsel to search for a solution. Due to new theories the scientists have postulated, a short time later it is decided to correct the shielding problem with the errant reactor and continue with the project. The new "super" power source may give them more options to deal with.
Three years later, Earth's human population slowly decreasing, a more complex plan emerges. Due to the facts, that no ova membranes survived the accident and none can currently be produced, it is currently impossible to genetically engineer or even to simply clone any human on Earth. It is decided that, using new space and astronomy technology the "super reactor" has enabled, the human race should explore the stars to find a solution. In addition, if they increase technology rapidly enough, they may be able to avoid most accidental deaths and dramatically decrease the aging process, thus lengthening the time available to search for a solution.
They are amazingly successful. They find numerous planets, many with complex life forms. Some planets even have climates similar to Earth's with life forms that are somewhat genetically similar to Earth's. Unfortunately, the ova membranes are found to be completely incompatible with humans. The search for a more suitable donor continues.
Many years later a suitable donor species is found. The planet's climate is very similar to Earth's and the life form is a humanoid (in scientific terms) that will probably by partially compatible. The geneticists earlier came to the conclusion that genetic engineering would probably have to be used to hybridize the human species with any semi-compatible donor species--using mostly Homo Sapien DNA engineered with the donor's to allow ova membrane compatibility. The overall process of making the human species sexually self sustaining would require a massive genetics program over many years.
The very old humans aboard these ships look somewhat different than they originally did due to the use of certain "anti-aging" techniques. Only a small percentage of the original population from 2023 is still alive and they are anything but immortal. Onboard their now highly advanced space vessels they decide to attempt to make contact with the humanoids on the planet and persuade them to aid the people from Earth in repopulating the human species.
The attempt does not go as planned. First they contact several "pre-industrial" persons in the outlying area of a large city. They take several persons on board their landed ship, show them around and attempt to begin the initial experiments. The humanoids become extremely agitated and frightened. The Earth humans let them go, deciding to try others from the population. Again and again the humanoids want to be let go. On one occasion a band of sword welding humanoids surprise several of the Earth humans killing most of their ship's crew. Further investigation allows the space faring humans to conclude that a religious figure of a humanoid town had instructed the people to surprise and kill every "evil sky person" that they encountered. The Earth humans are further forced to conclude that the humanoid's overall mythical ideas make the humanoids unwilling to undergo the experiments.
A potentially ethical situation is at hand for the few surviving humans from Earth.
