In a future timeline, the earth was visited by a species known as the Slarkovs.
Having lost their home planet and in search of a new one, earth was the next habitable planet for them.
Humans and Slarkovs made a deal with each other in
return for the Slarkovs living on earth.
The Slarkovs traded their technology and knowledge for a new home.
They were similar to humans except for some of them who had slight differences so fitting into the society wasn't a problem.
Over the years Slarkovs and humans began to mate
with one another and reproduce offsprings.