In the year 2611, a Salsene colony ship landed on the planet LV-314. While investigating a mysterious crash site, one salsene was attacked by a facehugger and impregnated.
Within days, 90% of the salsene colony had been killed by the xenomorphs, and over a hundred saliens were born. A few salsenes managed to escape the world, but one had been impregnated by a salien facehugger. Thus the saliens spread to other planets.
Predators occasionally hunt saliens, as their ability to use weapons renders them quite formidable. In fact, it has been speculated that these are among the most dangerous xenomorphs of all.