History (conjectural): It is claimed by a scientists that the species originated in the Large Magellanic Cloud (called the Magellanic Galaxy in Species II), but no real evidence has been given to that assertion. It is also stipulated that they were a "cancerous" race that visited Mars millions of years ago and annihilated all life on its surface, (which was Earth-like at that time) before leaving a remnant of DNA in its soil. This DNA was intended to be picked up by other visitors so their species could continue to infect other planets in like manner.
Two years later, the first successful manned mission on Mars was infected and took over by the original alien life form, which had layed contained in a Martian rock sample. It transformed the surviving astronaut into the male version of the species and started to look for a new version of Sil which had just been created in a governmental lab. They were killed after they mated and their offspring survived.
Source: Species I, II, III