Alien Origin

What follows is my personal canon for the Space Jockey as seen in Alien, with some influence from Prometheus. 

Foreseeing their eventual extinction over the next few thousand years, the Space Jockeys send their ships to the furthest reaches of their known galaxy. Each ship is tasked with finding a habitable world to colonize.

A prehistoric planet Earth is found by one of these ships. It's determined to be an ideal world for terraforming. There's one problem: the planet is already inhabited by a pre-industrial race of humans. Specimens are collected and taken back to the homeworld for study.

There, the Jockeys design their plan. After years of research, the humans' genes are edited into the DNA of a species of parasitic orgnism, to make the humans a viable host for it. 

Research! 

The union of the two species creates a new one, an Alien xenomorph that will be used as a biological weapon to eliminate the human presence on Earth.

Development! 

The plan is simple. The creatures will collect the humans in hives of secreted resin, using them as hosts to birth new Aliens and continue their lifecycle. 

Once deposited, the hive resin grows like a fungus and takes it's own shape. Given time, it will form a huge silo. 

Programmed by the Jockeys with an insatiable need to kill and procreate, they will not stop until their supply of hosts are exhausted. 

Eventually, the Aliens themselves will perish from lack of sufficient prey, leaving behind their organic hives, which will serve as a foundation for the Jockeys to grow their biomechanical cities. 

Gentrification 

A lone Pilot with a singular is integrated into a specially built ship. It leaves the homeworld for Earth with a cargo full of Alien spores. But something goes wrong. The ship never makes it to Earth. It lands -or crashes- on a moon that will be designated as LV426 hundreds of thousands of years later, when the crew of the Nostromo would find the derelict spacecraft....

I tried

Meanwhile, the surviving human specimens that were collected ages earlier have evolved into a new race who worship the now extinct Space Jockeys as their Creators.