The atmosphere is composed of 71% of nitrogen, 21% oxygen, 3% carbon dioxide and traces of argon and other gases. The air on LV-223 is mostly identical to Earth's atmosphere, but contains a higher concentration of carbon dioxide (CO2) making it toxic for humans to breathe outside (Earth levels of CO2 currently average about 0.04% for comparison).
It is uncertain if there were any existing life forms before the Engineers, but at some point a group of them constructed an outpost on the planet. It is suggested by Janek that it was some type of military base set up by the Engineers to build biological weapons before some incident compromised their operation. There is also a worm-like lifeform found within which is speculated to have been indigenous to the planet. After going through a mutation process similar to Fifield's, it's believed the worm-like lifeforms become the lethal Hammerpede.
At some point approximately 2,000 years before, the Engineers established a temple on LV-223 and initiated a plan to wipe out humanity for reasons unknown by releasing a pathological agent known as Chemical A0-3959X.91 – 15. However, they apparently lost control of the pathogen and led the Engineers there to die off. Four surviving Engineers took refuge within their ship's Hypersleep Chambers, but only one of them survived.
In 2089, archeologists Elizabeth Shaw and Charlie Holloway discovered a star map among several unconnected ancient cultures within a cave during their expedition on the Isle of Skye. They interpreted this as an apparent invitation from the Engineers. Shaw then contacted Peter Weyland, the elderly CEO of Weyland Corporation, for a proposal in order to gain enough funding for an expedition to follow the star map. Weyland agreed with the proposal and funded the creation of the scientific vessel USCSS Prometheus to follow the map that led them to LV-223.
In 2093, after the Prometheus' two year voyage, the ship arrived on LV-223. The Prometheus crew explored the moon's surface where they came across the Engineer's structure. The crew entered the structure and discovered the bodies of some Engineers, as well as several "urns" stored within the structure. The crew's interference caused the pathogen stored within the "urns" to melt. A few crew members were exposed to the pathogen, and in the ensuing chaos most were killed. The android David successfully found the last living Engineer and informed Weyland, who was secretly aboard the ship the entire time. Weyland and his team went to the Engineers' ship to commune with the last Engineer however, upon waking, the Engineer decapitated David after a brief conversation and killed Weyland along with the entire party, save Shaw, who managed to escape.
The Engineer reactivated his ship and headed for Earth to carry out his original mission to wipe out the human race. The remaining crew later sacrificed themselves and the Prometheus to destroy the Engineer's ship. The angered Engineer survived and attacked Shaw within Prometheus' lifeboat. Shaw released the Trilobite she had extracted earlier from her womb as the result of having sex with Holloway who was infected by the pathogen, which restrained and impregnated the Engineer while Shaw escapes. Shaw recovered David's remains from the crashed Engineer ship, the two activated another Engineer ship and left LV-223 for the Engineer Homeworld. In the crashed lifeboat, the impregnated Engineer died, birthing a Deacon.