This was taken from a comment I saw on another site, and I did a little research and everything seems to check out! :
Wow.To give some historical/mythological background that may shed some light, Ridley stated the SJ culture was based on Persian Myths. This would be Sumerian/Akkadian/Hindu. This is all taken from the Atra Hasis.
Creators - "G"ods- Annunaki - Dragon Humanoids (Naga, Dragon Kings,)
Helpers - "g"ods - Igigi - Engineers. (Android like living beings....BIOmechanical humanoid. Key features- Pale skin and large black eyes. Also known as watchers, Grigori, and Archons) (in many summerian texts they are actually referred to as "Pilots". Pretty much the Annunaki Air Force.)
When the Annunaki began terraforming the earth, they had the Igigi do the work for them. After a few thousand years the Igigi revolted and went on strike. The Annunaki then decided to create humans to do the work for them.
They sacrificed one of the rebel Igigi named Geshtu to use his blood and dna to make human beings, by mixing it with elements native to the earth.
(In the movie, this can be explained by the the different oval spaceship at the beginning representing the spaceship of the Annunaki)
(It can also be explained by the concept art that leaked from the official book this week)
(According to wikipedia it also says this about the Igigi: "Though sometimes synonymous with the term "Annunaki," in one myth the Igigi were the younger gods who were servants of the Annunaki, until they rebelled and were replaced by the creation of humans." This is reflectled exactly in the concept art below!)
Even though the humans were created and did the work, 1/3 of the Igigi still werent satisfied and sought revenge for Geshtu, so they rebelled again against the Annunaki Lords and began breeding/mixing with the human females creating Nephelim. This is what sparked the Prime Lord Enlil to flood the earth. Some humans were saved by Enki, the Lord responsible for the sacrifice of Geshtu and the creation of humans. Enlil and the rest of the annunaki decide to return home and let the humans develop on their own. Enki and his family stay behind. The Igigi are forced to leave earth as well. The remaining rebel Igigi are imprisoned on a planet on the way back to the homeworld and it is said as punishment and as a mark they are altered into a demonic appearance, no longer retaining the Angelic appearance.
Enki and his crew are probably the ones leaving the maps for humans to find, along with the ones helping humans advance throughout time.
The sacrfice engineer is Geshtu
The lone engineer is most likely Marduk or a servant/worshipper of Marduk.
The xeno is Mushussu, a creature Marduk fashioned and used as his pet.
The "Engineers" we see are trying to destroy Earth are of the Igigi rebels who view earth as their own. They have always despised humans because the Annunaki saw us as more in their likeness than them. IT's possible that the Igigi have long since destroyed or taken over the annunaki and the homeworld, and Earth was like going to claim the prize or spoils.
They mustve used to the Xeno's to win this war and through its perfection it has began to destroy and infect the Igigi who manufacture and transport it, creating more Mushussu.
the xeno in Alien is most likely an older pilot igigi birthed Mushussu egg crossed with human or a future Annunaki birthed one which would explain the size difference in hosts.
It is mentioned in several lesser stories that Marduk created the Mushussu out of using the essence of the Gods' (Annunaki) he killed as a symbol of his conquering and being able to control them... ie the mural.