After watching Alien Covenant its now apparent (to which nobody really looked into) is that the variations we have seen in all films all point to a similar conclusion. That is, the eggs have all been a different variant than the one in the 1979 classic and the concepts of mutation and evolution in Covenant leads me to believe the following:
1. The environment (primordial or earth-like) in which the eggs develop results in different types of face huggers.
2. How the eggs are produced determines what type of facehugger. These can be produced from either Human, Android, engineer (small) or an engineer (large- I'm lead to believe these have biomech suit dependencies). I'll go into how in number 5.
3. In order to get the Bio mech version found in Alien 1979, one must create an egg through a large engineer and be in the confines of a primordial environment or un-earth like environment (such that the gravity may be different hence the dripping of liquid upwards).
4. We have seen in Alien 3 the absence of a REAL chestburster and much like Covenant the body of the burster is fully developed. This is not due to those HOST factors as most believe but the actual SUPER FACE HUGGER that was obtained on LV-426. Size of the facehugger doesn't matter, what matters is HOW that egg got made.
5. How eggs are made (this is just a theory) but I believe that
Spoiler
David experimented with a spore inside Shaws body hoping to produce answers to Davids reproduction queries. Hense, Shaws torso opened and head morphology.
If could be possible that an egg grows out of the body like a PLANT. David makes several of these but NONE from an engineer as were lead to assume ALL engineers were killed in the bombing scene.
This is the important part in getting the HR GIGER xenomorph. The egg must be birthed from the LARGEST engineer possible (the ones that were statues in Prometheus and Covenent.) and MUST be stored in Un-inhabitable HOT environments (remember the egg chamber? This blue light could have kept the eggs in incubation for a long time.)
6. There were SPORES on LV426 Juggernaut as depicted in the matt painting HR GIGER made.
7. Although there were no dead facehuggers around in that Juggernaut on LV426 (large ones), it could be assumed that the engineer was experimented on much the same way Shaw was. THAT is why there is so much calcification on that beast much the same way
Spoiler
Shaws body had displayed with some few odd stuff around here head. this could lead to those peices becoming part of a calcification process not yet understood.
8. Xenomoprhs from Ressurection and Covenant and AVP had a more organic look to them. Possibly meaning that they would survive and thrive in more earth like conditions. The Xeno in 1979 could have survived much better in space, space stations, or in juggernauts or on more primordial earth conditions. I say this because non-earth like conditions have less gravity, less oxygen, more methane, more extremophile type conditions.
That being said, I feel I know where the story should go to in the next movies if we are to get closer to how the 1979 movie creature was made.
