Everything Xenomorph revolves around the black liquid. This liquid has it's origins in the beginning of Prometheus, where unknown Engineer consumes a black liquid, disintegrating himself and contaminates the water of an unknown world. This unknown world is most likely LV-223, not earth as so many theorize. His mutated DNA mingles with bacteria in the water and it rapidly spreads turning the water and his DNA black. This new mutated version of the black liquid infects an indigenous species of worm on LV-223 and gives rise to a new species, Hammerpedes,
the first Xenomorph.
Now one may wonder why the Engineers DNA had such an aggressive effect on the water on LV-223. I believe this is caused by the Engineers attempts to improve themselves by engineering... their own DNA to evolve faster, something they have done for many thousands of years. This would explain how the black liquid and Xenomorphs are capable of such rapid evolution and adaptability. Just to go further down the Engineer path. I also believe that the Engineers are in fact engineered humans and not a species of their own. They are the lost civilization of Atlantis and Atlantis is the cityship they built before they left earth, hence why it was never found, this will be explained in Prometheus 2 if I know my movies right.
Anyways, the black liquid contained inside the ampules that infects Holloway is not the original pathogen, it's an evolved form directly sampled from the Hammerpedes. This becomes clear when Shaw prematurely and artificially gives birth to the Trilobite,
which have characteristics of the Hammerpede. This would not be the case if it was the original pathogen. Hammerpedes like all Xenomorphs adapt and take on characteristics of it's host. This means that the trilobite is part human, part hammerpede. Since Shaw prematurely removed the Trilobite it is still in it's larvae stage and not yet ready to burst from its host, this means it's still capable of adapting and evolving to a host. Which it does later on when re-impregnating itself into the Engineer. The Proto-Xenomorph who is later successfully birthed is therefor a 25% mix of worm & trilobite , 25% human, 25% Engineer and, hold on for this one, 25% biosuit from the engineers organically laced biosuits. A small detail many people tend to miss. So how does this translate into the Xenomorphs famous appearance? No fear, I shall have point and tell below.
- Xenos and worms-
Worms are sub-terrestrial and therefor lack eyes. However, they have an ingenious way of "seeing". They have specialized cells on their outer skin that are so sensitive they can actually detect slightest change in light, heat or chemical composure around them. You can imagine it by closing your eyes and looking into a lit lamp. Move your hand in front of your closed eyes and you can see the small changes in light intensity. So basically worms has an array of tools to use to navigate: Nightvision, heat vision and sensors to pick up slightest chemical in the air, clearly superior to our eyes. Hammerpedes are also worms, they probably have a similar type of vision as earthworms do and this got passed on to the humanoid Xenomorphs. This is also why their skin is smooth on their heads, to improve sensory adaptations. The hammerpedes look is also adapted into chestbursters.
-Xenos and Humans/Engineers-
This is probably the most apparent characteristic. They became bipedal with arms, legs and spine, just like humans and engineers. I would theorize that the humanoid features of Xenos is dominant because the Trilobite larvae inhabited two humanoids and therefor took on more humanoid characteristics than worm like. Their intelligence and ability to learn is also most likely caused by their double stay in two intelligent humanoid species.
-Xenos and biosuits
This is the sneaky one, but it's not a theory, it's fictional fact. This is where Xenos get their bio-mechanical look from. As can be seen below.
This is most likely the adaptation that gives Xenos their tough outer layer that is temperature resistant.
-Xenos and Trilobite-
I believe that the shape of the Xenos heads are related to the fact that Shaw removed the Trilobite prematurely, while it was still in the larvae stage. You can see the iconic head shape on the small larvae in the scene when Shaw removes it. Below is a picture to aid you.
-Conclusion & TLDR-
I also think it is from the Trilobite Xenomorphs got their extendable jaw. At the end of the movie when the Engineer is fighting the Trilobite we can see that the Trilobite has extendable tentacles, smack a teeth laced mouth on a tentacle and we got ourselves a party! Their head shape was also adapted from the larvae to accommodate the length of their extendable jaw.
Xenomorphs are just a happy accident caused by poorly written characters carelessly behaving like, well, humans. The Engineers had very little to do with Xenos besides delivering the original pathogen to LV-223.