Tonight's topic is... Space Jockeys, and Engineers.
The Setup: The Space Jockey, the big corpse seen in 'Alien', was one of the big mysteries in the movie. What was his story? Were there more of his kind? What's their connection to the Aliens? Ultimately they're never seen again in the Alien movies, but the expanded universe (EU) didn't leave that topic untouched. One shows up as early as the first sequel comic series from Dark Horse Comics, and then they've shown up intermittently throughout the comic series. They also play a reasonably prominent role in the novel 'Aliens: Original Sin', set after 'Alien Resurrection', where it's revealed that their species name is the "Mala'Kak" and that they're generally jerks. This is reasonably consistent with their other EU appearances - the one that shows up in 'Aliens: Earth Hive' (Aliens: Book One) ends up revealing that he wants to take over Earth and terraform it for use as a new "homeworld" for what's left of his people. He goes on a bit of a rampage after the humans figure out what his plans are, and his ship ends up getting blown out of the sky by multiple missiles.
Aliens: Apocalypse paints the Space Jockeys as a sort of noble, tragic, dying race that was wiped out by the Aliens, but that's not wholly incompatible with what's shown in Earth Hive or Original Sin - our only real source of info in Apocalypse is an insane person who's interpreting hieroglyphs that the Jockeys wrote about themselves, so it stands to reason that they'd paint themselves in a good light.
The Problem: 'Prometheus' comes along and promises to shine more light on the Space Jockeys and their role in the universe. Turns out they're not big elephantine creatures, but actually albino humanoids who dabble in "playing god" with genetics and various races and whatnot, and essentially created humanity. It would seem that this wipes out the Space Jockey backstory presented in the EU pretty much wholesale.
The Solution: The Space Jockeys and the Engineers aren't the same species.
The skeleton in 'Alien' is just that: a skeleton. It's got teeth, it's got eye sockets, it's even got what appears to be a tongue.
Even Ridley Scott acknowledges that this was originally intended to be a skeleton.
What we get in 'Prometheus' is... different to say the least. The Engineers in their suits, while markedly larger than humans, are still substantially smaller than the Space Jockey corpse we're familiar with. Not only that, but their proportions are very different - the Engineers are very human-proportioned, while the Space Jockey's arms are very long, the shoulders sit up higher near the head, and the head itself is large and bulbous.
Beyond this, while the Engineer designs are similar to the Space Jockey designs, they aren't quite the same. From a moviemaking perspective that's because the Prometheus stuff wasn't designed by H.R. Giger, so it lacks that certain biomechanical element that any non-Giger work lacks when it tries to emulate his style. Other artists have come close, but it's always lacking something and 'Prometheus' is no exception. The Engineer designs are Giger-esque, but seem to err more on the side of "mechanical" than "biomechanical". Everything is a bit too smooth, and looks more like someone trying to ape Giger's style (which in reality is exactly what's going on). Interestingly enough, I've seen other non-Giger Space Jockey art that comes closer to the original Giger work than anything in 'Prometheus'. The Space Jockey seen in the 'Alien' Heavy Metal graphic novel adaptation, or the "Pilot" structures in AvP2, or the Space Jockey structures in 'Aliens: Apocalypse', all of them look more like the original 'Alien' designs than what was in Prometheus.
So the Engineers are trying to copy the style of another race, the Space Jockeys. My thought is that the Space Jockeys are the Engineers' creators, or "gods". In 'Prometheus', Shaw even mentions the possibility of the Engineers having "gods" that created them.
This would be thematically appropriate given the "meet your maker" recurring theme in 'Prometheus'. The humans seek to meet "god", and Weyland in particular wants to *become* a god himself and surpass the gods. They meet the last surviving Engineer on the planet and learn that the gods created humanity in their image, and end up learning that their gods weren't nice people and were pretty much jerks, so Shaw rejects "god". This cycle plays out similarly with David, who was also created in "god"s image, "god" in this instance being humans. He learns that humans are jerks too, so he rejects them. Perhaps the Engineers sought to meet "god", the Space Jockeys. Upon meeting them they learn that they were not created in god's image, and instead chose to imitate their gods visually via their technology - that would explain why their suits and ships look like Space Jockey corpses and ships, but are still noticeably different -- they can approach god, but can't mimic it perfectly. They also ended up learning that their gods are big jerks (this is in-line with other Space Jockey depictions in the EU, incidentally) and reject them and ultimately surpass them, becoming gods themselves. They end up creating humanity (and in their hubris, create humans in their own image - just as in our hubris we created androids like David in our image), and eventually we seek to meet god, learn that god sucks, surpasses god, and create our own new form of life in our image... which also seeks to understand "god", learns that god sucks, and seeks to surpass god. It's all one big cycle, and the Engineers are just one iteration of it.
I guess if you take away anything from all of this, it's that civilizations that go on to create other life forms end up being colossal jerks and really disappointing their creations when they meet each other.