Ha, I was right. The thing behind the head is not only really an altar, it is indeed where the engineer sacrificed himself to generate the black goo which would be used to create the xenomorphs, as shown on the mural.
Sacrificial altar, with bowl. In the movie they replaced it with a green stone??
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Wait, what. Why would they change such a major visual detail in the movie? This really introduces more questions about the altar. If it was originally a bowl like the one from the opening, it would have certain specific implications. But since they deliberately changed it into something else, that implies they want to suggest something different. But instead it's just a random prop in the background which they never bother going back to again. Wut????