Glenn wrote:
When Ridley made Alien in 1979 he had the life cycle of the alien already planned (and Filmed) these scenes were cut in the final version , but it was widely know ... Egg , Facehugger, Chestburster , Alien , . The Alien would then grab the crew , and impregnate the crew and they would turn in to a egg , thus starting the cycle again (these scenes would be put back in the Directors cut a few years ago) Scott also said in interviews in 1979 that the alien would take the characteristics of the host , if it came out of a dog the it would be like a dog ..etc
When Cameron came along to make Aliens , he stated in interviews he did not like Scott's life cycle of the Alien , so he wrote the Queen alien in to the script, and since then Every Director has altered the Life cycle / and Look of the Alien since ,
As you say: Ridley had a good idea of the Alien's lifecycle in ALIEN. It would seem that in Prometheus he wanted to make like the other directors and changed it's lifecycle for the point of changing. Except he didn't want to make it clear if it was to be another creature, a proto-alien or the Alien itself so he left his bets hedged (to allow for the 0-2 sequels to Prometheus, that he may or may not be asked to make, to still vaguely line up in the Alien universe).
Different directors have different ideas (and the same director can change their mind), but
my own opinion is that Ridley has forgotten the elegance of the Alien (both in appearance and life-cycle). If he had enough respect for his previous work (and the fans?) he would have taken care to make this very definitely a different creature rather than mess with the Alien by linking it (even tentatively) with an ugly thing whose lifecycle was apparently dictated by how the available footage edited together rather than being thought through.
And I don't mean that it's linked genetically, by some distant heredity or even by bioengineering prototype and redesign "in universe" - that may or may not have been the intention - but in the realm of fictional monsters it is related too closely to the Alien by it's actual characteristics.
Reviewers and pundits say that some fans hated it because there wasn't an Alien in it - I have reservations about Prometheus because there WAS.
But that's
my own opinion - I've always had more reverence for the Alien in my mind than I do for Ridley Scott or any of the other directors.