That xenomorph sure matured quickly. What, about 5 minutes? I know it didn’t take long in Alien either but it sure seemed fast. And what was that cocoon the mature alien was coming out of? I guess it’s just something we’ve never seen before
The believability of the alien lifecycle in
Alien is a critical aspect of why the film worked so well and its enduring appeal. As Ridley Scott himself said regarding H.R. Giger's creation: "I think Giger has an extra quality, and one of the most frightening things is the quality of reality, combined with a sort of his own form of fantasy. And I think that's what makes it stronger, is the reality, not the fantasy."
I re-watched it last night to verify I had these facts straight.
In the film, once landed, Dallas, Lambert, and Kain set out to find the source of the signal while Parker and Brett began the repairs needed to get the Nostromo back into space. We know from one of the scenes that Parker informed the senior officers that the repairs would take "at least 25 hours." We know from later scenes that Parker and Brett were both tired and short-tempered from doing the work, and that Dallas was exasperated and impatient to leave as soon as it was feasible to do so. There was also some time spent when all the crew were evaluating Kain's initial condition and dealing with the aftermath of the acid incident. All of this happened while still on the planet.
We don't know exactly how much time elapsed from the point where Kain was infected to the Nostromo getting back into space. However, given the extent of the repairs and all the other stuff which occurred on the planet, it seems it was at least a full day, possibly two. Once back in space, Kain was in stasis for quite a while before his condition apparently improved. Shortly thereafter, the crew joined for dinner where the chestburster appeared.
After Kain's death, there was a burial which was attended by the full crew. There was also sufficient time for Ash to design and build the motion tracker while Brett built the "cattle prod." Then the search got underway, where Brett was abducted by the now full-grown alien. So, from the time Kain was first infected to the point where we first see a fully developed alien is approximately 2-3 days.
That's very fast by the standard of any terrestrial organism, but it's within the realm of believability given the bizarre nature of the alien's biological composition and quasi-parasitic nature.
Now I don't mean to sound unduly harsh, but Fede Alvarez either completely misunderstood the lifecycle of Giger's alien as it was presented in the original film, or he just didn't care and morphed it into a generic monster because that was convenient for the dumbed down action movie Hollywood wanted. Either way, the end result is a video game which apes some of style of
Alien, but none of the substance.