alien franchise - Why Do Facehuggers Fit Human Faces So Perfectly?

As seems to happen so often when we read one question here, another comes to mind.

While I liked the original Alien and the first and third sequels (and saw all four), I never wanted to see Alien vs. Predator and I'm not up with all the details. And when reading this question it reminded me of another question that's bothered me for a few decades.

The facehugger seems to have evolved specifically to target a human face. It always seemed odd the Space Jockey's ship crashed, since it had a trunk, like an elephant, and the facehugger wouldn't work on that kind of face. Even on Earth, there are examples of species that a facehugger won't work on, like an elephant, or likely on a large snake (there's nothing to grab on to).

Is there any reason given, in-universe, for the reason why a facehugger seems to be such a perfect fit to humanoids? Is there a reference to a common host species on the alien's homeworld that is enough like humans that there was a parallel evolution? Or do we find out, at some point, perhaps, that humans were an intended target from the start?

Or is this just due to convenience from a production standpoint, with no in-universe explanation?

Answer

The Facehuggers are actually a pretty versatile design, so I'm not sure I agree that they seem tailored to human-sized heads.

Their legs are considerably longer than they would need to be to cling securely to a human face, as is their tail, as we can see here:

In that shot, the second finger joint reaches to the back of the skull. However, they have a third joint beyond that, which we can see here:

There does seem to be a size difference between the Facehuggers in those two images, but both can clearly hold a head significantly larger than a human head with just their "fingers".

Similarly, their tail can wrap around something much thicker than a human neck. This can be clearly seen in both images.

Additionally, we've seen evidence from the movies (particularly the AVP movies) that aliens are also suited to "impregnate" the Predator species, even though their skulls are considerably wider, and their necks are thicker:

Finally, there's no reason to believe that the only way they can implant eggs is to wrap their "fingers" around a humanoid head. In the first Alien movie, it was demonstrated in the very first appearance of a Facehugger that they could almost instantly burrow through very tough materials, including the helmet of a space environmental suit.

It is entirely plausible that the Facehuggers could attach by burying the tips of their "fingers" directly into the flesh of aliens whose "mouths" were on a different type of surface, even a relatively flat plane. Similarly, the tail could be used to secure itself to a variety of smaller surfaces.