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I have a species that I wish to create whose life cycle is similar to a Xenomorph from the Alien series. By this, I mean to create a species wherein:

  • A Xenomorph, or Species A, the base creature, by some method (egg-laying, birth, etc.), produces a Species B, a Facehugger
  • The Facehugger, by some other method, (parasitism in the movies), breeds the true baby stage of a Xenomorph, Species A. In the movies, the Facehugger implants a chestburster into a host that cocoons into a Xenomorph.

My question is, is there any plausible way for a lifeform like this to be forced to evolve this way? I am fine with an advanced civilization engineering the ecosystem to push pressure into this kind of species, but I am wondering what factors might lead to such a species evolving, and if there are any close real life parallels.

The end result being:

Lifeform A giving birth to Lifeform B which then gives birth to Lifeform A's infant stage.

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    $\begingroup$ The 'Aliens' in the franchise of the same name did not appear to be plausible until the most recent installments of the franchise revealed that the Aliens were actually a bioweapon designed to exterminate humanity by a humanoid alien species, and further engineered by a rogue human-made synthetic person to the form they have taken for the majority of the franchise. $\endgroup$
    – Monty Wild ♦
    Commented Aug 31, 2019 at 8:11
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    $\begingroup$ The Aliens appear to possess traits that would be highly unlikely to evolve, including the ability to grow rapidly in the presence of electrical power sources (use of external power to assist with the fixation of carbon from gaseous carbon dioxide), and a self-limiting reproductive cycle designed to cause the demise of the Aliens shortly after they cause the extinction of any large vertebrates. $\endgroup$
    – Monty Wild ♦
    Commented Aug 31, 2019 at 8:17

3 Answers 3

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Alternation of generations

"Lifeform A giving birth to Lifeform B which then gives birth to Lifeform A's infant stage": funny that this is exactly how the lifecycle of plants works. But then I have always said that plants are aliens.

All plants exhibit an alternation of generations. A gametophyte produces gametes (eggs and sperm); when two gametes of opposite sexes merge, the results, as in animals, is a zygote, which will grow into an embryo. But here the similarity with animals ends: the embryo wil grow into a sporophyte, which will produce spores. Spores do not need fertilization (that's why they are called spores and not gametes). In suitable conditions, a spore will germinate and grow into a gametophyte, and the cycle will continue.

A schematic depiction of the alternation of generations in plants.

A schematic depiction of the alternation of generations in plants. Drawing by Peter coxhead, available on Wikimedia. Public domain.

  • In some primitive algae, the sporophyte and the gametophyte generations are equal, but this is boring, because the look alike. In land plants, the alternative generations are unequal and they do not look alike.

  • In mosses, the dominant generation is the gametophyte; the sporophyte grows (semi-)parasitically on the gametophyte and looks nothing like it:

    Moss sporophytes

    Moss gametophytes (the dominant generation, at bottom) and sporophytes (the tall stalks). Photograph by user Requestanaccount, available on Wikimedia under the Creative Commons Attribution-Share Alike 4.0 International license.

  • In ferns and seed plants, the dominant generation is the sporophyte.

    In ferns in particular, both generations are capable of independent life. A fern produces spores which germinate and grow into a prothallus, which produces gametes (sperm and eggs) which, when joined into a zygote, will germinate and grow into a fern.

    The sporophyte generation of *Onoclea sensibilis*   "Beads" (spore-bearing structures)   The gametophyte generation of *Onoclea sensibilis*

    Left, the sporophyte generation of the bead fern, Onoclea sensibilis. Photograph by user peganum, available on Flickr under the Creative Commons Attribution-Share Alike 2.0 Generic license. Center, the "beads" (spore-bearing structures) of Onoclea sensibilis. Photograph by H. Zell, available on Wikimedia under the CC BY-SA 3.0 license. Right: the prothallus (gametophyte generation) of Onoclea sensibilis, with a young sporophyte growing (the frond on top). Photograph by user Peter coxhead, available on Wikimedia under the CC BY 3.0 license.

  • In seed plants, the sporophyte is the dominant generation. The gametophyte generation is tiny, very much reduced, and incapable of independent life. (Fun fact: pollen grains are not gametes, but spores. If they fall on a receptive stigma, they germinate and grow into a teeny-tiny gametophyte which will produce sperm cells to fertilize the eggs.)

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    $\begingroup$ There are no animals that have alternation between sporophytes and gametophytes, there are animals that go through cycles of two body types having each other as offspring, like with xenomorphs laying facehugger eggs and facehuggers implanting xenomorph embryos. See this page on Cnidaria which calls this "alternation of generations", although the wiki page says other terms like "metagenesis" are used (it denotes one phase being sexual--we don't know if xeno queen mates with drones though) $\endgroup$
    – Hypnosifl
    Commented Aug 31, 2019 at 13:29
  • $\begingroup$ @Hypnosifl: The question asks about "lifeforms", not specifically animals. $\endgroup$
    – AlexP
    Commented Aug 31, 2019 at 13:32
  • $\begingroup$ True, but it also asked for "any close real life parallels", seeing this in an animal is perhaps a closer parallel and makes it a little more plausible evolutionarily than it would be if were seen exclusively in plants. $\endgroup$
    – Hypnosifl
    Commented Aug 31, 2019 at 13:38
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    $\begingroup$ @Hypnosifl: Maybe you did not fully consider what this answer says. This answer says that here, on Earth, an entire kingdom of lifeforms has a lifecycle with an alternance of generations. By consequence, it shows that the posited lifecycle is not at all impossible or far fetched. Therefore it is easily imaginable that on another world there would be animals with such a lifecycle. $\endgroup$
    – AlexP
    Commented Aug 31, 2019 at 15:03
  • $\begingroup$ It shows it's not "impossible", but on the question of whether the it's "far fetched", that's a matter of degree, no? Your examples certainly make it less far-fetched, but one could argue it's still somewhat far-fetched for various reasons. If the behavior were exclusive to the plant kingdom then we might imagine there are evolutionary reasons this is only likely to evolve in sessile organisms, so I think animal examples give a slight further reduction in far-fetchedness. But it still might be seen as fairly far-fetched if reproduction in both phases is asexual, unlike in all real examples. $\endgroup$
    – Hypnosifl
    Commented Aug 31, 2019 at 16:33
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Lots of animals, in particular certain species of wasps, use ovipositors to implant eggs into a host animal of some kind. These often have different appearances as they reach different stages of their life cycle. What you're more likely to see is something like this:

  • Adult stage of organism deposits eggs into host.
  • Larval stage of organism devours host from inside
  • (optional) Host uses shelled out carcass as cocoon before emerging as adult OR
  • Larval stage of organism emerges from host after consumption, to hunt more prey before pupating and moving into adult stage.

The facehugger->Xenmorph->egg->facehugger change is bizarre, because the face hugger is a much simpler organism and is the one that emerges from the eggs, then ostensibly implants a larva or egg into another organism. It's a very overly complex lifecycle that doesn't really have any corollary.

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    $\begingroup$ It makes a lot more sense if you assume face huggers don’t actually lay eggs then die, but are instead a husk left behind by the pupescent xenomorph as it seeks somewhere warm to gestate. I mean, it’s icky, but... $\endgroup$
    – Joe Bloggs
    Commented Aug 31, 2019 at 13:16
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    $\begingroup$ Not that there isn't much about Xenomorphs that isn't icky. $\endgroup$
    – Benjamin Wandio
    Commented Sep 3, 2019 at 10:36
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It is one organism; the "facehugger" is just a complex first exoskeleton

I don't think alternation of generations is necessary to explain this, since the facehugger phase doesn't really behave much like an independent organism - it does not eat, grow, or mate with other facehuggers. It is simply a complex original exoskeleton that is discarded once it is no longer needed.

There are quite a number of parasitoids that have life cycles not entirely unlike the Xenomorph. Most of the time, the parent deposits the eggs close to the host if not injected directly through an ovipositor, or alternatively brings a host organism to the alreaminimizing the effort the larva needs to expend to pass to the host.

However, a more complex, independent injection mechanism could be justified if the host organism and the adult phase of the parasitoid are not active at the same time. Perhaps they evolved together, but the hosts stopped being active in certain seasons due to climate shifts, and the parent could not guarantee that it would be able to procure food for the eggs by hatching time. This would encourage the evolution of eggs that could outlast the parent and hatch on their own when exposed to a nearby host, and also provide an advantage to larvae that were better at burrowing into their hosts independently.

Perhaps they evolved as a more standard parasitoid, where the parent would provide the eggs with a paralyzed host but the larva would still need to burrow into the host on its own. It would only need the "burrowing" mechanisms in its first molt; once it had successfully implanted itself into the host it could shed these and begin its metamorphosis into its adult phase.

As the burrowing mechanisms became more sophisticated and the larva more independent, they could eventually develop sensory organs for pursuing a host and injecting themselves into its gut directly. Alternatively, the facehugger was similar in form to the original larva, and the second molt became less sophisticated in order to make it easier for the first molt to inject it into the host.

Note: This assumes that the Xenomorph evolved alongside its chosen host. An organism that can successfully parasitize a host from another planet is much less plausible.

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