In the book The Host by Stephenie Meyer, a parasitic species (Souls) attach to the back of the spinal cord of alien creatures to control their brains, but I find this a wee bit unbelievable. As it is unlikely that a wide range of aliens would have a spinal cord, what feature of the nervous system is likely to be found across a wide variety of alien species?

  • 1
    $\begingroup$ Last time I checked, Flowers don't have spinal cords $\endgroup$ Commented May 5, 2016 at 4:00
  • 1
    $\begingroup$ I had thought of a similar question for a story involving mental possession but you have beaten me to it. If, as XandarTheZenon implies, the Souls have also taken over plant-like creatures who don't even have a nervous system, it is even more perplexing. I deliberately haven't read Host for fear of unconscious plagiarism but the problems with any such "universal parasite" that can latch on, physically or mentally, to a wide variety of alien species, are (1) how would any such creature know what to do with an utterly new species, and (2) why would it want to? Why would it evolve to do that? $\endgroup$ Commented May 5, 2016 at 5:59
  • $\begingroup$ By "control their brains" do you mean influence the thoughts and memories of the host, or just control the body movements? Because even if the underlying neurochemistry is the same, actual thought processes, memory storage, etc could make this a much more difficult task than just pulling the strings on a meat puppet (who is internally screaming). $\endgroup$
    – Jason K
    Commented Jun 14, 2016 at 16:57
  • $\begingroup$ @Lostinfrance I highly suggest you read it, it's very good. To your questions; (1) - The Souls connect through the neck or somewhere near the brain. (2) - The Souls reproduce very, very rapidly so colonizing planets is the easiest solution. $\endgroup$
    – TrEs-2b
    Commented Jun 30, 2016 at 1:34

5 Answers 5


First of all, this is not science fiction. Read up the life cycle of the sheep fluke, which at one stage takes over the brain of an ant to ensure it can be eaten by a sheep. Currently, there's a journalistic panic about toxoplasma in undercooked meat taking over human brains to make them turn themselves into cat food (I kid you not). This sounds like something made up by the soybean industry.

But to get to the point:

  • Some alien animal life is certain to develop bilateral symmetry. This is a winning design on earth, and has arisen independently among insects and vertebrates.
  • Bilateral symmetry requires a segmented body, each segment having a specialized function, with its own neural controls.
  • The front end must have sensory organs, if only to find food.
  • Sensory organs require neural coordination and control.
  • This predisposes to the development of an important neural collection at the front end.
  • This collection will have to communicate with the rest of the body, if only to keep it moving towards the food.
  • This means that nerve tracts will have to extend from the front backwards.
  • If this animal lives on a planet with minerals, it will develop a skeleton, if only to segregate the minerals it can't use and can't get rid of. And which planet doesn't have minerals?
  • This skeleton will evolve to protect the squishy nerves.
  • So rather than being unlikely, a spinal cord with a bony shell seems almost an almost inevitable feature of a sufficiently complex organism.

Of course, we also have asymmetrical animal life forms on earth, the most successful being the octopus. But having no skeleton, the octopus is confined to the sea, leaving the land free for those other animals with the spinal cords.

  • $\begingroup$ I loved the step by step logic of this answer. $\endgroup$ Commented May 5, 2016 at 7:59
  • 2
    $\begingroup$ While interesting, the step logic is empirically wrong. Because there are quite complex organisms without backbones.The reason for this is pretty simple, there are just too many steps. Even if every step taken in isolation had a 90% chance of being true for any given evolutionary path (Which seems a bit high for some of them), there would still only be a 40% chance of any given evolutionary path to wind up with a backbone. $\endgroup$
    – Saidoro
    Commented May 5, 2016 at 15:05
  • $\begingroup$ If you had read to the end of the answer, you would have seen that I mention complex animals without backbones, so my answer is not intrinsically wrong. And a 40% chance would seem pretty good, taking into account the probability that the number of habitable planets is guesstimated to be in the billions at least, just in this galaxy. $\endgroup$
    – frank
    Commented May 5, 2016 at 18:15
  • $\begingroup$ @Saidoro, I understand that the probability of a chain of events decreases with each step, but that doesn't make the chain impossible, just not certain. It's a matter of finding your niche. If I were doing a presentation to the Board in my role as Soul Vice President in Charge of Species Expansion and I were able to tell them that 40% of species in this galactic sector were available for "improvement", I'd be feeling pretty secure in my job. $\endgroup$ Commented May 5, 2016 at 18:17
  • $\begingroup$ @frank, our comments crossed. $\endgroup$ Commented May 5, 2016 at 18:17

Trying to get a handle on this question, I took a look at Wikipedia to see what a nervous system actually does:

At the cellular level, the nervous system is defined by the presence of a special type of cell, called the neuron, also known as a "nerve cell". Neurons have special structures that allow them to send signals rapidly and precisely to other cells.

It sends signals. That is the truly universal part.

They send these signals in the form of electrochemical waves…

Electrochemically. While that aspect is not absolutely essential, surely this method is likely to be among the fastest sorts of signalling that could evolve in a physical body. "Electrochemical signalling" is the closest thing I can offer to an answer to your question as currently formulated. However I think your question may include an overly-limiting assumption.

…traveling along thin fibers called axons, which cause chemicals called neurotransmitters to be released at junctions called synapses. A cell that receives a synaptic signal from a neuron may be excited, inhibited, or otherwise modulated. The connections between neurons can form neural circuits and also neural networks that generate an organism's perception of the world and determine its behavior.

Junctions. Modulations. Connections. All of this is where Information Theory meets Neuroscience, neither of which I know much about. But, along with the concept of signalling itself, isn't information processing, rather than any material property, the rock-bottom essential property of consciousness that the Soul or other Invasion of the Body Snatchers type of mental parasite (or demonic possessor, or Granny Weatherwax) must hitch on to?

So as well as, or instead of, a physical form, our parasite/symbiote exists in… er... information space or something and… er…

OK. That's really lame. But something like that.

As the first paragraph of the excellent answer by "frank" above says, real life evolution came up with something as weird as the lifecycle of the various sorts of fluke. Perhaps if we could describe how a being would evolve to take over the bodies of multiple drastically different species from different planets, we would be able to better theorize on how exactly it does it.

  • 1
    $\begingroup$ This is a good answer because it describes HOW the nervous system might work across alien species, so the alien parasite has a common framework to manipulate. While the layout of the nervous system may differ, if it is similar on a biological level the parasite can hijack it and learn to control it. $\endgroup$
    – Jason K
    Commented Jun 14, 2016 at 16:54

Likely to be found across a wide range of "animal-like" species: a central controller and head comprising a sensory cluster and feeding orfaces.

The "brain" needs to send and receive signals across the rest of the body. A central cord might not be the only way to do it: look at a starfish or octopus.

There have been discussions here about multiple brains or distributed systems. But they still connect to each other and to everything else.

same: signal distribution with a hub.
don't assume the same: the single-run topology.

same: some kind of cables carrying signals.
variety: how they work. Will your mechanism be able to tap in?


It depends on the type of planets that the aliens come from. If it the planets are similar to earth then having alien creatures with spinal cords is likely, as most creatures on earth have spinal cords.

Having read Host I don't think that they (Souls) only inhabit creatures with spinal cords. Wanda talks about living in a "seaweed" type creature.

The souls probably have multiple ways of talking over a creature's body. Attaching to the spinal cord is just the way that they take over human bodies. They probably use a completely different way when dealing with other creatures.

  • $\begingroup$ " They probably use a completely different way when dealing with other creatures." No doubt, but I think the point of The Real Bat Man's question is how the Souls know what way to use over a wide spectrum of creatures. $\endgroup$ Commented May 5, 2016 at 5:45
  • 1
    $\begingroup$ @Lostinfrance they have advanced technology they probably abduct several individuals of their targeted species and study their biology and then come up with a method of assimilating them. $\endgroup$ Commented May 5, 2016 at 12:36
  • 1
    $\begingroup$ Most creatures do not hace spinal cords. Your experience is with vertebrates on a meter scale. That is not typical. $\endgroup$
    – JDługosz
    Commented Jun 14, 2016 at 15:57

Unfortunately, aliens being alien, there are so many possible ways that signals could be transmitted around an organism that the souls couldn't realistically have a hope of being able to interface with them all. One might use electrical, the other might use chemical, the third might use light, and so on.

The souls are a scientifically implausible species. Magic must be involved.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .