Hard SF : Aliens : Alien Anatomy

Alien Anatomy

An alien from a tool-using species will presumably have some kind of appendages to do manipulations and some sort of body part(s) for locomotion. Also, to handle tool-using activities and locomotion, its body will probably need strength from something analogous to a skeleton. But this is rather vague and those could be arranged in numerous ways.

Is A Torso Needed?

It seems to me, those appendages and locomotion parts will probably be extensions to a "body". In the abstract, one could imagine an alien that was shaped something like the letter X -- all limbs and no torso or other central part. I don't see why it must be impossible for body organs to be in the appendages, but it seems unlikely to me.

  1. If the most crucial organs (things comparable to heart, lungs and brain) were in the appendages, it might follow that the appendages would have an exoskeleton (to protect the organs) rather than an internal skeleton (less protection). On Earth, most animals large enough for considerable brains use internal skeletons for most of the body, but have internal bone structures in the head and torso to protect major organs. Even turtles -- evolutionarily successful enough to have been around for 200 million years -- have a shell around their torso, but their legs have internal bones (not a hard leg covering). Perhaps part of the reason why leg exoskeletons are rare is difficulties having joints that are soft enough to bend but strong enough to provide support. Especially for manipulative appendages, one would expect a need for significant flexibility.
  2. Theoretically, extraterrestrials could have both an internal skeleton and a hard covering over its appendages. If the covering had developed for protection rather than as a supporting structure, it could be in separate parts over areas between the joints. The joints would be uncovered to allow flexibility. I suspect the presence of supportive bones going down the central axis of the appendage would result in limited space in desirable locations in the appendages for body organs. One way or the other there would need to be a functionally logical coordination in the positioning of the bones, joints, exterior and organs.
  3. One might suggest each appendage have the equivalent of a rib cage to protect organs. The simplest version of this would seem to be like our rib cages in that our spines don't go through the center of the torso like an axis going down the middle of a cylinder. If it did, it would need something like spokes going out to the ribs which are towards the outer edge of the chest cavity. If an alien's appendages didn't have bones going down the center of the appendages, but had it going along the "back" of the appendage with ribs going around the cross-section, this would presumably have a number of implications for the capabilities of the appendage. For instance, we have muscles in front and back of our upper arm bones. To move the arm one way one set of muscles contracts; to move the other way, the other set of muscles contracts. An appendage with ribs seems to need some other mechanism to move the appendage in either direction. (Our lower arms and legs have two parallel bones to allow swiveling, this also doesn't seem to work with a rib design.) I'm not sure whether having a bone going down the "back" of the appendage rather than down the center would give the same quality of support. Earth animals have long bones running the length of the upper and lower leg / arm. These may not be as good for attaching a series of ribs, compared to ribs being associated with a series of shorter bones making up our spine. On the other hand, a series of shorter bones may not work as well for the support needs of appendages. (A series of shorter bones like our spine might make an appendage more flexible, but weaker and with more risks of dislocations.) Factors like these could influence alien body structure.

At least when dealing with the relatively soft tissues many of our organs are made of, having them placed in an appendage that has muscles moving and shifting into somewhat different shapes as the limb works could be harmful to the organ or interfere with its function. Perhaps, this could be compensated for if the organs were tougher and more rigid. That wouldn't work well for organs such as a beating heart or lungs that inflate and deflate. Such an alien may need to function in a way that does not require flexible organs or has some form of protection around them. (Or, it might be necessary to have something unlike our muscles to move the appendages without affecting the areas set aside for organs.)


Having a central body area might not have to mean the kind of torso Earth mammals tend to have. A starfish has a central area between its arms, but it is not as distinctly separate as a torso. What we need to keep in mind is whether the central body, appendages and the connection between them meets the locomotion and manipulative needs of an intelligent, tool-using species. A body design too close to that of the starfish is not likely to meet those needs.

One of the primary assumptions we work under is that intelligence and tool use tend to be associated with (and tool use requires) manipulative ability. Even before animals had any significant intelligence (fish, amphibians, reptiles), body flexibility (one might think of it as an early form of manipulativeness) developed in Earth life. This includes a flexible torso by means of "the back bone" - actually being a series of bones. Although our torsos can't bend to as great an angle as a leg, a leg has flexibility at fewer places. There are understandable reasons for this. In Earth life, the spinal cord goes through the spine and too much bending of the spine might be harmful for that reason. Even excluding that, too much bending might effect internal organs in the torso. These kinds of factors will have to be considered in an alien's central body. Some flexibility would be desirable, if the central body is rigid, the appendages will probably have to make up for that in some way. An exoskeleton is not excluded, but may be discouraged. A rigid back bone is likely to be uncommon. But some supporting structure that limits the amount of bending seems probable.

Consider the above issue of the spinal cord. Assuming an alien has a centralized brain-like organ, it will need to communicate with various other parts of the body. This will be true regardless of whether the brain is situated in a head, in a torso or in the feet. An alien need not have a single main nerve cord passing through a back bone. But there is a logic and economy to having a central item from which secondaries only branch off when needed. I think this is how our circulatory system is designed. Certainly, it's how the phone and electric companies try to arrange their wires rather than having separate small wires going from their facilities to each home. And if you're going to have a central nerve cord, nature may try to find some structure to protect it. It doesn't have to be in the back bone, but some reasonably straight path with some form of protection.

I've been using the term "back bone". Does it have to be in the back? The first question is: Does the alien have a "back"? It may not be necessary to have a two-sided design as opposed to a cylindrical, trilateral or such design. But there may be implications. If a cylindrical torso had a spine going up along the edge of the cylinder, one might still think of it as a back bone. If the spine went up the center of the cylinder, it seems it would need some structure to extend the supportive benefits to the outer part of the cylinder, which will probably be a more complex form to develop.

My first reaction was that at least for a species evolved from a lineage molded by a [bilateral] quadruped form, gravity makes it reasonable to have most of the torso below the support of a skeletal structure going the length of the torso. Although this sounds sensible to me, I'm not sure that that explains why fish seem to have that general structure. Perhaps much of the reason quadrupeds are in that form is simply because fish happen to be. (I'm not really sure what influences led to fish having spines on top and rib cage below.) Having a rib cage sticking upwards above a back bone doesn't strike me as promising, but perhaps there's nothing to prevent having two smaller rib cages, one on either side.

General Design

In addition to having manipulative appendages and locomotive part(s), a tool-using alien will need sensory organs, sensory interpretation area, memory saving and retrieval, decision-making and other thought processing. Plus the more basic life functions of getting and assimilating sources of nourishment, etc.

The Brain

There are considerations that may favor a centralized brain-like organ. Intelligence and tool use seems to require awareness of and interaction with the environment surrounding the being. That suggests sensory organs and processing. The simplest designs for animals is one in which it has a front -- it cannot move equally well in any direction without first turning to face its front in that direction. For animals with a front, it is preferable to have sensory organs situated towards the front facing more-or-less in that direction. Additional sensory organs could be placed towards additional directions, but that is extra work for evolution. I'd guess it's easier for a brain and sense organs to evolve in proximity to each other (later on it may not be as essential). Having the sensory organs too rigidly facing in one direction could be a disadvantage, so some flexibility in varying the direction of the sensory organs could have a benefit. In Earth life, these factors have contributed to having most sensory organs and the brain placed in a head that is able to turn from side to side and up and down. This may not be the only solution to maximizing those factors, but alien anatomy will probably have to incorporate ways to act on most of those issues.

What would be the implications of a brain in head that did not have a skull surrounding it? (And what issues would there be in anchoring eyes and ears in place without a skull?) If a brain was in a torso that did not have an exo-skeleton, would the brain need a skull-like container around it inside the torso?


Tool-use seems to make limited sense in an organism that doesn't substantially move from place to place. An organism that is rooted like a plant would have limited materials within reach to make tools from, would have limited parts of the environment within reach to use any tools on, could not meet others as it traveled and learn new tool uses from them, etc. A mobile organism is unlikely to get enough nourishment and energy for its activities from photosynthesis, osmosis and such. Therefore, aliens will probably need bodies equipped to consume foods, digest them and eliminate waste byproducts.

For the kinds of reasons given above about why body organs may tend to be in something more like a torso rather than being situated in appendages, food will probably not be consumed at the ends of appendages (hands, feet, etc.) That would either require the food to travel the length of the appendage to get to the main body or the digestive organs would have to be in the appendages.

The "mouth" would probably not be situated on the "back" of a torso. Assuming one has manipulative appendages that could reach a "mouth" on the "back", it's not impossible but unlikely. Unless the alien has sense organs allowing it to "see" what it's appendages are doing by its back, this could be discouraged. If the alien has an exoskeleton over its torso, it won't be practical. If it has a "back bone", there are potential issues with placing that and the "mouth" and digestive organs. Generally, having to depend on appendages to get food to one's "mouth" is probably not the most desirable arrangement. For an alien that has "sides" where only some appendages can reach, it may be uncommon for a "mouth" to be on the "side". A "mouth" on a "neck" (a specialized eating appendage) might have more options where it was placed, but would probably either need its own sensory organs or be situated where sensory organs could "watch" it.

The advantages of being able to "see" the food being brought to the "mouth" (or the "mouth" being brought to the food) is likely to mean the "mouth" will be in some place convenient to the sensory organs. If the sensory organs are in a "head", this could tend to place the "mouth" in the "head" or in an area easily visible to the "head". Assuming there isn't a coordinated arrangement of sensory organs, it will probably not be at the "rear" of a horizontal (for instance, quadruped) alien or at the "bottom" of a vertical (for instance, biped) alien.

Digestive Organs

It seems unlikely to me that aliens will eat some living thing and be able to incorporate actual living parts of that thing (for instance cells) into its body structure. Without doing something like that, it seems aliens would have to break down what they eat into smaller parts (for instance molecules) somewhat like we do. Breaking down food into appropriate molecules the body needs is a matter of chemistry. That is, this kind of digestion is a chemical process requiring the appropriate chemicals to cause chemical reactions capable of creating the desired nutrients. In theory, an alien could ingest these chemicals along with the food. However, appropriate chemicals will not necessarily be lying around to do this. If chemicals capable of breaking down flesh were lying around, evolution would probably develop some protection from those chemicals. Also, if these chemicals are anything like stomach acids, it's not necessarily something you want to fill your mouth with and swallow down your throat. It may be more common for aliens to have digestive organs that use chemicals produced by the body to break down food.


Many of the arguments in this article, in effect, advocate a body structure with significant similarities to Earth life. No doubt, life on any planet has to conform to the same laws of physics, mechanics, chemistry, etc. However, as reasonable as these arguments sound, we also need to keep in mind we may be working under "obvious" assumptions that turn out not to be absolute. It's good to maintain some suspicions about theories that make aliens look too much like humans.

The point of the article is to present an approach that bases possible alien anatomy on practical feasibility rather than on a creative imagination. Some creative imagination may help us uncover some unnecessary assumptions made above, but only a practical approach will help us identify which imaginings are really feasible.