One finger, one thumb, one arm, one leg, one nod of the head, sit down, stand up, keep moving We'll all be merry and bright


I want a creature that literally has one of everything.

All of the above plus:

One eye, One nostril, One nipple, One foot, and so on.

The creature moves by hopping (of course) and it manipulates things with a combination of its tail and its one hand (which has one finger and one thumb).

I only want to consider macro features. It would be ridiculous to have only one brain-cell, although some people might think that applies to me.

If we assume that this is some future type of Earth-creature that has evolved over thousands or millions of years from a kangaroo, are there any insurmountable biological reasons to prevent it only having one of every major organ. It seems to me that it could have one liver, one lung, one kidney etc. and survive perfectly well.


Anatomically and medically is there any reason that such a creature's macro-scale internal and external structure could not be viable?

Assume the creature is pretty much bilaterally symmetrical on the outside and probably on the inside as well.

I'm interested at the organ level rather than the cellular level.


As I mentioned the creature is descended from a kangaroo. From the side it would look exactly like a kangaroo. From the front it would look like a one-eyed, one-eared, one-legged kangaroo. I'll see if I can produce a picture although this may not be easy.

How the creature evolved

First of all the legs fused centrally. This is because a present-day kangaroo always uses its legs in unison when travelling, two separate legs became unnecessary. The arms merged and the nostrils merged. I haven't yet come to grips with the fusing of two eyes into one. It may be better to preserve two so as to allow binocular vision. The genitals of both sexes migrated to the front and the anus to the back. Mating is done face to face. The reason for this is to allow the newly born joey to crawl into the mother's pouch. The joey as you know is quite primitive at birth and it retains some of the old kangaroo features to allow it to climb. As it develops further the new physiology develops in a reiteration of what happened during evolution.

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    $\begingroup$ Well I guess this will likely be some sort of snake. $\endgroup$
    – Trevor
    Dec 17, 2018 at 19:32
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    $\begingroup$ I've already said it is descended from a kangaroo. From the side it could be mistaken for one. It progresses by hopping along on its one leg. $\endgroup$ Dec 17, 2018 at 19:33
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    $\begingroup$ right, and when it gives up trying to stand up in the wind, it crawls, eventually a new generation has a longer foot with more bones to help the crawling, and eventually you have a one eyed snake. $\endgroup$
    – Trevor
    Dec 17, 2018 at 19:35
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    $\begingroup$ @Trevor D one-eyed sort :) $\endgroup$
    – Alexander
    Dec 17, 2018 at 19:37
  • $\begingroup$ @Alexander .... I didn't catch that.... feels shame $\endgroup$
    – Trevor
    Dec 17, 2018 at 19:38

4 Answers 4


One Eye , One Ear, One Nostril and One Tooth is going to make it extremely hard for your creature to survive.

Firstly it doesn't have depth perception. It can't tell if something is really small or very far and likewise if that thing is very large or very close. This means you creature will have to interact with objects to determine its size. This can be problematic if they were looking for water, food, prey or predators because they won't be able to figure out how far or close they are without having a physical interaction and this can also confused them if the size of the body of water, food or predator can vary in size. They would also have a pretty limited field of view, and would need to constantly turn their head to ensure they can see anything around them.

Secondly One Ear means they won't be able to pinpoint where a noise comes from. This means if you were to call them, they would need to spin around to find out what was the source of the sound. And if there are multiple possible sources, they would need to move around to determine which one created the noise (assuming it reproduces the same noise in the first place). This basically means they need to hunt or run based on their vision which is already lacking depth perception and has a limited field of view.

One Nostril is a bit different. This is going to be tough on your animal because if that nostril is blocked or sensitive via some form of object or illness you have to breath through your mouth.

One Tooth (Assuming you want this and not one mouth) is also a killing blow. You can't chew food with one tooth. You need at least two to be able to either cut, slice and tear meat or chew and grind plants into a pulp for easier digestion. Even with two, you run an extremely high risk of losing a tooth due to injury, bad dental practices and being stuck with soft foods or swallowing your food whole (Mind you, with one or two teeth, you risk losing them and not being able to bite something to grip onto it).


What you've created is called prey, it will die very quickly as it is unable to traverse terrain, or flee from predators, or chase prey on it's own. This sort of evolutionary mistake usually dies off and a stronger, usually a more mobile version succeeds.

Bipeds (creatures with 2 legs) are very rare in nature for good reason, it's not a good form of locomotion. 1 leg is even worse. Your creature would be forced to expend more energy just to stay standing and ready to flee, and it would not be able to move fast enough to catch prey, or agile enough to keep itself safe. It doesn't matter what it's organs arrangement is, it won't survive without other bipedal members of it's race taking care of it.

I can give you an example. enter image description here Somewhere in this birds evolutionary line, it decided standing on 1 foot is a good idea. But even it will use it's other foot when it's time to flee. This bird can stand on one leg, but it still needs it's other leg to survive.

  • $\begingroup$ One-legged hopping is possible, youtu.be/H0o_XM_vynU?t=20. Over millions of years (after the Australian Dingo has become extinct, leaving few or no predators) I don't see why a one-legged kangaroo shouldn't move as well as an ordinary one. However, I'm asking specifically about its anatomy rather than its method of escaping. $\endgroup$ Dec 17, 2018 at 20:09
  • $\begingroup$ When you say "biped", are you referring to something with two legs, and no other limbs, or simply something that has two feet? I'd agree that the former is rare and probably has some read disadvantages, but the latter has many, many examples of competitive forms. $\endgroup$
    – Harris
    Dec 17, 2018 at 21:25
  • $\begingroup$ @Harris Yes 2 legs, I'll clarify $\endgroup$
    – Trevor
    Dec 17, 2018 at 21:47
  • $\begingroup$ Sorry, the distinction I was asking about was whether or not you're excluding creatures that also have limbs that aren't legs. $\endgroup$
    – Harris
    Dec 17, 2018 at 21:49
  • $\begingroup$ With regard to defending itself, the present-day kangaroo can hop whilst steadying itself with the tail and deliver a very nasty kick youtu.be/ZsrNPmCFpW0?t=8. I imagine a kangerone would do something very similar. $\endgroup$ Dec 17, 2018 at 22:07

Your creature wouldn't even be able to stand up.

If there's only 1 toe on 1 leg, you have 1 tiny point of ground contact on which the creature has to balance it's whole weight on. Yes, I know it's possible to balance a weight over 1 point, but it takes continnuous effort and thereby uses up far too much energy.

Add in a Kangaroo tail and now you have 2 points of ground cantact. Congratulations, instead of toppling over to any given direction, your creature now topples over to the left or right. From a purely physical point of view that's an improvement, but from an evolutionary point of view it's just as bad.

The minimum number of ground contact points to stand stable is 3. The creature would have to support it's body with it's one arm, which might be considered breaking the rules since it serves more as a leg than an arm.

But now let's imagine this creature hopping like a Kangaroo. Since there's only one leg, it's quite hard to steer and slipping with this very unstable, one-toed food directly leads to falling over and being eaten.

If some of them should actually make it to adulthood, how are they going to arrange their bodies in order to copulate without falling over?

To sum it up: your creature cannot possibly eat enough nutritious food to compensate for the insane amount of energy needed to simply stand still and cannot possibly run away from predators.

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    $\begingroup$ (1) A foot can be wide and/or long. Humans with enough muscle strength can stand up from a sitting position on one leg. youtu.be/Ye_QqyfOaR8 This animal has evolved over a long time to do it. (2) The tail doesn't have to be straight. It can curl round and make a wide circular base or form an L-shape. (3) I don't know how good your imagination is but I believe ordinary female kangaroos have to move their tail out of the way to copulate. They can squat on the ground meanwhile. $\endgroup$ Dec 17, 2018 at 20:14
  • $\begingroup$ @chaslyfromUK True, but humans have more than one toe. In fact, the natural shape of a healthy foot results in 3 points of ground contact: the ball under the big toe, the ball under the pinky toe and the heel, making it a rather stable construct on itself. But even with this stable foot many people struggle to stand on one leg for more than a minute. $\endgroup$
    – Elmy
    Dec 17, 2018 at 20:19
  • $\begingroup$ Update on copulation. They do it face-to-face in a standing position. As in many animals, the male's ejaculation happens very quickly. The thrusting is done by rapidly hopping up and down. (Sorry, excuse the graphic details. I only include them for completeness). $\endgroup$ Dec 17, 2018 at 20:55
  • $\begingroup$ Re standing up. Kangaroos spend most of the day (if not the night) standing up. I'm not aware it costs a lot of energy. Balance can be achieved when standing by curling the tail slightly. There is no reason they can't lie down - even on their sides. Their tails are heavy and strong and can be used as a lever. In fact kangaroo tails have been shown to act as a limb. newscientist.com/article/… $\endgroup$ Dec 17, 2018 at 23:29
  • $\begingroup$ I'm still not convinced this species could survive. I've thought about one very thick leg on which it could move like a walrus but that would make it extremely slow and would handwave evolution away. $\endgroup$
    – Elmy
    Dec 18, 2018 at 7:03

I'm going to ignore the 'why' this animal evolved this way, because it appears to be throwing away many evolutionary advantages.

Anatomically and medically is there any reason that such a creature's macro-scale internal and external structure could not be viable?

I'm taking viable to mean as it could live and breed in the wild if there were no predators (as mentioned in the comments of another answer).


As long as it had one of each organ, it could breath, pump blood, digest food and excrete waste products.


To keep the animal balanced (while stationary and while moving) I think the tail would have to adapt to be more of a counterweight and probably a mechanism for turning, as I can't see other mechanism for turning around other than pirouetting while stationary (which would require some rotational force to get started) and a side anchor when moving, e.g. when hopping the animal would drag it's tail to one side to pull the animal around (sounds painful, so there is probably some thick skin on there).


I imagine that most of the leg would work in a similar way for forward movement and leaning backwards and forwards with limited sideways rotation at the top of the joint. The foot will be wider to support the animals weight and the front toe would be much wider as it is essentially replacing eight toes. As there are no predators, the front toe would be more hoof like to enable grip and pushing off (grooming is going out the window here). Kangaroos have a semi-thumb on their feet, so I imagine that this would move around to the back of the foot to help support the animal.


I'm struggling to come up with a realistic shoulder joint that wouldn't adversely affect breathing. It would have to be situated on the sternum of the animal, maybe with supporting bones coming from where the shoulders currently rest. The extra bones and muscles required for this would make the animal appear more barrel chested than a present day kangaroo.

Kangaroos will use their arms (technically front paws) for balance when leaning over to feed on grass (and shrubs depending on the type), so I assume this future animal will do the same. The tail will play a greater role in keeping the animal upright. The animal may have to eat with it's head to the side to maintain a tripod of tail, leg and arm.

Eyes, Ears and Nose

One nostril wouldn't make that much difference from an external perspective, but would probably play havoc with it's sinuses.

The question proposes that keeping two eyes maybe desirable

I haven't yet come to grips with the fusing of two eyes into one. It may be better to preserve two so as to allow binocular vision.

I agree with this, as having one eye would be a great disadvantage for depth perception and peripheral vision. I would also imagine that it needs both ears as well for similar reasons (hearing in more directions and being able to locate the source of a sound). I can't imagine where a single ear would go, on top of it's head maybe? It's not going to do much good there. Besides, if the animal is to appear as a kangaroo from the side, it will need a ear on either side of it's head.


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