Say that you have a medium-sized, bipedal, vertebrate-like creature. It no longer has the need to have arms, so they atrophied until they disappeared entirely.

Now imagine an animal that shares similar morphology to the other one, except that its two legs have, over time, fused to become a single, muscular leg. It is saltatorial, hopping across its open habitat like a kangaroo.

How would this evolutionary transition occur? What need would a two-legged animal have to fuse its legs into one? I'd prefer if answers weren't "the common ancestor had a birth defect and became genetically isolated", but rather something of an explanation as to how monopedalism would become advantageous to something.

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    Mind that a biped has already an unstable position. A monoped would just make it worse. – L.Dutch Sep 13 at 16:59
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    snakes are as close as you get – A. C. A. C. Sep 13 at 17:05
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    Maybe it swims/slithers? – JustSnilloc Sep 13 at 17:06
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    @JustSnilloc you should expand that into an answer. – RonJohn Sep 13 at 17:09
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    How about for hopping? If it has a tail, that would help to stabilize it. – John Locke Sep 13 at 23:46

Closet thing I can think of is our friends the dolphins and whales. Essentially they evolved from land based animals that gradually returned to the water to become fully aquatic over millions of years. Due to disuse, the hinds legs eventually disappeared, although some species do have a vestigial pelvis bone. The tail became the predominant method of propulsion.

If such a species had evolutionary pressure to return to land, maybe they could evolve to use their muscular tails to propel themselves across the ground? It's highly unlikely though

The monoped can make much higher and longer jumps, including over small rivers, thick brush and other obstacles.

The bipedal ancestors lived on similar terrain and were hunted by predators similar to lions and wild dogs. The bipeds couldn't outsprint the felines on flat terrain nor outlast the canine predators. The most successful strategy was heading for obstacles and jumping over them, slowing down the predators.

This meant a strong natural selection for the ability to jump high and far, meaning more and more stabilization and impact-absorption was needed. Specimens with more muscle tissue between the legs (upper first then lower too) thrived until they looked like a single leg from the outside.

Inside the leg there are still two sets of bones, with a mass of muscle and sinew between them that stores the energy of ground impact like a spring and then releases it on the next jump. Stability is enhanced by a strong tail (also used for standing up) and toes that can be splayed wide when standing still.

The final touch is the monoped's ability to curve it's head, neck, back and tail into a wheel shape, moving long distances efficiently by accelerating and then rolling when the terrain is smooth, maintaining speed with the occasional kick.

Sirenomelia often leads to fast deaths, even among humans with modern medical care. Also, we've all seen birds which have lost a leg or foot, but this takes a serious toll on their lifespan (I raise chickens and pigeons). Having a single leg, as others have pointed out, would be a serious disadvantage in most circumstances.

BUT since this isn't what you asked, and your actual question doesn't seem to require land based movement (just your description), let's look for an answer elsewhere in the animal kingdom. There are species of snails which have a single foot and use this to jump away from predators. Most mollusks also only have a single foot appendage, so similar situations aren't completely unheard of.

Taking this a step further, it is feasible that a species of amphibians evolved down to two legs (like the Lesser Siren, and there were subspecies of these which gained an advantage when they suffered from sirenomelia. What is that advantage? Well two legs fused together in a manner which allows for webbing between the legs and feet would give considerably more surface area for swimming faster. Or possibly the bones fused together in a way which allowed the toes to be much more rigid and used as a weapon for defense or a tool, such as cracking open mollusk shells.

The longer I thought about it, the less likely it is to ever happen.

Your creature definitely needs to have a strong and heavy tail to balance the body and use like a limb while standing up, since they have no arms.

The problen is the pelvic bone. Every land dwelling creature on earth has a more or less circular bone structure in the pelvis with the hip joints attached somewhere at the sides. The femural part of the hip joint always points sideways, even in humans. Having only one femur attached to the center of the pelvis is probably not stable enough to be able to hop.

And where do the intestines go? In every creature on earth the intestine and birth channel have to pass right through the pelvis. Your hopping alien would need a circular pelvic bone with a hole big enough to pass a fetus through it, while being strong enough to absorb the forces of hopping and landing without damaging the bone or soft organs.

The biggest no-go is evolution, though. You would have to gradually move both legs ever closer together until they start fusing at the hip. Now keep in mind that the hip joint is a ball-and-socket-joint to enable the movement of the legs in (almost) every direction. No matter how the femural part of the hip joint is designed at this point in evolution, fusing two ball-joints together yields one hinge joint. Your creature would most likely be unable to lay down, stand back up or steer its hopping because it couldn't move its leg to the sides.

Sorry, but not gonna happen.

What need would a two-legged animal have to fuse its legs into one?

They have big feet, and lying on their backs with feet up provides shade from the noonday sun.

(EDIT: thanks to @L.Dutch for point out the Dufflepud inspiration.)

https://en.wikipedia.org/wiki/Monopod_(creature)

From the Nuremberg Chronicle of 1493:

enter image description here

And C.S. Lewis' interpretation: http://narnia.wikia.com/wiki/Dufflepud

enter image description here

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