# Could a single row of legs be useful?

Could it be useful for a species to evolve a single row of many legs on the underside of the body? The specific organism is amphibious, and has no paired structures on the outside. It does have unpaired fins along its midline, which would be the origin of these legs

• How many legs? Two? Commented Sep 15, 2021 at 20:35
• My suspicion is that they could be made useful, even if the creature might never be a sprinter. The best way to answer it definitively would be (unfortunately) a simulation sophisticated enough to test various configurations and see if any rise above some arbitrary threshold. Get me a \$200k grant and I'll get to work on that. Commented Sep 15, 2021 at 20:58
• Would it depend on the direction of movement - two legs one front and one back for a creature walking sideways would effectively be the same in most ways as a "normal" bipedal creature in terms of mechanics and stability, especially if the species already has ~360 degree vision. Commented Sep 16, 2021 at 14:43
• If your creature live on Arde, or any other planet in the Planiverse, then a single row of legs is the only possible way to do it! Commented Sep 16, 2021 at 16:35

## probably...

The biggest issue is stability. Conceptually, it looks like this viewed head-on:

If it continuously tips over on its side, it's not going to survive long.

Since this creature is described as having "many legs", I'm going to assume it is proportionately long suggesting an eel with legs or a millipede with only half its legs. That form suggests a possible mitigation for the stability issue. If this creature never walks straight forward but always follows a serpentine path, the head-on via changes to something like this:

This, unfortunately, introduces an efficiency issue. continuously walking in serpentine path wastes a lot of energy when compared to going straight. So, maybe we just grow a proper foot to widen the base a bit. Now the head-on view looks like this:

To make this work, it'll need to evolve an ankle to handle uneven terrain. That's not impossible so it could work. But there's an arguably easier and more efficient fix. What if it pointed its center-mounted legs to alternating sides? Maybe all of the odd-numbered legs lean right, and all the even-numbered legs lean left. That gives us a head-on view like this:

Ok, problem solved. Your amphibian can now walk on land which is a definite advantage and worth doing. It even had two different approaches so they'll probably both exist. Two species for the price of one!

But...

Nothing's that easy. Over time, our angle-leg version is going to find it more efficient to move the legs out from the center. Like this:

And now, unfortunately, we've evolved two rows of legs. Oops.

conclusion

This is definitely doable but it's going to face pressure from more efficient designs (bilaterians dominate animal life forms for a reason). If this design is important to your project, I'd recommend making your setting right after a mass extinction event when evolution tries all the crazy stuff.

...

and no.