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On plain Pilar forest biome of planet X, you have mostly 3 types of vertebrates:

  • those who burrow under the ground
  • those who run through the widely separated trees
  • those who live above ground level, calling the colossal plants home and hardly coming down.

In this scenario, one creature adapted to both climb efficiently and running on the plains.

It has 4 relatively long limbs with durable non retractable claws. The hind limbs are digitigrade and mostly similar to something like a zygodactyl emu, capable of sustaining the creature while it uses its arms to grab prey. It's forelimbs end in short palms with 4 digits (2 of them being opposing digits) and has 2 elbow joints, the first bending backwards and the second one forwards,somewhat like the following, very poorly drawn, example:

enter image description here

Now, would this forelimb structure really aid a creature that climbs and runs? I know it doesn't happen here since no ancestor had such arms, but I'd like to know if this could truly be a good evolutionary trait.

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    $\begingroup$ If that is what their ancestors have, then that is what they will have, evolution is not going to remove entire segments of a limb without basically removing the functionality of the limb entirely. the arm of a primate and arm of the first terrestrial vertebrates have the exact same gross limb layout. $\endgroup$
    – John
    Mar 22, 2020 at 13:24
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    $\begingroup$ @John dinosaurs had more joints and bones and than their bird counterparts, that is due to many bones fusing together as they evolved, being no longer necessary. As this was also observed in the evolution of many other animals, including our hands (our last two fingers are already undergoing a fusion in their muscles and tendons, which is why they can't truly move independently) I thought it wouldn't be too far fetched for my creatures to have one of the joints loose flexibility and eventually fuse to the arm after a few million years. Please tell me if there is a mistake in my assumption $\endgroup$
    – ProjectApex
    Mar 22, 2020 at 13:33
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    $\begingroup$ but they still have the same number of bones, fusing mobile limb joints is something that requires strong pressures, like flight were every milligram of tissue has a high cost, and the bones only ever have to serve one function for billions of generations. birds are one of the rare animals to fuse limb bones so there is no reason to assume it will happen.You can justify a lot of weird limb structure as long as you can justify the first terrestrial vertebrate having it, and indeed there are semi-terrestrial fins that could evolve into your layout, (eusthenopteron for one) $\endgroup$
    – John
    Mar 22, 2020 at 18:13
  • $\begingroup$ @John I see, well it's not like I hadn't already decided to cut off one elbow and stick to a single one already, since Nuclear wang made a great point, but thanks nonetheless for the insight on the process of fusion of bones and joints $\endgroup$
    – ProjectApex
    Mar 23, 2020 at 2:17
  • $\begingroup$ Good part of the climbing strength in humanoids and monkey like creatures comes from the long head of the tricep muscle which is as big as the latissimus dorsi so around 1/2 of the climbing power comes from the upper arm.... Decrease the size of the upper arm to add a new Bending point and you pretty much sacrificed a good chunk of the climbing power. Well unless you extend the tendons of the tricep muscle up to the wrist, but I think this would decrease mobility again. $\endgroup$
    – user75689
    May 14, 2020 at 9:41

6 Answers 6

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An extra arm joint does not improve mobility.

The human arm has 7 degrees of freedom (axes of rotation) - we have 3 in the shoulder, 1 in the elbow, and 3 more in the wrist. This allows us to put our hands at any reachable position in space in any orientation. This configuration actually already has one redundancy, as it only requires 6 degrees of freedom to achieve the same range of motion. Adding even more joints to the arm adds more weak points, increasing the overall fragility of the arm, while adding absolutely nothing to the range of motion achievable. It does add a bit of extra redundancy, allowing a wider range of motion even with a broken joint, but since you typically avoid using an arm with a broken joint anyway, this redundancy has very limited value.

There's very likely a good reason why no animal has evolved limbs that look like this - the singular, slight advantage of redundancy does not outweigh the disadvantage of increased complexity and fragility of the arm.

See: https://www.ncbi.nlm.nih.gov/pubmed/22255087

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    $\begingroup$ Plus fewer joints means longer lever arms for a variety of actions. $\endgroup$
    – Carl Witthoft
    Mar 20, 2020 at 14:41
  • $\begingroup$ Thanks a lot, I guess that I'll stick to a single elbow then. Just one last thing, which I didn't get much in detail in the question: the forelimb has an extension at the elbow region, which I planned to work as a better anchor point for the muscles, as it seems present in many canids. Should I remove that as well or that might be something good? $\endgroup$
    – ProjectApex
    Mar 20, 2020 at 15:04
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    $\begingroup$ @ProjectApex That sounds reasonable, bone protrusions do appear in the animal kingdom as anchor/leverage points for strong muscles. Some animals have what's called a sagittal crest, basically a bone mohawk that provides the structure needed to allow for tremendously strong jaw muscles. An animal with exceptionally strong limb muscles might have analogous structures in the arms/legs. $\endgroup$
    – Nuclear Hoagie
    Mar 20, 2020 at 16:28
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    $\begingroup$ Note that this only applies if "position and orientation of the hand" within some small region of space is all that you care about. Having an extra joint allows things like moving your arm while gripping an object, or selecting paths for your hand that meet secondary objectives, like not spilling a container that you're holding. $\endgroup$
    – RLH
    Mar 21, 2020 at 1:23
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    $\begingroup$ The claim that no animal has evolved liek this is in contradiction with @TedWrigley 's answer, wherein a pretty straightforward example of a limb like this is presented. $\endgroup$
    – AmagicalFishy
    Mar 21, 2020 at 17:59
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The legs of most ungulates are actually multi-jointed in the way you describe. The knee — between the femur and tibia/fibula — makes one joint, and what we think of as the horse's lower leg is actually an extended tarsal bone with the ankle joint (at the calcaneus) always elevated. Ungulate 'feet' are actually single or doubled toes: solid or cloven hooves.

skeletal anatomy of a horse rear leg skeletal anatomy of a horse rear leg

This design gives the ungulate leg extra power that translates into speed and maneuverability. But this comes at the expense of flexibility: ungulate legs only move in one direction, with minimal lateral freedom.

It's not impossible for a non-sprinting creature to develop limbs with two joints, but it requires some odd contexts. For example, flying creatures like birds and bats have extended their finger bones to act as structural bones, making the wrist effectively a second arm joint, and the aye aye has developed an elongated finger with special joint mobility in order to reach around 'corners' to get at grubs. I'm sure we can all think of situations where we just can't get our fingers to bend the right way to reach something we've dropped down a crevice, but I find it hard to imagine an environment so structurally complex that there would be an evolutionary advantage to develop an extra joint for reaching along twisting trajectories.

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    $\begingroup$ All that has to happen is for creatures with this limb structure to evolve in to a climber. Evolution works with what it has. $\endgroup$
    – John
    Mar 22, 2020 at 13:03
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    $\begingroup$ @John, Yes. I was just thinking that there's no particular advantage to this limb structure for climbing that might create a selection bias. but you're right that a creature which already had this structure (because it evolved as a sprinter) would likely just adapt it to climbing. $\endgroup$
    – Ted Wrigley
    Mar 22, 2020 at 15:08
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Spider legs have seven segments:

Diagram of spider legs anatomy

Source: https://en.wikipedia.org/wiki/File:Spiderlegdiagram.png

The last segment has claws, which is how the spider clings to surfaces.

Tarantulas can be as large as rats and they are still able to climb on surfaces quite well. If your creatures are about that size, having more joints should be no problem when it comes to climbing or whatever.

If they are larger, though, see L.Dutch's answer.

Being vertebrates, your creature has one more point to break ligaments. It takes only one elbow going bad to make the whole limb go bad. In the long run, evolution may favor less elbows because of that. So for the extra elbow to last geological eras, it must provide some advantage - sexual selection, or increased range of movement for the limb that the creature needs somehow.

If you happen to have a copy a Spore, you can see how such a creature would run. Proceed to the second stage, sketch your creature in the game and then run around the island ;)

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One can throw stones at great distances with such limb.

https://en.wikipedia.org/wiki/Spear-thrower

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    $\begingroup$ In the case of spear thrower, is not the number of articulation that matter, is the length of the thing. Think how much you need your elbow joint to use one. $\endgroup$
    – Adrian Colomitchi
    Mar 22, 2020 at 5:53
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I think it would be an obstacle to a creature climbing.

The reason is simple: if you hold yourself on a object, your arm, forearm and object, or better your shoulder, your elbow and your wrist form a triangle which, as we are taught since primary school, cannot be deformed, assuring a firm hold.

Any geometrical figure with more than 3 sides instead can be deformed easily, therefore it won't be able to easily hold on the object.

If the object is a tree or a boulder, it means more ease of falling down and getting hurt. Not the best situation for being able to reproduce.

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  • $\begingroup$ I think it depends on the size of the creature. Spider legs have seven segments each and they are adapted for climbing. $\endgroup$
    – The Square-Cube Law
    Mar 20, 2020 at 14:30
  • $\begingroup$ @Renan, and they have a built in safety strap, too $\endgroup$
    – L.Dutch ♦
    Mar 20, 2020 at 15:05
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    $\begingroup$ The idea of an undeformable triangular structure requires that it actually be a triangular structure. Two rigid long objects connected at a single joint is not one of them, so unless there is some rigid thing held in the hand and secured in some way to the shoulder directly, which is not apparently the means you describe, no such undeformable structure exists. It's the entire point of bending your knees when landing, the muscle groups can absorb shock by gradually allowing greater bend, significantly changing the distance between the hip and ankle/foot, because nothing directly links them. $\endgroup$
    – Nij
    Mar 21, 2020 at 10:01
  • $\begingroup$ @Nij, your reasoning is faulted: when you land you don't have a stick between your buttock and your foot ball. If you had it that would make the triangle. I clearly said that the triangle is made when you hold to something, so that there is no loose end. $\endgroup$
    – L.Dutch ♦
    Mar 21, 2020 at 10:27
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    $\begingroup$ Holding something in the hand is not enough. It has to also be secured to the shoulder. Otherwise you don't have a triangle at all - just two long things forming a chain from a body to an object. $\endgroup$
    – Nij
    Mar 21, 2020 at 10:49
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An Ibex (ungulate) is a great climber up extremely steep rocky surfaces and a jaguar (digitigrade) is great at climbing trees, I think using all four limbs to climb may be necessary, leaping up the surface as both animals do but once in a safe area the forelimbs could act as arms again.

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