I’ve noticed that among invertibrates, there is a wide variety to methods of locomotion- crawling on lots of legs, sliding on mucus, etc. But when you get to vertibrates, its quadripeds/bipeds. Similarly, among invertibrates, you dont see little bipideal creatures about.
Does this have something to do wtih having an exoskeleton vs an endoskeleton? Or is it a matter of scale- one method working better at small sizes than large?
I would think a big, heavy animal would benefit from having six legs- each leg wouldn’t have to be as big and heavy, their weight would be better distrubuted, redundancy if one leg gets wounded, etc.
So why dont we see giant centaur-like mammals about?
Vertebrates evolved from lobe-finned fish with paired pectoral and pelvic fins – two sets, not three. There’s nothing to build a third set of limbs from.
Glib answer. For the same reason that you don’t see any four legged insects or six legged spiders. That’s the number of legs our ancestors had.
The common ancestor of the arthropods had a segmented body plan which was a really good way to pave way for a multitude of leg arrangement in its descendants. Add a few hundre segments here to get a millipede, fuse a few and drop half a dozen legs and you have an insect.
We inherit our four legs from aquatic ancestors, and what made two pairs of fins such a success for them or what, if anything, prevented another set of fins from appearing and/or succeeding. I can’t tell.
Still, if I’m not mistaken, insects have had six legs and spiders have had eight for longer than there’s been terrestrial vertebrates in existence. So it’s not like they change the number of legs more often than us.
You mean tetrapods (land vertebrates) in the first sentence. The establishment of a vertebrate body plan with two pairs of appendages goes much further back. The early ostracoderms had paired pectoral fins. The later placoderms were the first to develop a second pair of fins in the pelvic region. Modern sharks and bony fish both may have evolved from placoderms, and retained the ancestral condition of two pairs of appendages. And land vertebrates derived it from their aquatic ancestors.
It’s most likely fundamental developmental constraints that have kept vertebrates from evolving additional sets of limbs. Although vertebrates have a somewhat segmental body plan, it’s not nearly as flexible as in the arthropods.
It should be mentioned that invertebrates include dozens of different phyla, which include many different kinds of basic body plans. Vertebrates represent just part of one phylum, and have one basic body plan.
There are some insects that are functionally four-legged. It’s easier in general to lose appendages than to gain them (e.g. snakes). So the real analogy would be eight-legged insects or ten-legged spiders.
A reminder about how evolution works might be in order: Evolution doesn’t cause the emergence of useful traits. Random processes cause the emergence of traits, and if the trait happens to be useful, evolution causes it to stick around. For any question of the form “why don’t we have this trait?”, the general answer is “because that trait never happened to show up to be selected for”.
Putting that a different way: evolution is neither directed nor telelogical. There is no plan, no ultimate aim for any given species: just what worked in the past long enough to be kept in the present, which may or may not carry on to the future. Humans are not more “highly evolved” than chimpanzees or rats or bumblebees or even bacteria; all have simply evolved in different ways.
There are plenty of vertebrates which are not quadrupeds or bipeds. There are vertebrates with no legs (snakes, some lizards, and most fish) and also vertebrates which are functionally tripeds (kangaroos and some fish).
Vertebrates have a lot more modes of locomotion than quadripeds/bipeds.
Snakes, eels and so forth crawl with no legs.
Hagfish crawl and burrow on mucus.
Fish swim, variously using their dorsal fins, tails, caudal fins, pectoral fins or even gills.
Tripod fish walk ion three legs formed from the pectoral and anal fins.
Mudskippers hop and clkmb using their tails and pectoral fins
Cetaceans swim by arching their spines.
Macropods have evolved a pentapedal posture using their tail as a fifth limb.
Colugos glide.
Birds and bats fly.
Pangolins are tripedal.
Golden moles swim through loose sand using their heads and forelimbs.
And so on and so forth. There’s no justification in a claim that vertebrates have only two modes of locomotion.
Others have noted, the ancestral tetrapod had four legs, but that doesn’t go any way at all towards answering the question. It’s simply sidestepping it.
The ancestral uniramous arthropod had at least 16 pairs of legs. That doesn’t stop modern uniramia from having anywhere from 700 to 4 pairs of legs. The ancestral chelicerate had only 3 pairs of legs, most modern species have four pairs and some ancestral species had up to 30 pairs.
So clearly the number of legs the ancestor has doesn’t mean squat. While the ancestral arthropod had 2 pairs of legs, that doesn’t explain why 99% of of their descendants also have two pairs, while less than 5% of the descendants of the arthropods groups retained the same number of legs. So the real question is why the vertebrate limb count is so anomalously rigid. Pointing out that it *is *rigid is just rephrasing the question without actually answering it.
The reason that the vertebrate limbs are so invariant is because of the way that our bodies are constructed. Most invertebrates are constructed on a modular design. There’s a head, followed by a body segment with one or two pairs legs, then that segment is repeated n times, where n is anything from two to 750, and finally terminated with an anal segment. That design plan is most easily seen looking at the millipedes and centipedes, where it is followed very closely. With relatively minor exceptions, every segment is identical.
In the case of some arthropods groups, such as the insects, the plan is modified, but not greatly. Instead of simply having umpteen repeating segments, they essentially have x thoracic segments, followed by n abdominal segments, then an anal segment. But even there, the segments are fundamentally the same. WHile the thorax with its wings and legs might look radically different to the abdomen, they are genetically the same, and with minor tweaking you can cause the abdominal segments to grow limbs.
That modular body plan is why the arthropods find it so easy to vary the number of limbs. A minor mutation in one gene will add a whole new body segment, complete with a functional pair of legs, and a similarly minor mutation will remove one. IOW it is trivially easy for athropods to add or remove as many legs as they like.
In contrast, vertebrates are not modular. We evolved from modular ancestors, but we lost that body plan a long, long time ago. The only place that we retain modularity is our tails, which is why vertebrates can grow or shrink their tails as easily as arthropods can add or subtract limbs.
The rest of the vertebrate body plan is much more complicated than the ancestral modular system. Limbs are positioned based upon distance from other limbs and from midline. So it’s almost impossible for vertebrates to add functional limbs with minor mutations. And so of course we didn’t. We tend to be very conservative in our body plans.
Oh, and it’s also no surprise that invertebrates show so much more diversity than vertebrates. Vertebrates are a fairly small group, and invertebrates are everything else. It’s only because we happen to be members of that small group that we think it’s a meaningful distinction, but a mollusk might just as well wonder why there’s so much more diversity among non-mollusks than among mollusks, or an arthropod why there’s so much more diversity among non-arthropods, or so on.
What appear to be extra legs in vertebrates are almost always a partial conjoined twin. The embryo either partially divided, or else a potential twin got partially absorbed by its sibling. The legs are not attached to the rest of the skeleton in a functional way, or integrated with the nervous system sufficiently that they can work in locomotion.
And of course, several of those images aren’t even that, but shots of two separate animals from a funny angle, or Photoshop jobs, or such. One of them is a picture of a tick stuck to a perfectly normal dog.
Wow, great answers! I never thought about the ‘modular’ design of arthropods. You also reminded me of something-
Our shoulder joints are very different than our hip joints, and with an endoskeleton you have a complicated framework on this. I guess with an internal skeleton limb structure gets really complicated, especially where it joins the body.
Its fascinating how evolution works both ways- something might exist on an animal because there’s a damn good benefit to having it, OR no downside to it being there.
Oh well, so much for my dreams of having a six legged cat.
Wait, stop the presses! We can grow tails?! Why hasn’t anybody told me this? How soon? (Seriously though, does that mean humans could grow tails within a couple of generations, or more like a thousand generations?)
Well, technically we have tails, the coccyx. Ours just isn’t sticky-outy or equipped with muscles. And if you look at the embryos of many animals, at a certain phase of development, they all look very similar; its like the keel gets laid down the same way, its just the details that define the greater differences.
I always thought tails were simply an extension of the spine, really. They’re so common, I’d say we’re in the minority of vertibrates that dont have an exposed tail.
