Incubus: Incubus: 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.

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.

So why dont we see giant centaur-like mammals about?

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.