Dear Everyone.

F'starters, I'm NOT a scientist, or creationist, or anything-ist, it's just I've noticed this years and years agon, been searching for the reason WHY, off and on, ever since. Haven't even heard this mentioned and that's interesting in itself. Hokay, here goes!

Insects and everything going on down from there - different body shapes, lots of legs, all to fit different specialized niches. Everything ABOVE insects - the SAME TEMPLATE. Octopuses and starfish aside, EVERY SPECIES is head, torso, limb at each corner of torso. Fish, yes, stand one on its tail and it has two 'arms' - fins - two 'feet' - tail fins. Whales, dolphins, vestigial limbs inside their bodies. Yet I've never read, anywhere, WHY everything's the same basic design. Why don't you get animals with 6 limbs, say, 4 to walk on, 2 to pick things up with? And why isn't this more of a talking point, why does everyone take it for granted this IS so without wondering WHY it's so? I mean I've got Asperger's so I might be missing something obvious, but I'm not seeing it!

I mean - even dinosaurs. Do you get big dinosaurs with more than 4 limbs? Even T-Rex has vestigial arms on the top corners of his torso. Everything else I've seen - head, neck, torso, limb at each corner of torso.

Now I'm NOT trying to prove anything with this question. "See? This PROVES there must be a creator!" "See? Proof of Parallel Evolution!" - nah. I'm not doing any of that. It's just I've noticed this and was wondering if one of you geniuses could tell me why, above a certain cutoff point, everything SUDDENLY became cut from the same basic template. Octopuses and starfish have always fascinated me because they're the ONLY TWO creatures I could find above insects that are designed differently. (If I've missed any others, might well have done, tell me!) The main thing I'm puzzled about is why you don't see this talked about more, have I missed the Utterly Obvious to the Rest of Humanity Reason? If yes, Mr. Asperger's here's spent a lifetime missing Totally Obvious Things, usually to his detriment. Just tell me WHAT'S totally obvious and I'll go away.

Yours puzzledly,

Chris.

4 hours ago, ulrichburke said:

Dear Everyone.

F'starters, I'm NOT a scientist, or creationist, or anything-ist, it's just I've noticed this years and years agon, been searching for the reason WHY, off and on, ever since. Haven't even heard this mentioned and that's interesting in itself. Hokay, here goes!

Insects and everything going on down from there - different body shapes, lots of legs, all to fit different specialized niches. Everything ABOVE insects - the SAME TEMPLATE. Octopuses and starfish aside, EVERY SPECIES is head, torso, limb at each corner of torso. Fish, yes, stand one on its tail and it has two 'arms' - fins - two 'feet' - tail fins. Whales, dolphins, vestigial limbs inside their bodies. Yet I've never read, anywhere, WHY everything's the same basic design. Why don't you get animals with 6 limbs, say, 4 to walk on, 2 to pick things up with? And why isn't this more of a talking point, why does everyone take it for granted this IS so without wondering WHY it's so? I mean I've got Asperger's so I might be missing something obvious, but I'm not seeing it!

I mean - even dinosaurs. Do you get big dinosaurs with more than 4 limbs? Even T-Rex has vestigial arms on the top corners of his torso. Everything else I've seen - head, neck, torso, limb at each corner of torso.

Now I'm NOT trying to prove anything with this question. "See? This PROVES there must be a creator!" "See? Proof of Parallel Evolution!" - nah. I'm not doing any of that. It's just I've noticed this and was wondering if one of you geniuses could tell me why, above a certain cutoff point, everything SUDDENLY became cut from the same basic template. Octopuses and starfish have always fascinated me because they're the ONLY TWO creatures I could find above insects that are designed differently. (If I've missed any others, might well have done, tell me!) The main thing I'm puzzled about is why you don't see this talked about more, have I missed the Utterly Obvious to the Rest of Humanity Reason? If yes, Mr. Asperger's here's spent a lifetime missing Totally Obvious Things, usually to his detriment. Just tell me WHAT'S totally obvious and I'll go away.

Yours puzzledly,

Chris.

I suspect the key to your question is in the term "vertebrates", more strictly these days called "chordates". The common body plan you mention is due to evolution of all chordates from a common ancestor, an early fish-type thing. The legs evolved from fishy fins. Fish, amphibians, dinosaurs, modern mammals are all in the chordate family, having the same basic body plan. So that's why.

But in fact there are a lot more alternative body plans in the animal kingdom than just the chordates and the arthropods that you mention (insects, crustaceans and so forth). Both athropods and chordates are examples of a "phylum", which is a biological classification based on the basic type of body plan. A few examples of other phyla would be molluscs (shellfish, squid, octopuses etc) echinoderms (starfish, sea urchins, with 5-fold radial symmetry) and annelids (segmented worms). But there a lot more besides. You can read more here: https://en.wikipedia.org/wiki/Phylum

It is true that all the large, land-dwelling animals are chordates, which is why you can get the impression that the majority of animals have the same body plan. But if you don't limit your consideration to size, or to dwelling on land, you get a very different picture, with a lot more variety.

Edited April 23Apr 23 by exchemist

As exchemist said, the animals in question are all descended from a fish-type thing, and evolution modifies body parts, so four fins means four limbs. The question becomes why do fish have four fins. I suspect it’s because more than that wouldn’t be beneficial. Appendages create drag, which slows you down.

But some animals did modify other body parts to be useful. Some animals have prehensile tails, and some have a prehensile snout, and prehensile lips and tongues exist.

7 hours ago, ulrichburke said:

Why don't you get animals with 6 limbs, say, 4 to walk on, 2 to pick things up with?

Because the transition from four limbs to six requires the precise coordination of thousands of individual genetic and behavioural adjustments.

Statistics greatly reduces the chance of such a revolutionary change occurring all at once (see Saltation), especially in organisms above a certain level of complexity.

On the other hand, achieving the same through many thousands of incremental changes is only feasible if each generation is at least as 'fit' as the previous. Which means that eg. an extra quarter of a leg must offer some survival advantage exceeding the extra energy needed to grow it, and having more bits that can potentially go wrong (see Exaptation). Not impossible, it's how we got our limbs in the first place, but still a major hurdle. Evolution is blind: even if six limbs were better than four, evolution has no way of knowing this in advance and therefore no reason to head off in that direction.

Maybe you're not trying to prove 'creation' or 'evolution', but I suggest things only make sense if you consider evolution,
Aquatic animals are forced to go onto land, they have two options, slithering on the ground, or supporting themselves on appendages to stay off the ground. Four appendages is the most economical and stable method.
Then evolution continues ...
Land animals evolve larger capacity brains, and need to grasp things. Their brain is now capable of balance, so again economy of resources diversifies two appendages for grasping things.

Evolution has always been about environmental forcings ( the need to do specific things ) in the most resource economic way possible. If something isn't needed, it isn't developed by evolution.

https://en.wikipedia.org/wiki/Homology_(biology)

53 minutes ago, MigL said:

Aquatic animals are forced to go onto land, they have two options, slithering on the ground, or supporting themselves on appendages to stay off the ground. Four appendages is the most economical and stable method.

And there's a clade where 8 is even better (especially if you never know when you're going to lose a couple in a fight or accident).

https://www.livescience.com/animals/spiders/why-do-spiders-have-8-legs

"Those legs are actually part of their mouth," Nipam Patel, a developmental biologist and director of the Marine Biological Laboratory, which is affiliated with the University of Chicago, told Live Science. 

Because spiders, insects, crustaceans and millipedes all evolved from an ancestor that likely had a segmented body with a set of appendages on each segment, these species are just highly modified riffs on that basic plan. According to Patel, all arthropod appendages — including legs, antennae and even mandibles (the jaws) — can be traced back to a stubby lobopod appendage. 

Take a mantis shrimp. It swims with a bunch of little legs on a segmented abdomen. On the cephalothorax (a fused head and thorax) are its walking legs, and then near its mouth are little appendages that not only make up its jaws but also sweep food into its mouth to help it eat....

Then, there are spiders.

"If you look at a spider embryo, it looks exactly like an insect embryo," Patel said. "Except it only grows the legs on its head. But instead of using those as mouthparts, it uses them to walk."

The reason spiders walk with appendages from their faces goes back to lobopods and the original chelicerate body plan. While modern arthropods are spoiled for specialized appendages, the lobopods were wormlike creatures with many sets of roughly similar appendages. 

Initially, all of the legs were the same," Heather Bruce, a research associate at the Marine Biological Laboratory, told Live Science. "But then the first appendages became differentiated for being a sensory appendage, like for sensing and grabbing food."

From that point, the spider's chelicerate ancestors began to diverge from the other groups. In the ancestors of insects and crustaceans, the lobopod's multitasking front appendages lost their grabbing and feeding ability and became specialized sensory structures called antennae. But for chelicerates, those same appendages lost their sensory capabilities and became fangs.

5 hours ago, exchemist said:

The common body plan you mention is due to evolution of all chordates from a common ancestor, an early fish-type thing.

To me, the recurrent laryngeal nerve is the best evidence that the statement above is accurate, that evolution is at work here. That early fish-type thing had an organ that performed a gill function, and when branches of this species went onto land, evolution repurposed that organ to eventually become the larynx. Unfortunately, as the organ travelled over a long time, the recurrent nerve got wrapped under the aortic arch of the heart. There's no good reason for such a long nerve to connect larynx with brain, but that's how it evolved. And all chordates have this setup, even giraffes, where the recurrent laryngeal nerve is around 15 feet long!

50 minutes ago, TheVat said:

And there's a clade where 8 is even better (especially if you never know when you're going to lose a couple in a fight or accident).

Covered by 'economy of resources'.

As for spiders, why evolve more legs when when 'jaws' or food grasping appendages can be re-purposed ?
Again, 'economy of resources'; nature is resourceful, but frugal.

Yes. Was just expanding a bit on the concept, as arthropods provide interesting examples. (Haven't seen it yet but I think the alien in the current hit film Project Hail Mary is sort of spider-like - I'll be interested if they explore the conceptual territory around alien ecosystems where primordial species go a different direction)

5 hours ago, MigL said:

Evolution has always been about environmental forcings ( the need to do specific things ) in the most resource economic way possible.

And existing designs can place a constraint on subsequent ones. This is how genetic relationship work. I.e. traits that are complex cannot be easily undone or reversed. But to specifically address questions like this:

14 hours ago, ulrichburke said:

WHY everything's the same basic design. Why don't you get animals with 6 limbs, say, 4 to walk on, 2 to pick things up with? And why isn't this more of a talking point, why does everyone take it for granted this IS so without wondering WHY it's so? I

As exchemist points out, this is because the animals you are referring to all have a common ancestor which share the same body plan. But I think you might be a bit confused about this point:

14 hours ago, ulrichburke said:

bove a certain cutoff point, everything SUDDENLY became cut from the same basic template. Octopuses and starfish have always fascinated me because they're the ONLY TWO creatures I could find above insects that are designed differently.

You seem to make a hierarchy here (above and below something), but this is an inaccurate way to see things. A cat is not above an ant, for example. Evolutionary, everything that exists at a given time exist in parallel (to state the obvious) and are not hierarchically ordered. What you might think of is how far back the lineage between ant and cat have split, which would be about 700 million year ago. I.e. there was an ancestor 700 million years ago, that split into different lineages that, over time, become what we now can see as ant or cat.

So based on that, there is no sudden cut-off in that perspective. However, what has happened is that at some point animal with the body plan you mentioned evolved and they have split into further and further species, but they did not all come into the existence at the same time. Tetrapoda (four-limbed vertebrates) evolved around 390 MYA and are the ancestors of amphibians and amniotes, including dinosaurs as well as mammals.

Now, if you go back to lineages that existed in parallel or earlier to tetrapoda, you will find many other designs. You mentioned insects, which are derived from arthropods which go back about 540 million years. Cephalopods (to which the octopus developed) in parallel around a similar time.

All these groups belong to the bilateria, animals with a bilateral symmetry, which where first members with estimates as ranging to about 700 million years ago. So we have a long history of animals that are not four-limbed, but with some basic symmetric body shape.

Beside bilateria, other lineages included porifera, ctenophora, cnidaria and placozoa. Those have a very different structure and include sponges, jellyfish, corals, comb jellies and so on. These are the weirdos were folks might not intuitively recognize as animals, and where some are just blobs.

So going back to the why, it is history and relatedness, but there is not sudden cut-off point as such. Only really a point where the body scheme first existed and if it still exists now, it just means that their descendants survived to this day.

1 hour ago, TheVat said:

Yes. Was just expanding a bit on the concept, as arthropods provide interesting examples.

OK.
But can we stop talking about spiders ?
Always feel like something is crawling on me when discussing spiders.

Although I am reminded of the Predator aliens, with their extended, horizontally opposed mandibles.
( the original movie may have indicated they were related to scorpions )
And one episode of Star Trek TNG suggested Lt. Barclay's ancestors were spider-like creatures, although he looked very human.
( Genesis S7 Ep19 )

1 hour ago, CharonY said:

All these groups belong to the bilateria, animals with a bilateral symmetry, which where first members with estimates as ranging to about 700 million years ago.

Bilaterians only have to have bilateral symmetry in embryonic or larval form to be in the clade. Echinoderms are included due to BS as larvae, even though they change to radial symmetry in adulthood. When they achieve, in some cases.... stardom.

1 hour ago, MigL said:

But can we stop talking about spiders ?
Always feel like something is crawling on me when discussing spiders.

Then we should probably steer clear of pyroglyphidae, too. 😄

https://en.wikipedia.org/wiki/Pyroglyphidae

VACUUMING MATTERS.

Snouts.

These are not bilateral and not every creature has one or if they do uses it for digging.

In fact you have pointed out one particular similarity, but said nothing about the many many difference between creatures. Why do some have fur, some have skin, some have olfactory senses in the nose , other elsewhere, some have eyes, some are blind, some have very hollw bones because they fly (talking about bilateralism, bilaterism is good for flying things and most swimming things, although I understand from D Attenborough that there are some fish that lie one one side only on the bottom and are not bilateral.

There was a recent popsci book that explains all this, I will post a reference when I can remember it.

24 minutes ago, TheVat said:

Bilaterians only have to have bilateral symmetry in embryonic or larval form to be in the clade. Echinoderms are included due to BS as larvae, even though they change to radial symmetry in adulthood. When they achieve, in some cases.... stardom.

True, though with the exception of echinoderms I think the symmetry is maintained in generally maintained in other groups even as adults. But as with many phenotypic classification schemes, things might be weird.

On 4/23/2026 at 2:49 PM, ulrichburke said:

Everything ABOVE insects - the SAME TEMPLATE. Octopuses and starfish aside, EVERY SPECIES is head, torso, limb at each corner of torso. Fish, yes, stand one on its tail and it has two 'arms' - fins - two 'feet' - tail fins. Whales, dolphins, vestigial limbs inside their bodies. Yet I've never read, anywhere, WHY everything's the same basic design.

I typed "Why are reptiles, birds and mammals four limbed?" into a search engine and got abundant references to Tetrapods and their evolution. Amongst those near the top were links to explanations of why that body plan is so prevalent and has persisted so strongly.

I find it odd that you have not found and read any of those.

Lots of links and references from a simple search to the fact that most species of extant fish are not tetrapods, despite the four fins if you don't count the tail form is very common. But six fins and tail are common too.

Most living fish are 'ray finned' and are not tetrapods. Early tetrapods 'fish' (fish being a body style rather than a taxonomic category) were 'lobe finned'. Lungfish and Coelacanths are examples of 'lobe finned' fish more closely related to tetrapods - but aren't descended from them.

Size and it's relation to gravity probably has a great deal to do with limb number's, since more than four limbs only exists below the weight of bigish spider, on land.

1 hour ago, dimreepr said:

Size and it's relation to gravity probably has a great deal to do with limb number's, since more than four limbs only exists below the weight of bigish spider, on land.

See Arthropleura

Arthropleura, from Ancient Greek ἄρθρον (árthron), meaning "joint", and πλευρά (pleurá), meaning "rib", is an extinct genus of massive myriapod that lived in modern-day Europe and North America around 344 to 292 million years ago,[2][4] from the Viséan stage of the lower Carboniferous period to the Sakmarian stage of the lower Permian period.[2][5] It was a millipede, and was capable of reaching at least 2 metres (6 ft 7 in) in length, possibly up to over 2.5 metres (8 ft 2 in), making it the largest known land arthropod of all time.

1 hour ago, dimreepr said:

Size and it's relation to gravity probably has a great deal to do with limb number's, since more than four limbs only exists below the weight of bigish spider, on land.

There are limitations on size for insects, arachnids and arthropods that aren’t tied to gravity (e.g. respiration). One that is is the problem of scaling up exoskeletons, but having more legs would tend to mitigate that.

Tetrapods adapting to land and filling up niches has to be considered as well. A heavy organism owing to its exoskeleton, supported by a lot of legs probably can’t move quickly and needs a lot of food, so it’s not going to compete well with anything that’s faster and more efficient.

Big arthropods were peculiar (if not exclusive) to the Carboniferous period, due to the high oxygen levels, rather than gravity.

Number of appendages seems to be more of a developmental accident, as most members suggest. Embryonic development tends to follow body plans long past initiated.

We still have some pretty large ones in the tropics

(Apologies if there's a forum ban on myriapod porn)

13 minutes ago, sethoflagos said:

We still have some pretty large ones in the tropics

(Apologies if there's a forum ban on myriapod porn)

There is also the coconut crab, which grows up to 4kg weight.

1 hour ago, sethoflagos said:

(Apologies if there's a forum ban on myriapod porn)

As long as they're using protection. As exampled by the pill bug.

https://en.wikipedia.org/wiki/Armadillidiidae

I don't think @MigL will like the way this is turning...

42 minutes ago, TheVat said:

As long as they're using protection. As exampled by the pill bug.

https://en.wikipedia.org/wiki/Armadillidiidae

Surely that’s just a woodlouse, isn’t it?

4 hours ago, exchemist said:

Surely that’s just a woodlouse, isn’t it?

Armadillidiidae is a family of woodlice, a terrestrial crustacean group in the order Isopoda. Unlike members of some other woodlice families, members of this family can roll into a ball, an ability they share with the outwardly similar but unrelated pill millipedes and other animals. This ability gives woodlice in this family their common names of pill bugs[1] or rolly pollies.

(All that family shows convergent evolution with the myriapods like the millipedes, in morphology and pilling)