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The fixedness of body plans varies widely across different types of Earthling organisms. At one extreme, you have things like tardigrades, for which every individual of any given species has exactly the same number and arrangement of individual cells, differing only in size; at the other extreme, you have things like mycelial fungi, which don't really have any consistent large-scale shape.

At various points in the middle of the spectrum, you have typical animals and plants: all animals of a given species tend to have the same high-level shape (e.g., number and arrangement of limbs), even though they differ in small-scale details and may be different sizes, while plants tend to be similar in terms of the shapes of specific organs (leaves, branching structure, etc.), but can modularly assemble those mid-range features into wildly different large-scale structure--i.e., it is easy to find pairs of animals that are nearly identical to each other, but good luck finding two trees that grew in exactly the same shape! This modular construction conveys numerous advantages to plants, most notably the fact that they can sustain massive injuries and still survive--lop a third of the limbs off a typical tree, and it won't care, 'cause it's got or can grow spares. Lop off a third of a dog, and, well... you've got an animal cruelty case and a lot of blood to clean up.

So, can we make a plant-style modular-type body plan work for more animal-like creatures? How would something like that evolve?

Requirements

A modular animal must:

  1. Be mobile.
  2. Be heterotrophic--whether vegetarian or carnivorous doesn't really matter.
  3. Be constructed largely out of repeatable, interchangeable, and redundant organ complexes, such that damage and regrowth is both possible and expected.
  4. Have its detailed large-scale shape determined in large part by environment and injury history, not fixed by genetics.

And for the sake of narrowing the scope:

  1. Live on land.
  2. Exist in a size range typical of mammals--say, somewhere between a housecat and an elephant.

This does not, however, necessarily mean that a modular animal can't have critical specialized organ systems, like a single head or single digestive tract; after all, separate the crown of a tree from its roots, and it will die (unless each separate part is in good conditions to regenerate the missing half, of course)--each part may be modular, but that doesn't mean the different parts are arbitrarily divisible.

A list of all Anatomically Correct questions can be found here: Anatomically Correct series.

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This question has an open bounty worth +250 reputation from Measure of despare. ending in 3 days.

The question is widely applicable to a large audience. A detailed canonical answer is required to address all the concerns.

Whilst the current answer offers a starting point, the question stipulates "on land", a canonical answer will take this into account.

  • $\begingroup$ @Chickensarenotcows Added specifications for size and land-living, but I am intrigued by what existing creatures you have in mind. $\endgroup$ – Logan R. Kearsley Aug 12 at 21:35
  • $\begingroup$ I was thinking hydras, planaria and slime-molds, way outside the current scope of the question. As I see it, the biggest issue is respiration and oxygen distribution: cut the torso off a mammal, the legs might just want to run away, but heart and lungs are nowhere nearby to help. I'll think about it, but have nothing to add at this point. $\endgroup$ – Measure of despare. Aug 12 at 21:44
  • $\begingroup$ How would something like that evolve? It wouldn't, fractal animals had this and died out. Land living constraint is too much considering everything started in the sea for good reason. It would never evolve far enough to get on land $\endgroup$ – Kilisi Aug 14 at 0:30
  • $\begingroup$ Your question is bit confusing, do you want to physically be able to swap parts, or just swap them out at the genetic level? Plants are not modular they are totipotent which is a very different thing. If you just want something that can regrow after severe damage including loss of entire limbs and organs look at echinoderms. $\endgroup$ – John Sep 16 at 19:21
  • $\begingroup$ @John I want them to be able to swap parts out at the developmental level, like plants do. $\endgroup$ – Logan R. Kearsley 2 days ago
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Oops, someone did that (sorta)

I’d like to introduce the siphonophore, a colony organism made of specialised individuals called polyps (or sometimes zooids). Examples include the long strings of polyps. enter image description here

The hideous mass of many different polyps.enter image description here

Or perhaps the more familiar, blue bottle jellyfish.enter image description here

But it isn’t a jellyfish, it’s just related. The sail is an individual organism, and so are each cluster of stinging tentacles, the feeding polyps, and reproductive polyps. Each individual serves one purpose and is utterly useless on its own, having to rely on the other polyps to perform the actions it lacks.

This concept can easily be blown up into more complex (and less gelatinous) colonies. In fact, a certain future biology documentary has explored this a little (https://speculativeevolution.fandom.com/wiki/Ocean_phantom).

The problem of living on land means you may need ‘lung polyps’ ‘locomotive polyps’ and ‘gut polyps’ but otherwise vascular connections and more solid construction may not be too far-fetched. The other option is simply to make them small (I understand this doesn't meet one of your checkpoints). Gas exchange isn’t an issue for insects and frogs.

@Chickensarenotcows mentioned planaria, which have an advantage similar to some worms where they can be completely bifurcated and, as long as a certain portion of the body is left intact, completely regenerate. The way this is achieved in siphonophores and partly in planaria is segmentation - basically modularization of the body. Examples of similarly segmented life (without the regenerative ability of course) are centipedes and worms. A merging of these more complex, terrestrial body plans with the colony zooids/polyps of siphonphores could yield a believable organism of your description (if arthropleura is anything of an example).

P.S. Sorry for the enthusiasm, I just really like these guys.

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  • $\begingroup$ The other big issue is coordination, if the same parts are not occurring together over and over the chances or evolving the correct control mechanisms are laughably hard. You are literally at the tornado assembling a 747 territory. If you want regenerative ability look at echinoderms a sea cucumber can literally fire its internal organs out its anus then just grow new ones later, or you have star fish which you can cut into sections and as long as they can still feed will regenerate completely.. $\endgroup$ – John Sep 16 at 19:14
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The problem is that plants were able to evolve the way they did because if you cut them, the only criteria the new limb needs is that it has access to sunlight. This makes the random developmental generation of new material without strong negative consequences.

Doing this for an animal, which requires locomotion, would be extremely difficult. That's why animals like lizards regenerate limbs, but they follow a very specific structure that allows them to walk the same way every time it happens. If they generated a random limb, varying is size and shape, it would make walking very difficult and they would die off very quickly after that. Hence, not very appropriate for evolution.

I propose an entirely new kind of animal with a unique organ or a section of the brain completely devoted to calculating how to use and adapt to its randomly generated limbs. This would allow that animal to have random locomotion patterns, causing each individual of the species to be very difficult to track by predators and thus would benefit the evolution of the species. Some locomotion patterns would be more helpful than others, and as such those patterns would over time be removed from the rules that the generation of new limbs would follow. (ie having limbs of vastly different lengths would be avoided)

This animal would be similar to insects, in the sense that they would not have a circulatory system, because loss of blood could cause death and would inhibit the generation of appendages. Rather, upon losing a section of the body, the animal might go into a hibernation stage dedicated to generating new limbs and training the brain how to use those limbs.


Summary of why my model works:

1. Be mobile  

My model moves using randomly generated limbs.

2. Be heterotrophic--whether vegetarian or carnivorous doesn't really matter.  

My model works for all kinds of animals, regardless of diet.

3. Be constructed largely out of repeatable, interchangeable, and redundant organ 

complexes, such that damage and regrowth is both possible and expected.  

Other than a lack of circulatory system and a unique organ or dedicated area of the brain, my model does not specify any organs that cannot be regrown.

4. Have its detailed large-scale shape determined in large part by environment and  
injury history, not fixed by genetics.

My model supports its evolutionary development, as well as the ability to generate limbs in a way that can adapt to the environment. For example, developing limbs suitable for climbing, limbs for running, or a combination of both and any other possible uses.

5. Live on land.

My model can be land-based, water-based, or flight-based.

6. Exist in a size range typical of mammals--say, somewhere between a
  housecat and an elephant.

No size restrictions need to be placed on my model. However, the larger the animal, the longer the amount of time limb regeneration would require (leaving the animal vulnerable), which suggests that evolution would tend to lean towards developing the species into smaller creatures.

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I think the comments about Echinoderms are actually a good start to get on the right track. It is already the phylum most closely related to the phylum Chordata (which includes mammals), and many of them have similar, if not exact, traits you are describing.

  1. Be mobile. No question there. Many species of Echinoderms are mobile, if slow.

  2. Be heterotrophic. Again, no issues there, Echinoderms definitely eat stuff.

  3. Be constructed such that damage and regrowth is both possible and expected. A notoriously important characteristic of many of the most commonly recognizable Echinoderms.

  4. Have its detailed large-scale shape determined in large part by environment and injury history, not fixed by genetics. Here is where we start to diverge from known Echinoderms, and have to borrow from other forms of life. I see 2 main options here, depending on just how mobile the animal is, and just how specialized it's different organ structures are.

Option 1: it is very slow moving, and rarely moves when it can avoid it. It remains in place, foraging any food source in the area immediately within its reach, and stays put until the local food source is entirely exhausted. Whenever possible, it grows toward food it can detect, rather than moving toward it. Its appendages for eating and for locomotion are both very small, very numerous, and interconnected (it eats any food it steps on through its feet). When remaining in its current location is not possible, it uses starfish type locomotion (possibly in conjunction with snail/slug type locomotion) to move to a new food source, and repeats the process. Size/shape of the individual is determined by the shape of the food sources it encounters, as it (at least somewhat) grows in to the shape of that source while it feeds.

Option 2: Slightly faster moving, but still slow. A generally rounded or spherical shape, but with no set numbers of appendages. I imagine something the shape of a sea-urchin, with a random number of spines. More specialized locomotion appendages are present, but other appendages can be recruited for locomotion for faster movement in emergencies. Existing appendages are converted, or new ones grown, as needed, after injury or accident. The key being "as needed". If a missing limb isn't slowing the animal down, or keeping it from feeding normally, etc, no need to regrow it at all. If repeat attacks occur, defensive appendages could be converted to locomotion "permanently", or vice versa, depending on the types of attacks and the results.

  1. Live on land. Sea Cucumbers already have a similar external body plan to slugs, and starfish can reach rocks at high tide that are out of the water at lower tides, and stay there until the tide returns, so there is some (little) precedent for out of water (if not fully land based) animals with some of these characteristics.

  2. Size range somewhere between a housecat and an elephant. Sunflower sea stars can get about 4 feet across from arm tip to arm tip (eye-balling some google images, it looks to me like just the body, without the arms, is close to 2 feet across), and sea cucumbers can get more than 6 feet long

As far as evolution, it seems to me to be no great leap from tide pools to fully land-based lifestyle, both to evade predators in the tide pools themselves, as well as reach land-based food sources that tide-pool-locked species can't reach, especially if those food sources are similar enough to require relatively small change in digestive function. The first step would probably be water retention, to avoid dehydration on land, followed by specializations for respiratory functions, and then locomotion and food source specialization would be next.

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  • $\begingroup$ The OP has asked for plant-style and specified that he does not want organisms that can simply "regenerate". He wants organisms that can generate in a way similar to plants, where their environment directly influences the regeneration process. $\endgroup$ – overlord 8 hours ago

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