Questions about the realistic implementation of repeating parts/segments (and Williston's law)

So, one thing I’ve wondered about Thrive is the viability of creatures with many similar parts, such as millipedes. There are no real-life creatures to my knowledge that started with only 2 legs and kept adding more, a la Spore. How could Thrive be more realistic in its depiction of evolution of body parts?

For some background, most organisms have ended up with their characteristic arrangement of parts by first evolving many similar parts and then specializing or losing those parts over time. This is a general rule of biology known as Williston’s Law (best non-Wikipedia source I could find on this name is a crummy old book from the early 1900’s: Water reptiles of the past and present : Williston, Samuel Wendell, 1851-1918 : Free Download, Borrow, and Streaming : Internet Archive).

Examples of Williston’s law include:

  • The reduction in number and specialization of teeth in mammals compared to other vertebrates
  • The specialization of arthropod limbs into different functions (feet, pedipalps, mandibles, pincers, and antennae were all homologous at one point)
  • The reduction in number and specialization of sarcopterygian (lobe-finned fish) fins into legs in tetrapods
  • The reduction in number of bones in the cranium as vertebrate skulls evolved

This poses a huge problem for a Spore-like DNA currency system. If legs have to be added using the old-fashioned Spore method, then basically everything would probably evolve into bipeds or monopods first and then grow completely new limbs later if the need arises. Obviously, this isn’t very realistic.

I don’t know what the end-all be-all solution to this would be, but I had some ideas including the ability to modify multiple body segments at once with no additional fees, or something to that effect. Tradeoffs could be involved too, with more legs meaning more stability but less speed for instance.


if it were me, i would have it to where making new parts is extremely expensive, and not something to just do willy-nilly, sort of like adding a nucleus in the microbe stage. the much more affordable and safe option would be to modify already existing parts.

I mean, every part needs to start somewhere, no matter how many you have. Limbs could start as fairly cheap but useless knobs of musculature and then gain segments, branching, and hard parts over time (I’m pretty sure that’s the plan already, at least, because that would again mirror real life). The complexity of the limb doesn’t really factor into the segmentation issue, though, because Williston’s law would apply either way.

edited the topic title to prevent confusion (this isn’t just a thread about limbs, I was just using them as an example and there’s already threads about legs)

Good quality post. I was thinking about a similar problem as well, in that within the terms of evolution, evolving another pair of limbs costs a loooot, which is the reason why most limb adaptations focus around the tweaking of existing limbs rather than adding new ones. If we just had a system like Spore for example, where you can copy an entire arthropodal leg and just drag it on your organism for a couple of mutation points, I don’t think we would have a realistic experience.

I think you bring up the optimal solutions: have the simple limb “nub” roots (joints) be pretty cheap but not very effective, requiring further specialization to develop into well-developed legs/arms, and having each root of a limb requiring their own upgrades to reach a certain point in limb “technology” rather than just copy and pasting whatever upgraded limb you have purchased. So instead of placing an entire limb, you’re placing a joint which requires a commitment in both MP and time to make anything of it.

We can also have these various upgrade “levels” behave differently in optimizing movement. By this, I mean having it so that the limbs of, say, a trilobite operate differently than that of a bee so that each require differences in the way you approach them. So for the trilobite limbs (I think they have very simple one jointed limbs?), you optimize their speed and efficiency by adding a bunch of them, but for similar efficiency in the leg of a bee, you should probably have less so that each leg has more range of motion to function the way it is supposed to.

Does that make sense? To put it another way, different types of limbs behave differently so that whereas having 10 pairs of simple trilobite limbs gives you +10 speed or something, having 10 pairs of more-developed insect limbs would give you +3 speed instead due to differences in morphology, and would give you +10 speed if you had 3 pairs of them. For simple inefficient limbs, you can spam them, but as a limb involved in movement get more complex, it generally becomes a better strategy to reduce the frequency of said limbs. This incentivizes players to go crazy in the Cambrian Explosion era of their Earth while encouraging them to trim things if they choose to become more advanced in the way their creature’s locomotion works.

The same concept can be applied to other important parts - simple teeth warrant a higher quantity, but more specialized and advanced teeth require more space and thus require some trimming. Fins can be numerous in the water depending on the nature of how your aquatic organism moves, but on land, the number of limbs needed for maximal efficiency drops a bit. And we can make it easier for arthropods to add something resembling a limb than it is for vertebrates, and vice versa for any unique vertebrate features.

That’s probably a good solution. I think it would still be good to have some sort of system allowing for placing or upgrading multiple limbs. Maybe it could be that “unlocking” parts (or upgrades for those parts) is way, way more expensive than applying them, so that you don’t run out of money before you can put simple claws on all of your pairs of limbs or something.

Note about trilobite limbs, though: they have several joints. The limb segmentation probably evolved before the exoskeleton did, judging by the limbs of lobopods.

Trilobite limb:
Trilobite Ventral Morphology

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Well, look, at the very beginning a large number of segments with limbs appeared, it was something like evolutionary experiments, but initially a lot of segments could well be useful, after all, we are talking about the period of the Cambrian explosion and the Ordovician radiation, and how I previously wrote under the post about reproduction, it would be possible to make quite interesting mechanics:
the more an organism exists and the more complex it is, the more conservative it becomes, as, for example, in the case of cells, the change of which becomes more and more difficult each time. Also, this can manifest itself in the fact that with each aromorphosis, the development of new parts of the body will become more expensive and the reduction of old ones will become cheaper. Thus, in the first couple it will be more practical to take a large number of segments so that there are more chances for their implementation in the future. For example, there is a simple chordate like a lancelet, one of its descendants begins to furiously develop new limbs and their belts, and already its descendants during radiation will change some, reduce some, and turn some into something completely different