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I am trying to understand if it would be possible for organisms to evolve by different means other than random mutations and natural selection. Eukaryotes were predicted to be formed by fusion with other organelles that got them their nucleus and mitochondria (endosymbiosis). Why can't this fusion process systematically continue on for larger organisms on a larger scale? For example why can't multicellular organisms detect other appropriate multicellular organisms in their environment as matches and fuse with them to form a more complex organism (in addition to traditional evolution also happening)?

Question: What biological factors prevent organisms from continuously fusing with one another? Is it possible even hypothetically for it to work?

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    $\begingroup$ Do you mean like the process that happens when fungus forms colonies, or something else? $\endgroup$ – Halfthawed May 27 at 18:34
  • $\begingroup$ @Halfthawed The process like endosymbiosis except systematically on a larger scale. The closest analogy I can think of would be "The Thing 1982" movie where the alien merges with other animals (communion). How could that or something similar be plausible? $\endgroup$ – LDR May 27 at 18:40
  • $\begingroup$ It completely isn't because single-celled organisms stop being practical once you get over a certain size? $\endgroup$ – Halfthawed May 27 at 18:52
  • $\begingroup$ @Halfthawed I don't mean a single cellular organism, I mean a multicellular organism whose entire cells have a mechanism to perform endosymbiosis. $\endgroup$ – LDR May 27 at 18:56
  • $\begingroup$ I'm confused. Don't very many fungi have a syncytium as their cellular structure? Weren't you conceived by the fusion of two cells? In other words, show what you have researched. $\endgroup$ – AlexP May 27 at 19:22
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Question: What biological factors prevent organisms from continuously fusing with one another? Is it possible even hypothetically for it to work?

It's possible hypothetically to work. Note the emphasis. A lot of things are theoretically possible, it's just that reality prefers efficiency, and this is not efficiency. Not anywhere close.

Also, just to clarify - this isn't evolution. Nothing close to it. Evolution is based off of reproduction, mutation, and changes. This is a single species which can take on multiple forms based on various growth rates, so something more akin to a species of fungus. I mean, somewhat akin to it. But you run into a problem very fast - specialization vs generalization.

In order for any living organism to survive, it needs to do a lot of things in order to survive as a species - get energy, defend itself, reproduce, etc. Now, a smaller organism has to be able to do everything - bacteria has one cell to get energy, defend itself, and reproduce. However, once you start getting bigger, you can start specializing, and specializing is awesome - human muscle cells can't digest complex molecules like the stomach can, they can't defend themselves like white blood cells against foreign invaders, they can't detoxify blood, etc. etc. - but what they can do is they can contract, and they can contract really, really well. An adult human is built up of cells that can't survive without all the other cells.

Let's take your 'fusion' creatures. They can survive on their own, presumably, which means that each creature is capable of doing everything they need to do. When they fuse, that means you're introducing redundancies everywhere. Imagine a pack of tiny humans stacking themselves up to resemble a crude adult human. This just isn't going to work - the 'arms' aren't going to be as strong as a normal humans because it's made up of tiny humans. The bones will be weaker, because it's not a single strong bone, it's a matrix of little fragile ones. It's just not going to end well.

Let's say that these things can do the incredible and reassemble themselves at the cellular level so when they fuse they can shift around the cells. Will that help? Well, no. Because that's not possible. You can't just move around cells with no consequences - if a human decided to move their lungs around, they would die, because they are missing their lungs. It's a colossal amount of energy besides, and there's no blueprint for the end result, and their are a lot more bad end results than there are good ones.

That said, it is a lot more possible on a small scale level and with very simple creatures - nothing more complex than, say, a sponge of jellyfish. Once again, it's not a very good method of creating a creature and runs into problems, but it's possible at that point.

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Maybe:

-Chimeras https://en.wikipedia.org/wiki/Chimera_(genetics) are organisms that are composed of more than one individual. The most common form of this is two related members of a single species fusing in the womb to form a composite organism (like twins). Your organisms would be evolving into a VERY close symbiosis, specializing into narrow functions, and the defects of any one symbiote would affect the whole organism. I think it could work, but it would be fraught with complexities that uni-species organisms wouldn't need to face. There would also be a REALLY tricky transition, as a huge number of things would have to be perfect at the starting conditions for such a fusion to happen. Most likely it would need to start as a result of an "island of Dr. Moreau"-type experiment to overcome the very high barrier to starting conditions.

  • The individual organisms would end up needing separate reproductive processes, so that if an "arms" organism reproduced, but not the "legs" organism, the resulting arms organism would lack the critical legs needed to survive. Most inter-species chimeras that were created would go extinct at this stage as only one set of reproductive organs would be present in normal chimeras. Where are you going to find another identical chimera for your first Mer-man, for example, with two sets of reproductive organs in just the right places?
  • Within a species, they can complement each other, so a fused person with a genetic abnormality in say, lysine synthesis may be able to obtain lysine from the parts of the body that do so. It would get more complex with different species, but the opportunities for benefit could be higher.
  • As the symbiotic relationship evolved, non-essential biological functions would devolve, and the resulting chimera would need both parts to co-evolve very closely. This is more 'moving parts' that have to be perfect to make a resulting functional chimera.
  • If the two species were significantly different, the immunology would be very complex. both organisms would need to not reject the other, limiting the ability to respond to disease states and expanding the chances of autoimmune problems. This would also be the biggest stumbling block to initial formation.
  • The first chimera would need to fuse at the embryology level, and you would need to justify how a viable fish embryo came to be in a human womb with a human embryo to give you your fish-man chimera (or whatever chimera you desire).
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  • $\begingroup$ You could also consider a vampiric-sort of graft chimerism, where an organism might harvest organs from other species. en.wikipedia.org/wiki/Graft-chimaera but this would get really messy in terms of immunology and how such a thing could evolve. $\endgroup$ – DWKraus May 27 at 20:43
  • $\begingroup$ If someone is looking for "hard science" on this question, it doesn't get any harder in biology for something that doesn't actually exist. It's conjecture based on likelihood and derived from current understanding. Biology is, by definition, soft on evolution of this kind. $\endgroup$ – DWKraus May 28 at 3:29
  • $\begingroup$ I believe cells mitochondria originated from a fusion between two types of cells that ended up being able to work symbiotically. This symbiosis fundamentally altered the course of life on Earth. So there is potential for large shifts in the evolutionary path by fusion of organisms, though it seems fairly uncommon for these fusions to result in evolutionarily viable organisms. $\endgroup$ – DerekG Jun 9 at 18:33
  • $\begingroup$ @DerekG The ancestors of Eukaryotic organisms likely phagocytized aerobic bacteria that were then able to survive intracellularly. The benefits for this would be large (maybe not as big as mitochondria). While this does represent a type of symbiosis, multicellular organisms are much more complex and the number of moving parts needed for two multicellular organisms to synch up like this is high (and immunology is a real barrier.) It would almost take design. Any ideas how to overcome the problems? It would make a good answer. $\endgroup$ – DWKraus Jun 9 at 21:00

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