Is there something about them that makes them less able to form more complicated, multi-Archea life forms? I know there are some colony Archea, but not true multi-cellular organisms. Though a colony is perhaps on the way there.
Or is this something where, we basically have no idea?
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The degree to which Cope’s rule applies at all in any consistent sense is a long-running debate, and a lot of models propose that it’s not so much a tendency for body size increase but a tendency for diversification in body size, which may look like a statistical increase if most clades start with relatively small-bodied species
I would say Cope’s rule among biologists is not widely accepted. You can see evidence of larger body sizes but you can also see reductions in body size, so what gives? In general I largely don’t think it is right even though there can sometimes be selection for larger body size. But since you can find selection for smaller body sizes as well, then the rule does not seem to really work. Many things have evolved to be small and stay small, the reasons for this can vary, but the point is those small things could be bigger, there is no physical thing preventing that from happening, but they still don’t
You are bringing up archea here but you could say the same about bacteria. Bacteria and archea are very adaptable and can evolve in very short time scales compared to say a mammal. In many ways these single celled organism are the most adapted to survive and to get larger is to actually reduce that survival fitness. Bacteria and/or archea can be found in just about any niche where life is feasibly possible. You definitely cannot say that about larger organisms. And if it were true that things evolve to be bigger, would you not expect those things that can evolve the fastest to do just that? And yet they don’t. So as a broad rule I don’t accept Cope’s rule premise. If it was much more narrowly defined that certain organisms may evolve a larger size to avoid predation you could at least make an argument, and yet small prey remain, so even then. If you want to be an organism that is most likely to survive don’t be an elephant, be a bacteria or archea. You have a better chance of surviving an apocalypse.