Now I really gotta clarify what I mean by this before I get flamed in the comments. What I specifically mean is that sponges look very similar in form and have not differentiated a whole lot compared to other animal species despite being around since the start and being a relatively successful organisms (the fact they’re still around is a surely testament enough). So by dead end I am more talking variety in form rather than success of natural selection, is there something about the sponge body plan/way of life that has kept them from making different varieties of forms compared to other animals? Would love to know what people think.
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I mean, while they have the same basic body plan, they have definitely diversified. Silica vs calcite spicules, some have calcite exoskeletons and other don’t, some have spongin, etc.
Just because sponges still exit doesn’t mean other organisms didn’t evolve from them.
For the extreme example, single cell life still exists, yet multicellular life evolved from it.
There’s only one question that drives differentiation through natural selection: “Are you better at reproducing than the rest of your species?” Sponges are doing fine as is. They’ve survived for hundreds of millions of years simply because there’s no pressure for them to need to fit their niche any better.
If the form works, then why would it change?