Well the common ancestor of all living fish would be more like a lancelet, which is nothing like a sea sponge. (it is bilaterally symmetrical and has a "notochord" which is like a primitive spinal chord.
The main issue I have with the parent's comment is the idea that "most species" evolved independently. No, most species have a whole bunch of other species very closely related to them.
There are a few major branches of fish, such as ray finned (most fish) and lobe finned, the taxanomic category of the latter also includes tetrapods, and only has a couple of species that are still actual fish, such as coelacanth and lungfish. And above that distinction, there are bony fish and cartilaginous fish, such as sharks and rays.
But still, even if fish is not monophyletic, it is not polyphyletic, which means all things considered fish are "connected" to other fish.
The main issue I have with the parent's comment is the idea that "most species" evolved independently. No, most species have a whole bunch of other species very closely related to them.
There are a few major branches of fish, such as ray finned (most fish) and lobe finned, the taxanomic category of the latter also includes tetrapods, and only has a couple of species that are still actual fish, such as coelacanth and lungfish. And above that distinction, there are bony fish and cartilaginous fish, such as sharks and rays.
But still, even if fish is not monophyletic, it is not polyphyletic, which means all things considered fish are "connected" to other fish.
https://en.wikipedia.org/wiki/Evolution_of_fish#/media/File:...