Wow. I would have bet on the sponge-sister scenario all night long. Sponges have no neurons. This finding implies that either neurons were “lost” on the sponge lineage, or neurons evolved convergently in comb jellies and us (parahoxozoa). Both possibilities seem pretty interesting.
It is highly likely that the common ancestor of the sponges and of the other animals except comb jellies did not look at all like a sponge, but it was a mobile ciliated animal, which was much more similar to a sponge larva (which is mobile) than to an adult sponge.
The sponges have developed their own way of life, where the adults are immobile and fixed and they feed by filtering the water, like many other groups of animals, for instance clams or sea squirts.
All such groups of sessile filtering animals have greatly simplified bodies and very reduced nervous systems.
Because the ancestor of the sponges was already much simpler than the ancestor of clams or of sea squirts, they are the simplest of all.
While sponges do not have neurons, they have most of the genes required by neurons and there are good chances that the ancestor of the sponges had neurons.
The fact that sponges are more closely related to humans than comb jellies is exactly analogous to the fact that the sea squirts are more closely related to humans than the lancelets.
While both comb jellies and lancelets appear more similar to humans than, respectively, sponges and sea squirts, that is only because both sponges and sea squirts are simplified due to their sessile water-filtering way of life.
> All such groups of sessile filtering animals have greatly simplified bodies and very reduced nervous systems.
Yes, as the joke goes, as the larva of an anemone settles into its permanent feeding spot it will consume its own brain, much like the professor that has finally achieved tenure.
I thought it was tunicates (sea squirts) because they are closely related to vertebrates and start off free swimming with a notochord. (I also heard the joke as being about Americans- they are curious and thoughtful while young, but then settle down, become couch potatoes in front of the tv and digest their own brains). Either way, it is a cute joke and interesting marine biology.
There is also the question of where to put Placozoa, they still might be the true sister group to all others. But these are so simple and little known that there might just not be enough data to know for sure. If we disregard Placozoa, I would also have bet on the sponge-sister scenario.
