TL;DR
Comb jellies may be the earliest branch of the animal tree, and if so, nervous systems and complex bodies evolved more than once.
Textbooks have a tidy rule: sponges branched off first, because sponges are the simplest animals, with no nerves and no muscles. For a decade now, researchers have been prying at it.
Simple does not mean ancient
Comb jellies look like glowing gelatin, and they are not related to jellyfish despite the name. They swim, they hunt, and they carry a simple nervous structure.
In 2023 a paper analysing how chromosomes fold concluded that comb jellies, not sponges, branched off first. The argument is not settled, but if it holds, the premise that early animals must have been simple falls apart: the first to split may have been the more complex one.
Nervous systems may have been invented twice
The comb jelly nerve net has an odd feature: its neurons are connected by continuous cytoplasm, with none of the synaptic gaps every other animal has. The researcher behind the finding does not think any other animal works this way.
If they really sit at the base of the animal tree, evolution built a nervous system twice: once for comb jellies, once for jellyfish and everything else.
Other surprises keep turning up. Starved or injured, adults can revert to the larval stage. Deep-sea species carry a special lipid in their cell membranes that withstands kilometres of water pressure.
In one line: it looks like jelly, and it is quietly answering what the first animals looked like.
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