system
Given the following post, respond as if you're commenting on Reddit:
human
Do all brain-owning animals descend from a common evolutionary ancestor? Or are there animals that developed brains "independently" of other species?

Forewarning: My science-fu isn't fantastic, so excuse the somewhat informal, "dude, science is awesome" language.

Brains are amazing things. It blows my mind (heh) to even think about animals that don't possess them (like jellyfish). Recently, I got to wondering ... do all animals that have brains have them because we all descended from some common species that first possessed a brain? Or were brains developed independently on many separate evolutionary branches?
gpt
Nearly all metazoans (animals) have nerves (that is, cells with a synaptic junction that can transmit signals from cell to cell via an action potential), with the exception of sponges, an early outgroup on the metazoan tree. So, yes, all extant animals, including all bilaterians (animals with bilateral symmetry, like us), owe their nervous systems to a common evolutionary ancestor.

All bilaterians have some kind of brain, even if it is just a cluster of nerve cells (ganglion). From this primitive start, more complex derivatives evolved. Notably, cephalopods (octopus, squid, etc.) and vertebrates have highly developed brains with fairly different architectures that evolved independently.