The young sea squirt swims the oceans. When it finds a comfortable rock to settle down on it attaches to it by the head and proceeds to digest its own brain – a brain which would be of no further use during an uneventful future of filter-feeding.
Sea Squirts, aka tunicates, also aka urochordata, are more than just a curio from marine biology. Sea squirts are more closely related to humans than any other invertebrate group – evolutionary biologists reckon that they resemble the ancient last common ancestors of all vertebrates.
The brains they have in the larval form are really just a rod of nerve cells, a notochord. But it’s this notochord, found in its most primitive form in the sea squirts, which defines the phylum to which all birds, fishes and mammals belong. We humans could, ultimately, be just a development on the larval form of these slimy plankton eaters.
Nicol suggested to me that this means there might be a genetic switch which could still be flipped in humans, and would give us a strong urge to press our heads to the nearest rock face, digest our brains and move no more.
I think maybe it’s already happened, except that the switch is memetic, not genetic. The rock is a sofa and the digestive juices responsible for atrophying our brains are the emissions from the TV.
An additional curious note about tunicates is that they use a rare metal, vanadium, to bind oxygen in their blood, rather than iron (like humans) or copper (like squid). What this means for the sofa/TV/brain digestion analogy I don’t know.