The sea squirt is an ugly creature. In its adult form it has a tubular shape that resembles a sponge or worm, and in its larval form it looks like a tadpole. Still, the sea squirt is one of our most ancient relatives. Its primitive nervous system makes it more closely related to humans than the sponges and corals it resembles. Scientists say a sea squirt tadpole approximates what an early human ancestor — the very first chordate — may have looked like some 550 million years ago. In this larval form, it has a primitive spinal cord and bundle of ganglia that act as a functional brain. This tiny brain helps it move selectively toward nutrients and away from harm. Like most oceanic creatures, juvenile sea squirts spend their time growing and exploring the sea.

Once the sea squirt grows to adulthood, it attaches itself permanently to a rock or a boat’s hull or pilings. It no longer needs to monitor the world as it did as a juvenile because the passing current provides enough nutrients for it to survive. Its life becomes purely passive. The adult sea squirt becomes the couch potato of the sea. In a surprisingly macabre twist, the sea squirt digests its own brain. Without a need to explore or find its sustenance, the creature devours its own cerebral ganglia. It’s like something out of a Stephen King book: “All work and no play make sea squirt a brain-eating zombie.”

The sea squirt is an example of a basic principle of nature: Use it or lose it. If a capability is not being used, it becomes an extravagance that is jettisoned or fades away. Either we grow and develop or we waste away.

//Stuart Brown, Christopher Vaughan, Play: How it Shapes the Brain, Opens the Imagination, and Invigorates the Soul