The following is a passage from from Play: How it Shapes the Brain, Opens the Imagination, and Invigorates the Soul by Stuart Brown. Stuart writes beautifully.
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. It’s 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 acts 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 into 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 squirts 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.
Most animals don’t go to extreme measures like the sea squirt, but the pattern remains the same. Most animals grow new nerve connections extensively only during the juvenile period. The sea squirt stops moving, and many higher animals stop playing, and the brain stops growing.
But not humans. The brain can keep developing long after we leave adolescence and play promotes that growth. We are designed to be lifelong players, built to benefit from play at any age. The human animal is shaped by evolution to be the most flexible of all the animals: as we play, we continue to change and adapt into old age. Understanding why many animals stop playing in adulthood, and why humans don’t, helps us further understand the role play has in adult life.
When we stop playing, we stop developing, and when that happens, the laws of entropy take over–things fall apart. Ultimately, we share the fate of the sea squirt and become vegetative, staying in one spot, not fully interacting with the world, more plant than animal. When we stop playing, we start dying.