Dreaming doesn’t have an evolutionary purpose per se. It’s just an emergent property of the piecemeal way our brains have evolved, from the older and more automatic systems out to the newer, learning-enabled systems. I’ve seen it suggested by several different scientists that most animals go about their waking lives in a state similar to the one we experience in dreams: centered in the present, with little notion of past or future, just strong sensations and automatic reactions to those sensations. The theory, then, is that our lizard brains take over in dreams, but instead of experiencing the real world, they explore the memories accumulated in the neocortex.
Carl Sagan sets out the idea in The Dragons Of Eden.
Human intelligence is fundamentally indebted to the millions of years our ancestors spent aloft in the trees. And after we returned to the savannahs and abandoned the trees, did we long for those great graceful leaps and ecstatic moments of weightlessness in the shafts of sunlight of the forest roof? Is the startle reflex of human infants today to prevent falling from the treetops? Are our nighttime dreams of flying and our daytime passion for flight, as exemplified in the lives of Leonardo da Vinci or Konstantin Tsiolkovskii, nostalgic reminiscences of those days gone by in the branches of the high forest? Could the pervasive dreams and common fears of “monsters,” which children develop shortly after they are able to talk, be evolutionary vestiges of quite adaptive baboon-like responses to dragons and owls?
Here Sagan doesn’t mean dragons in the fantasy sense; he means giant lizards and other hominid predators.
The seeming fact that mammals and birds both dream while their common ancestor, the reptiles, do not is surely noteworthy. Major evolution beyond the reptiles has been accompanied by and perhaps requires dreams. The electrically distinctive sleep of birds is episodic and brief. If they dream, they dream for only about a second at a time. But birds are, in an evolutionary sense, much closer to reptiles than mammals are. If we knew only about mammals, the argument would be more shaky; but when both major taxonomic groups that have evolved from the reptiles find themselves compelled to dream, we must take the coincidence seriously. Why should an animal that has evolved from a reptile have to dream while other animals do not? Could it be because the reptilian brain is still present and functioning?
In his book Beethoven’s Anvil, William Benzon goes deeper.
Jaak Panksepp, a specialist in the neuroscience of emotion, observes that while dream sleep is almost universal among mammals, it is lacking in fish and reptiles and only sporadic in birds. Suggesting that dream sleep is unlikely to have evolved from nothing, Panksepp speculations that the relevant core brain mechanisms “originally controlled a prmitive form of waking arousal. With the evolution higher brain areas, a newer and more efficient waking mechanism may have been needed.” Among these higher-brain areas, of course, is the neocortex, the chief repository of learning.
Panksepp goes on to speculate that dream mechanisms “originally mediated the selective arousal of emotionality. Prior to the emergence of complex cognitive strategies, animals may have generated most of their behavior from primary-process pscho-behavioral routines that we now recognize as the the primitive emotional systems… These simple-minded behavioral solutions were eventually supersseded by more sophisticated cognitive approaches that required not only more neocortex but also new arousal mechanisms to sustain efficient waking functions within those emergine brain areas.”
Dreaming is thus a kind of neural palimpsest: the evolutionary vestige of the system that once regulated the waking state but has since been overwritten. Panksepp concludes by suggesting that dreaming now serves to integrate the emotional impulses of old brain systems with the cognitive capacities of the new brain systems.
I would like to recast Panksepp’s speculation ins a more colorful way: when we dream, the ancient animals within go out romping in the neocortex. The core brain systems of our reptilian heritage treat the neocortex as an environment and set out to explore it. Imagine the neocortex as some lush jungle setting or peraps a grassy savanna, in which a small animal may bask in the sun, pursue prey, and enjoy a warm meal. To put it another way, dreaming is your inner lizard running free in the dance hall of the mind.
Benzon conjectures that music, at its best, is a kind of waking dream, a speculation I find highly plausible