How Dreaming Changed Human Evolution
Two dangers and three possible benefits of sleep for human evolution
Posted Oct 14, 2013
In our last blog on dreaming and evolution, we made the argument that more meat in the diet of australopithecines and better sleep ultimately helped them transform into the genus Homo. Regardless, of whether a primate (human or nonhuman) sleeps in a nest, in a tree, or on the ground, there are at least two properties of sleep that make it dangerous. First, slow-wave sleep (also known as stage 4 sleep and less frequently known as delta-wave sleep) is a deep, heavy sleep with a nearly complete absence of any reports dreams or thoughts from sleeping humans. Slow-wave sleep is actually a very old behavior, and it has been a characteristic of mammals, birds, and reptiles, going back at least hundreds of millions of years. In modern humans, it begins soon after the onset of a sleep period, and it is completed usually within the first couple of hours of sleep. Modern humans get about an hour of slow-wave sleep in an 8-hour sleep period, and when suddenly awakened from it, they usually deny dreams and or any thoughts whatsoever. Often they not only have no clue what time it is but sometimes report that it takes them a short time (seconds) to remember where they are or even who they are. They are also almost completely oblivious to noises in their environment, and this makes them particularly vulnerable to dangers like predation. Nonetheless, slow-wave sleep has probably evolved for some set of reasons, so humans appear to need it. For example, when humans are sleep-deprived and then allowed to sleep, they begin slow-wave sleep before they start dreaming (a.k.a. REM sleep).
The second dangerous characteristic of most primate’s sleep is dream sleep (rapid eye movement sleep or REM sleep). There are certainly other stages of sleep where humans report images, thoughts, or ideas. However it is only during a very light stage of sleep (Stage 1) when one's eyeballs are rapidly moving periodically that people report vividly active dreams, and they often recall flying, running, fleeing, driving, and a plethora of other dream activities. But here is where it gets really interesting. We are virtually paralyzed during REM sleep! This muscle paralysis is also called muscle atonia. It is apparently an evolved trait that protects our muscles from acting out the visions in our dreams. We have hypothesized that as hominins began making the full transition from sleeping in trees to sleeping on the ground, a suite of changes began. More meat in their diets allowed the fueling of bigger brains, as brain size was no longer constrained by vegetarian diets, and the high protein content in meat could serve as the fuel necessary for bigger brains. Meat-eating, perhaps, also changed the nature of tool use from simple use of stones to crack nuts and cracking bones for marrow to stone tools as weapons for defense and for predation. Ancient human types could go from being prey to being predators!
As we previously blogged, early human types undoubtedly slept better on the ground with the protection of many others and the protection of a shelter, rock overhang, or cave. We (Coolidge and Wynn) think that the liabilities of slow-wave sleep (being ‘dead’ to the world) and being paralyzed in REM sleep no longer had the deadly consequences associated with sleeping in a nest high in a tree. Specifically, we believe that the natural selection against REM sleep was reversed, and now sleeping on the ground under the protection of others and protection from bad weather allowed for greater amounts of REM sleep and probably a more integrated, single sleep period. Comparative studies of extant human and nonhuman primates supports this REM speculation. Humans’ percentage of REM sleep is about 25% of their total sleep. Chimpanzees (our genetically closest ancestor) have about 20% REM, and monkeys (a more distant ancestor) have from about 5% to 15% REM sleep.
So what are the evolutionary advantages of more REM sleep and the integrity of a single, extended sleep period on the ground? We published an article in 2006 where we hypothesized that there might be at least three prominent advantages: dreams might help prime us to be more alert for particular dangers when we are subsequently awake (threat rehearsal), dreams (and slow-wave sleep) might help consolidate and enhance the memory for visual-motor skills (like navigating in larger territories or stone knapping) and perhaps aid the processing of emotional thoughts and memories, and sometimes dreams are creative and give us innovative solutions to problems that might not come to us when we are awake (email us if you wish a copy). Also, interestingly, beginning at about 1.8 million years ago, archaeologists found a new type of stone tool, the bifacial, symmetrical handaxe, often with an oval or teardrop shape, with a cutting edge all around the circumference. It was no longer a simple sharp flake knocked off from a larger stone. Its makers may not have had a mental template for this beautiful stone tool, but they certainly intentionally shaped it with great skill and care.