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Key points
- Today, REM neurobiology functions to enhance cognitive creativity.
- Greater access to REM neurobiology may have fueled the cognitive evolution of our species.
- Widespread cultural creativity became abundant after we interacted with Neanderthals.
- We acquired traits from Neanderthals genes that increased access to REM neurobiology.
What is behavioral modernity?
"Behavioral modernity” is the phrase some evolutionary theorists use to refer to the point in time at which our ancestors developed all of the traits that make humans utterly unique relative to other animals, including our closest relatives, the primates. Most evolutionary theorists place that point at about 75,000 to 50,000 years ago.
Arizona State University archeologist Curtis Marean (Marean, 2015) argued that the social-cognitive bases for behavioral modernity that uniquely characterized our species were highly advanced cognition operating in a symbolic framework, hyper-prosocial proclivities, and a dependence on social learning. These traits or capacities can be summed up in the idea of “cumulative cultural evolution"—which simply means our ability as a species to develop shared social knowledge that builds upon itself via transmission from generation to generation.
Although all cognitive requirements for cumulative cultural evolution (highly advanced cognition operating in a symbolic framework, hyper-prosocial proclivities, and a dependence on social learning) can be identified in the archaeological record in Africa long before anatomically modern humans (AMH) dispersed out of Africa some 75,000 to 50,000 years ago, there is little evidence that that capacity really took off or became widespread among AMH populations in Africa before the dispersal out of Africa.
There is, however, plenty of evidence that cumulative cultural evolution took off among AMH populations in the upper paleolithic in the Eurasian ecologic context around 50,000 years ago. AMH populations dispersing out of Africa between 75,000 to 50,000 years ago and meeting other human populations, such as the Neanderthals, had to adapt to these new populations, new diseases, and colder climates.
Changes in REM Sleep
Recent analyses of Neanderthal DNA have demonstrated that migrating AMH peoples mated with some Neanderthals. Among the Neanderthal genes preserved in AMH populations are genes (e.g., ASB1, EXOC6) that enhanced REM sleep processes in several ways but also resulted in a higher risk for narcolepsy—a disorder wherein REM neurobiology is dramatically disinhibited, resulting in REM processes seeping into daily waking consciousness.
REM neurobiology, therefore, thrust itself upon or became available in a new way for AMH peoples of the upper paleolithic—indeed, it likely became an intrusive presence for some persons predisposed to dissociative states. Easy access to REM neurobiology also allowed better access to and employment of all of the things REM neurobiology makes available—including enhanced emotional learning and consolidation of emotional memories, enhanced neuroplasticity, enhanced creativity, enhanced linking of disparate ideas or connections, dreamy visionary states, advanced mental simulations of significant social interactions, the appearance of numinous or supernatural beings/figures, etc
Interestingly, relative to other primates, humans invest an unexpectedly higher proportion of their sleep time into REM (Nunn and Samson, 2018). In the upper paleolithic, when AMH populations entered the colder climates of Europe and encountered new human populations, the reduction in daylight hours and the colder climates meant that AMH peoples had to further adjust their sleep habits, and they likely did so by doubling down on their already unusual investment into REM. They upped their investment into REM with a little help from the new peoples they began meeting in the upper paleolithic—the Neanderthals.
Evolutionary researchers Micahel Dannemann and Janet Kelso, in a 2017 paper, reviewed Neanderthal genetic contributions to AMH populations. In addition to the well-known fact that we obtained (from Neanderthals) genes contributing to skin and hair traits, we also obtained genes that significantly altered AMH sleep patterns. One of the introgressed SNPs discussed by Dannemann and Kelso modifies the coding sequence of ASB1 (MIM: 605758; rs3191996, p.Ser37Lys; Material and Methods). Archaic alleles near ASB1 (tag aSNP: rs75804782) and EXOC6 (MIM: 609672; tag aSNP rs71550011) are associated with a preference for being an "evening person" and an increased tendency for daytime napping and narcolepsy, respectively.
As AMH populations met the challenges of reduced sunlight and colder climate in Europe, they acquired genes that helped them adapt to shorter day lengths and greater amounts of dark. Presumably, these factors also played a role in alterations in skin pigmentation and hair color. Dark skin is a liability in ecologic conditions with short daylight times.
But these adaptations to shorter daylengths had the side effect of enhancing REM, as well as increasing the risk for the disorder of REM, narcolepsy. To understand the significance of this enhanced risk for narcolepsy among upper paleolithic peoples, we will need to discuss narcolepsy a bit.
Narcolepsy and the Default Mode Network
First, please note that the collection of structures activated and de-activated during REM overlap to a significant extent with the so-called default mode network (DMN). The DMN includes posterior cingulate, precuneus, retrosplenial cortex, inferior parietal, superior temporal, hippocampal formation, and medial prefrontal cortex. Interestingly, when people sit around just doing nothing but daydreaming, this set of structures tend to get activated. It is as if REM neurobiology produces dreams when the individual is asleep and daydreams when the individual is awake.
Now, let's discuss narcolepsy—the disorder that may have become a more insistent reality for some peoples of the upper paleolithic if the gene in question (EXOC6) was operating in the same manner then as it does for modern populations: i.e., increasing the risk of narcolepsy. Narcolepsy is characterized by the irruption of REM neurobiology into daytime consciousness. Its symptoms are 1) excessive daytime sleepiness, 2) hypnogogic hallucinations, 3) “sleep attacks” or sudden paralysis (cataplexy) following a strong emotional stimulus like laughing or intense emotion, and 4) sleep paralysis or paralysis during the transition from sleeping to waking or from waking to sleeping.
In addition, people with narcolepsy very frequently experience lucid dreams, meaning that they are more often conscious of dreaming when dreaming in REM sleep than controls. They also recall far more REM dreams than do healthy controls. In short, narcoleptics appear to be more or less often, if not constantly, immersed in REM neurobiology on a daily basis and indeed cannot escape it effectively. To see the mental or psychological effects of REM neurobiology, we only need to look at narcoleptics.
Now because dreaming, especially lucid dreaming, has been associated with creativity, Lacaux et al. (2019) formally assessed links between REM neurobiology and creativity in 185 narcoleptics and 126 healthy controls. They did indeed find that narcoleptics evidence higher levels of creativity than controls as assessed in multiple ways. They were more creative on tests of past achievement and in-lab objective creativity tests that tapped imaginative, innovative, and researcher modes of creativity, as well as divergent, convergent, verbal, graphic, abstract, and concrete modes of creative thinking.
In addition, each of the classic symptoms of narcolepsy (cataplexy, sleep paralysis, hypnagogic hallucinations, lucid dreaming, and REM sleep behavior disorder) that are directly responsible for dissolving the normal boundary between wakefulness and REM sleep were found to be significantly associated with higher creativity scores. Notably, 43 percent of the 185 subjects with narcolepsy were frequent lucid dreamers compared to 3 percent of the 126 normal controls, and these lucid dreamers were among the most creative of the group.
In Summary
Ultimately, one brain network that likely contributed significantly to the rise of the capacity for cumulative cultural evolution among AMH peoples was the brain system associated with REM sleep. REM neurobiology became more accessible to AMH peoples due to the introgression of relevant genes from Neanderthal peoples. In essence, the default mode network became easily accessible or activated for AMH peoples who had mated with Neanderthals, and these were the people who fueled the creative explosion that gave rise to widespread behavioral modernity in the Eurasian upper paleolithic.
Now, please note that none of what I summarize above is settled science. This is all merely my hypothesis. The consensus among evolutionary anthropologists who know a great deal more about these matters than me repeatedly points out that AMH populations within Africa before dispersal out of Africa very likely had capacities for cumulative cultural evolution.
I certainly agree with this fact. I merely point out that evidence for widespread effects of cumulative cultural evolution among AMH peoples before they interacted with Neanderthals is meager—but after they interacted with Neanderthals, it is abundant. Therefore, something about that interaction dramatically spurred cultural creativity among AMH populations.
My candidate for that something is REM neurobiology. Cumulative cultural evolution may not have originated after that interaction between archaic humans, but it certainly jumped to a whole new level after that interaction. That jump may have been due, in my opinion, to new, more fluent access to the DMN and REM neurobiology.
