An Evolutionary Perspective

An Evolutionary Perspective
In order to evaluate the threat simulation theory of dreaming (of the kind found in REM sleep), it is useful to discuss it in an evolutionary context, and consider whether dreaming meets the necessary requirements of evolution by natural selection; namely, genetic variation, inheritance, and differential fitness. As for the first condition, there is evidence that REM sleep is genetically varied between and within species. REM sleep seems to be exclusive to placental and marsupial mammals (Winson, 1993). This suggests a particular phylogeny of dreaming, and that there was some point in time in which this characteristic was acquired and further spread to evolving species. Also, the amount of REM sleep placental and marsupial animals tend to require varies in a shared manner throughout their life cycle (Siegel, 1995), pointing towards an underlying genetic control over dreaming.
Likewise, different physiological processes occurring during REM must have
undergone processes of natural selection. Consider disorders in which people physically act out their dreams, and the potential dire consequences that could result from such disorders. Those who acted out their dreams may have put themselves at great risk. As the trait of physical inhibition during dreaming varies in humans, those individuals with the trait which inhibits paralysis during REM sleep seem to have been predominately removed from the current dreaming population, indicating also that the second condition of inheritance is satisfied.
When considering the third proposition of the differential fitness of dreaming in modern humans, it is important to understand the environment in which selection was occurring. Our human ancestors faced a number of challenges posed by interactions with conspecifics within and between groups (Foley, 1989), as well as in procuring food and protecting themselves from predators (Kaplan and Hill, 1985). In this environment, the ability to most efficiently react when a real threat is apparent would obviously confer a survival advantage. Evidence from mental imagery and dream studies suggest that rehearsal in the dream is treated as a real threat and, therefore, those individuals with these imagery skills to rehearse threatening scenarios should have an improved ability to deal with threat, making them more likely to be the progenitors of offspring. Through the survival and procreation of their offspring, this ability of, and propensity towards, imagery would be differentially passed on to future generations.
If dreaming was selected for because of its adaptive function, the general content of dreams should certainly reflect this, and consist of situations that allow the rehearsal of scenarios that ultimately lead toward increased fitness. This is exactly what is seen, with studies indicating that dream content is biased toward negative elements reflecting threat, as opposed to positive elements. Data collected from over 500 dream reports by Hall and Van de Castle (1966) indicate that about 80% contained negative emotions, while only about 20% contained positive emotions. These negative dreams are also disproportionably likely to contain threatening elements such as animals and male strangers in threatening encounters. The evidence points towards the overrepresentation of threatening events in dreams, which should not occur if dream content is random. Through appropriating and learning to deal with these threats in dreams, it is proposed here that an animal could increase its overall evolutionary fitness.