In addition to the need to eat, drink, and breath, humans and other animals also regularly experience the undeniable need to sleep. Sleep is now understood to serve many functions that range from helping the body’s healing process to aiding in memory and learning. But why did such a complex and whole-body experience come to exist? Are there observable intermediate steps indicating how sleep evolved? And did the phenomenon of sleep arise only once, or have different kinds of sleep independently evolved?
On the surface of the matter, sleep seems like exactly the kind of behavior that natural selection would quickly eliminate from any specie’s gene pool. Becoming unresponsive to the surrounding environment for hours at a time seems like a sure way to be devoured by lurking predators. At the very least, sleep takes away from the time available to devote to other essential activities such as searching for food, reproducing, or raising offspring.
So why did sleep evolve? There are a number of excellent hypothesizes currently circulating in the research community, all of which center around trying to identify what adaptive advantage slumber first bestowed upon life’ sleepless ancestors. Some researchers have hypothesized that sleep provides a way to conserve energy and resources during periods in which an organism would be particularly vulnerable to predators. Still others believe that sleep serves even deeper neurological functions such as helping to remove the buildup of toxins and waste products from the body, restore a homeostatic condition, and/or facilitate learning by “resetting” the connections between neurons.
A person who is interested in understanding why animals sleep might very well wonder when sleep evolved. That is to say, what was the first organism to ever get some shut-eye? It may be surprising to know that sleep is found in some of Earth’s more “primitive” or organisms. Invertebrates such as those from the well-studied fruit fly genus Drosphila have very regular circadian rhythms that include periods of activity and rest. Just like humans that miss a night of sleep, fruit flies that are deprived of their rest period will extend their next rest period seemingly to “make up” for the difference. Evidence such as this and that pertaining to other “sleeping” species of invertebrates indicates that sleep probably first arose sometime in the Paleozoic era around 500 million years ago.
In higher orders of animals more complex sleeping behaviors such as REM sleep evolved. It is known that both mammals and birds experience typical REM sleep which is characterized by darting of the eyes and dreaming. Cephalopods such as octopuses also experience increased periods of brain activity during rest, but the EEGs that they produce are strikingly different from those found in birds and mammals. Therefore many researchers hesitate to refer to cephalopods as experiencing REM sleep.
At a first glance, the presence of REM sleep in mammals and birds seemingly implies that both of these behaviors arose from a common ancestor. Both birds and mammals evolved from a reptile-like ancestral species many millions of years ago. Though species such as crocodiles and caimans have evolved modestly since that time, one might reasonably expect to find REM sleep behaviors in modern reptiles.
Some researchers believe that the spiking patterns they witness from the EEGs of sleeping caimans are indicative of a primitive version of REM sleep. This apparent intermediate step towards REM sleep in mammals and birds would confirm the hypothesis that REM sleep has arisen only once in the evolutionary history of these disparate clades.
However, others have failed to reproduce the observations of pre-REM sleep patterns in reptilian species. Such failings have led a number of experts to believe that the previous studies had mistaken EEGs from brief spurts of wakefulness to be REM-like sleep. These researchers argue that a lack of evidence of REM sleep in a common reptilian ancestor indicates that birds and humans evolved REM sleep independently.
By Sarah Takushi
Arizona State University
Handbook of Behavioral State Control: Cellular and Molecular Mechanisms
Map of Life
Rapid Eye Movement Sleep: Regulation and Function