Fruit flies, octopuses, birds and humans don’t seem to have much in common. Some live on land, some are in the water. Some fly, while others are earth-bound. Some are vertebrates, some have no backbones. These beings evolved separately and their common ancestors are far, far back in the evolutionary chain. But they can have one fundamental characteristic in common: they dream.
Almost all creatures are asleep, although there is some debate as to whether unicellular organisms such as paramecium do. But nobody really knows whyFor years, researchers have rung about theories that sleep aids in memory, growth, and learning – and it is clear that humans need sleep to function properly – but there is little else that is well understood. “Sleep is a big black box,” said Marcos Frank, a neuroscientist at Washington State University. Frank likens sleep to a mysterious organ: it is clear that it exists and is vital to animal health, but its exact function and the mechanisms that govern it are still unknown.
Even more puzzling, some species seem to have only one sleep state, in which their brains are relatively quiet, while other species seem to experience two types, a calm phase and an active state. In humans, the period when the brain lights up with activity is called Rapid Eye Movement (REM) sleep. It’s when we’re dreaming and when we’re the hardest to wake up.
Scientists had not observed this deeper, active sleep phase in amphibians or reptiles for a long time. So until recently, the theory was that it evolved later in history, through an ancestor shared by birds and animals. But in 2016, active sleep was recorded in lizards. In 2019, the state was described in squids, and in March, a team of scientists in Brazil published a paper in iScience identifying in octopuses. Cephalopods like this one evolved eons before the emergence of creatures that would have shared ancestry with both birds and humans. “There is no common ancestor there,” says Frank. Now scientists are wondering if this dormancy is more common than they initially thought, or if it evolved at different times in different species, the way wings and flight evolved separately in insects, bats, and birds, a phenomenon called convergent evolution.
Understanding the selective pressure that caused this adaptation and the preservation of the genes that code for it could help scientists understand what function dreams serve for the central nervous system and why sleep is important at all. “What does sleep do for animals?” asks Sidarta Ribeiro, a co-author on paper and the director of the Brain Institute at the Federal University of Rio Grande do Norte.
The first step in studying how animals sleep is to find out when they are actually sleeping. This is more complicated than it sounds. “Imagine you were on Mars and you found an organism,” says Frank. “How would you know if he was asleep or not?”
For mammals, scientists can implant electrodes in their brains to track how their neurons fire. But octopuses have a highly divided central nervous system. Rather than concentrating control of their nervous system in one brain, they have eight ganglia in their arms that often work independently.
Rather than using an invasive method, such as attaching probes to determine the sleeping state of the octopuses, scientists at Ribeiro’s Institute studied some of their behavioral characteristics. Sylvia Medeiros, a graduate student and the lead author of the study, tested the animals’ arousal thresholds. Three of the lab’s four octopuses were given a visual stimulus – a video of moving crabs. One received a vibrating stimulus, in the form of a light tap on the tank. Medeiros wanted to see how quickly they responded to stimuli when they were awake. Then she tested them when they appeared to be inactive and measured their response rates. Slower responses meant they slept more deeply.