Fruit flies, octopuses, birds and humans apparently did not have much in common. Some live on land, others are watery. Some fly, while others are on earth. Some are vertebrates, others have no backbone. These beings evolved separately and their common ancestors are far, far back in the evolutionary chain. But they may have one fundamental characteristic: their dream.
Almost all creatures sleep, but there is a debate about whether single-celled organisms like paramecium do. But no one really knows why. For years, researchers have been clinging to theories that sleep helps with 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 this big black box,” says Marcos Frank, a neuroscientist at Washington State University. Frank compares sleep to a mysterious organ: it is clear that it exists and is essential for the health of animals, but the exact function and mechanisms that control it are still unknown.
It is even more gratifying that some species have only one sleep state, during which their brain is relatively quiet, while others apparently experience two kinds, a quiet phase and an active state. In humans, the period in which the brain is illuminated with activity is called Rapid Eye Movement (REM) sleep. This is when we dream and when we are the hardest to wake up to.
For a long time, scientists have not observed this deeper, active phase of sleep in amphibians or reptiles. The theory, therefore, was until recently that it evolved later in history, via an ancestor shared by birds and animals. But in 2016, active sleep was recorded in lizards. Then the state is described in squid in 2019, and in March a team of scientists in Brazil published an article in iScience to identify it in octopuses. Cephalopods like these evolved centuries before the emergence of creatures that would have shared a lineage with birds and humans. “There’s no common ancestor there,” Frank says. Now scientists are asking whether this sleep state is more common than they initially realized, or that it evolved at different times in different species, how wings and flight evolved separately in insects, bats and birds, a phenomenon that converges evolution named.
Understanding what selective pressure has caused this adaptation and the conservation of the genes that encode it can help scientists understand what function dreaming serves for the central nervous system and why sleep is important at all. “What does sleep do to animals?” asks Sidarta Ribeiro, a co-author of the paper and 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. It’s more complicated than it sounds. “Imagine you were on Mars and you found an organism,” Frank says. “How would you know if it was sleeping or not?”
For mammals, scientists can implant electrodes in their brains to detect how their neurons shoot. But octopuses have a very widespread central nervous system. Instead of concentrating the control of their nervous system in one brain, they have eight ganglia in their arms that often act independently.
Instead of using an invasive method, such as committing sins to determine the octopuses of the octopuses, scientists from Ribeiro’s institute studied some of their behavioral traits. Sylvia Medeiros, a graduate student and lead author of the study, tested the thresholds of the animals’ arousal. Three of the laboratory’s four octopuses received a visual stimulus – a video of moving crabs. One received a vibrating stimulus, in the form of a light tapping on the tank. Medeiros wanted to see how quickly they respond to stimuli when they were awake. Then they tested it when it appeared to be inactive, and measured their reaction rates. Slower reactions meant they were asleep deeper.