What the octopus’s dreams tell us about the evolution of sleep
Fruit flies, octopuses, birds and humans seem to have no resemblance. Some live on land, others are aquatic. Some fly, while others fly to the ground. Some are vertebrates, others have no backbone. These creatures evolved separately and their common ancestors are very far in the evolutionary chain. But they may share one basic characteristic: they dream.
Almost all creatures sleep, although there is debate as to whether single-celled organisms such as paramezium do so. But no one really knows why. They have spent years researching theories that help them sleep memory, growth and learning — and that’s clear humans need to sleep to function well, but there is little else that is well understood. “Sleeping is this big black box,” says Marcos Frank, a neuroscientist at Washington State University. Frank equates sleep with a mysterious organ: it is clear that it exists and is essential for animal health, but its exact function and the mechanisms that control it are still unknown.
Even more surprising is that while some species appear to have a single sleep state, the brains are relatively calm, while others appear to have two types, the quiet phase and the active state. In humans, the period in which the brain is actively illuminated is called the Fast Eye Movement (REM). When we dream and we are the hardest to wake up.
For a long time, scientists have not seen this active and deeper phase of sleep in amphibians or reptiles. So until recently, in theory, it later evolved into history, through an ancestor shared by birds and animals. But 2016 was the year in which active sleep was recorded lizards. Then, in 2019, there was the statue described in squid, and this March, was published by a team of Brazilian scientists a paper iScience identifying in octopuses. Cephalopods like these evolved into eons, sharing a lineage with birds and humans before the creation of creatures. “There’s no way a common ancestor is there,” Frank says. Now scientists are wondering if this sleep state is more common than initially realized or if it develops at different times in different species, in insects, bats and birds that formed wings differently in flight, a phenomenon called convergent evolution.
Understanding the selective pressures caused by this adaptation and the conservation of the genes that encode it can help scientists dream about the role that sleep plays for the central nervous system and why sleep is so important. “What does sleep do for animals?” asks Sidarta Ribeiro, 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 when they are asleep. That’s more complicated than it sounds. “Imagine you’re on Mars and you’ve found an organism,” Frank says. “How would you know if he was asleep or not?”
In the case of mammals, scientists can implant electrodes in their brains to track how their neurons are fired. Octopuses have a highly distributed central nervous system. Instead of concentrating control of the nervous system on one brain, they have eight ganglia in their arms, which often act independently.
Instead of using an invasive method such as attaching probes to determine octopus sleep conditions, scientists at the Ribeiro Institute studied some features of their behavior. Sylvia Medeiros, a graduate student and lead author of the study, tested the excitation thresholds in animals. Three of the four octopuses in the lab were given a visual stimulus: a video of the crabs moving. One got a vibrating stimulus, knocking light on the tank. Medeiros wanted to see how quickly they responded to stimuli when they were awake. He then tested them when they appeared inactive, and measured their response rates. Slower reactions meant they were asleep.