For the first time, researchers have found the same types of brain activity that humans, other mammals and birds experience while sleeping in a species of reptile, according to a new study published online Friday in the in the weekly, peer-reviewed journal Science.
As reported in the newly-published paper, Gilles Laurent from the Max Planck Institute for Brain Research in Frankfurt, Germany and his colleagues observed the Australian bearded dragon (the Pogona vitticeps) and found evidence of both rapid-eye movement and slow-wave sleep patterns.
The discovery suggests that the brainstem circuits responsible for slow-wave and REM sleep are far older than scientists had previously realized, dating at least as far back as the evolution of the amniotes and the earliest days of the colonization of terrestrial landmasses by vertebrates.
Sleep mechanics could date back to the emergence of amenities
Amnitoes are a clade of tetrapod vertebrates that lay eggs capable of surviving outside of water, and this group contains reptiles as well as birds and mammals, Laurent and his fellow researchers explained Thursday in a statement. They first appeared approximately 320 million years ago and split into two groups, one that gave rise to mammals and another leading to birds and reptiles.
Bearded dragons emerged from the reptilian branch roughly 250 million years ago, much earlier than either the dinosaurs and the birds. If a behavior was observed in a lizard, bird and mammal, it likely would have existed in their common ancestor, so the team decided to study the brain of the Pogona vitticeps due to its simple design and similarities to more ancient creatures.
While attempting to learn more about the reptile’s cortical function, dynamics and computation, Laurent’s team monitored the brain activity of the creature while in it was in a resting state, and found that this activity shifted between two different states, which were later found to be patterns of REM and slow-wave sleep which oscillated continuously for between 6-10 hours, but which only lasted approximately 80 seconds per phase (versus 60-90 minutes in humans).
Mechanics may be different, but the results are the same
Like in mammals, the bearded dragon experienced a phase characterized by low frequency/high amplitude average brain activity and rare and bursty neuronal firing (slow-wave sleep), and one marked by awake-like brain activity and rapid eye movements (REM sleep), the authors said.
Furthermore, slow-wave sleep was found to be coordinated by the cortex and another part of the brain, although in reptiles it was the dorsal ventricular ridge, while in mammals, the region is the hippocampus. Based on their findings, the researchers believe that these mechanics originated in a common ancestor when the amniotes first emerged rather than a separate, convergent evolution of such sleep patterns in birds, mammals and reptiles.
“Positing convergent evolution (two or three times in amniote evolution) of a complex phenomenon such as sleep brain dynamics is a lot less plausible than imagining a common origin,” Laurent explained in a statement. “Given the early branching out of the reptiles, additional evidence from several of reptilian branches such as turtles, lizards, or crocodiles will only increase the probability that we are looking at a common origin.”
“The evidence, thus far, points to an origin of REM and slow-wave sleep at least as far back as the common ancestor of reptiles, birds and mammals, which lived about 320 million years ago,” he said, adding that he and his colleagues plan to continue analyzing this type of brain activity during both resting and awake states so that they can better understand the similarities of brain function in different types of vertebrates.
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