It all comes down to how the different groups of animals are related to each other.
Mammals, reptiles, and birds are all amniotes, animals with a special membrane around their eggs. They share a common ancestor that lived over 300 million years ago.
Mammals and reptiles diverged from that common ancestor in two distinct lineages. The reptilian lineage ultimately yielded the dinosaurs and they, in turn, yielded the birds. (Yes, birds are dinosaurs and are therefore technically reptiles, too).
Because birds and mammals are so far apart on the amniote family tree, it seemed like a more logical conclusion that the two groups had separately evolved slow-wave and REM sleep patterns.
“It could very well have happened that birds and mammals independently invented slow-wave sleep and REM sleep,” Laurent says. “But if there is evidence that [non-avian] reptiles also have REM sleep and slow-wave sleep, then the most plausible explanation is that it actually existed in their common ancestor.”
Although birds are reptiles, the Australian bearded dragon and other lizards are likely more similar to the common ancestor of mammals and reptiles. Lizards branched off early in the evolution of reptiles, Laurent explains. In fact, he says, the common ancestor of reptiles and mammals probably looked somewhat like a lizard.
And in recreating such an ancient evolutionary history, it’s important to look at animals that are as close to the common ancestor as possible.
This study provides “extremely strong evidence that the patterns of structure of sleep that we’ve seen in a broad range of species is reflective of something that evolved very early in vertebrate evolution and is shared across many – perhaps all – vertebrates,” Daniel Margoliash, a professor of organismal biology and anatomy at the University of Chicago, told The New York Times. “It forces us to think about the earliest evolution of these phenomenon. When did these aspects of sleep start, and what were they for?”
Simple sleep cycles
Slow-wave sleep and REM sleep alternate when an animal is asleep. When Laurent and his colleagues peered into the lizards’ brains, they identified electrical signals characteristic of just such a sleep cycle.
This same cycling happens in mammalian brains, including our own. But the lizards’ sleep cycles don’t exactly match ours.
In humans, a full sleep cycle lasts for an hour to an hour and a half. But in the dragon, it takes just a minute to a minute and a half. Furthermore, slow-wave sleep is made up of multiple stages. But in the lizards, it appears to be all one stage.
That doesn’t necessarily mean anything about how these sleep patterns evolved, says Laurent. But, it could suggest that the bearded dragon’s sleep cycles are indeed more similar to ones that the common ancestor of reptiles and mammals might have experienced.
“They are simpler, which is another good correlate of being ancient,” Laurent says of the lizard’s sleep patterns. “In general, through evolution things become more complicated.”
It’s just speculation, he quickly adds, “but the idea would be that what we’re looking at here is an example of a sleep pattern which is closer to the ancestral sleep patterns.”