The Evolution of Sleep: 700 Million Years of Melatonin
When the sun sets, the encroaching darkness sets off a chain of molecular events spreading from our eyes to our pineal gland, which oozes a hormone called melatonin into the brain. When the melatonin latches onto neurons, it alters their electrical rhythm, nudging the brain into the realm of sleep.
At dawn, sunlight snuffs out the melatonin, forcing the brain back to its wakeful pattern again.
We fight these cycles each time we stay up late reading our smartphones, suppressing our nightly dose of melatonin and waking up grumpy the next day. We fly across continents as if we could instantly reset our inner clocks. But our melatonin-driven sleep cycle lags behind, leaving us drowsy in the middle of the day.
Scientists have long wondered how this powerful cycle got its start. A new study on melatonin hints that it evolved some 700 million years ago. The authors of the study propose that our nightly slumbers evolved from the rise and fall of our tiny oceangoing ancestors, as they swam up to the surface of the sea at twilight and then sank in a sleepy fall through the night.
To explore the evolution of sleep, scientists at the European Molecular Biology Laboratory in Germany study the activity of genes involved in making melatonin and other sleep-related molecules. Over the past few years, they have compared the activity of these genes in vertebrates like us with their activity in a distantly related invertebrate — a marine worm called Platynereis dumerilii.
The scientists studied the worms at an early stage, when they were ball-shaped 2-day-old larvae. The ocean swarms with juvenile animals like these. Many of them spend their nights near the ocean surface, feeding on algae and other bits of food. Then they spend the day at lower depths, where they can hide from predators and the sun’s ultraviolet rays.
Maria Antonietta Tosches and her colleagues examined how different genes became active in the worm larvae. They discovered that some cells on the top of the larvae make light-catching proteins — the same ones we make in our eyes to switch melatonin production on and off. These same cells also switch on genes required to produce melatonin.
The scientists wondered if the worms were using this network of melatonin genes the way we do. To find out, Dr. Tosches and her colleagues tracked the activity of the genes over 24-hour periods.
They found that the worms didn’t produce melatonin all the time. Instead, they made it only at night, just as we do.
The scientists also found that this nightly surge of melatonin allowed the worms to move up and down in the ocean each day.
The worms travel by beating tiny hairs back and forth. During the day, they rise toward the surface of the ocean. By the time they get there, the sun has gotten so faint that the worms start making melatonin.
The hormone latches onto the neurons that control the beating hairs and cause them to produce a steady rhythm of electrical bursts. The bursts override the beating, causing the hairs to freeze and the worm to sink. When dawn comes, the worms lose their melatonin and start to swim upward again.
When it comes to melatonin, humans and worms are so similar that they can both get jet lag.
“If you take larvae in daytime and put them in darkness, they stay in their own daytime behavior,” Dr. Tosches said. The melatonin-driven cycle continues to determine how they swim. “They have a clock that’s controlling this,” she said.
That the melatonin network works so similarly in worms and humans suggests that it was what arose in their common ancestor. “It could have been the first form of sleeping,” said Detlev Arendt, a co-author of the new study.
David C. Plachetzki, an evolutionary biologist at the University of New Hampshire who was not involved in the study, called it “an exciting paper — it’s a very complete story.”
Still, he added that while the similarities between worms and humans were striking, there was more work to be done to confirm an evolutionary link. It would still be necessary to find melatonin playing a similar role in other animals.
“We just have this tantalizing hypothesis,” Dr. Plachetzki said. “But it’s a great hypothesis.”
The new study offers an intriguing idea for how our vertebrate ancestors adapted the melatonin genes as they evolved a complex brain.
Originally, the scientists argue, the day-night cycle was run by all-purpose cells that could catch light and make melatonin. But then the work was spread among specialized cells. The eyes now took care of capturing light, for example, while the pineal gland made melatonin.
The new study may also help explain how sleep cuts us off from the world. When we’re awake, signals from our eyes and other senses pass through the thalamus, a gateway in the brain. Melatonin shuts the thalamus down by causing its neurons to produce a regular rhythm of bursts. “They’re busy doing their own thing, so they can’t relay information to the rest of the brain,” Dr. Tosches said.
It may be no coincidence that in worms, melatonin also produces electrical rhythms that jam the normal signals of the day. We may sink into sleep the way our ancestors sank into the depths of the ocean.