As much as we may try to deny it, Earth’s cycle of day
and night rules our lives.
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 newstudy 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.
A highly-magnified view of young larvae of the marine
worm Platynereis dumerilii.CreditHarald Hausen 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’ve 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.
Continue reading the main storyContinue reading the main
storyContinue reading the main story 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.
Dr. Frank Talamantes, Ph.D,
Professor of Endocrinology (Emeritus)University of California
Santa Cruz, California, 95064
Residence: 83 Sierra Crest Dr.
El Paso, Texas 79902
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