Eerie Lampreys Hint at the Origins of Our ‘Fight-or-Flight’ Response and Sympathetic Nervous System

The jawless, parasitic fish largely haven’t changed over the last 340 million years, but they might be better sources for studying our own evolution than thought, a recent study suggests

a lamprey showing its round mouth with small, pointed teeth in circles
The sympathetic nervous system was thought to have evolved with jawed vertebrates. But lampreys—jawless, parasitic fish that suck out the blood of their hosts—have a simple one, per recent research. Megan Martik / Caltech

The sympathetic nervous system plays a pivotal role in our bodies’ automatic functioning by regulating heart rate, blood pressure, digestion, sweating and other crucial processes. It also controls our fight-or-flight response, the physiological reaction to dangerous or stressful situations.

Researchers have long thought that the sympathetic nervous system arose with the evolution of jawed vertebrates. But a recent study of sea lampreys—eel-like, primitive, blood-sucking fish—reveals that they too have sympathetic neurons. The findings suggest even the earliest vertebrates may have had a rudimentary version of this nervous system, shedding light on the origins of our own, researchers reported last month in the journal Nature.

“The conclusions are textbook-changing level,” Daniel Meulemans Medeiros, an evolutionary biologist at the University of Colorado Boulder who did not contribute to the findings but has worked with the researchers before, tells Science News’ Claudia López Lloreda.

“Studies like this help teach us how we were built over evolutionary time,” Jeramiah Smith, a computational biologist at the University of Kentucky who was not involved in the work, tells Live Science’s Joanna Thompson.

Vertebrates evolved around 550 million years ago, and the earliest jawed vertebrates, known as gnathostomes, appeared at least 425 million years ago. Today, more than 99 percent of all vertebrate species are gnathostomes.

But sea lampreys, which live in the northern and western Atlantic Ocean, are among today’s rare, jawless vertebrates. Their skeletons are made of cartilage, and they don’t have scales or fins. The parasitic fish latch onto a host using their suction-cup mouths lined with sharp teeth and suck the fish’s blood and bodily fluids.

Lampreys are ancient creatures—they have survived four major extinction events on Earth over the last 340 million years without major changes to their anatomy. In addition to lacking jaws, lampreys were also thought to lack sympathetic nervous systems.

Vertebrate embryos have a collection of stem cells called the neural crest that contributes to developing a variety of structures, including jaws, other skeletal parts of the head and the sensory, sympathetic and parasympathetic nervous systems. But because lampreys are jawless, scientists assumed their embryos did not have a neural crest and therefore did not develop sympathetic neurons.

When they started the study of lampreys, the scientists were looking for cells that are precursors to sympathetic neurons, and in jawed vertebrates, these develop from the neural crest. They weren’t looking for full-fledged sympathetic neurons, but unexpectedly, they found lampreys have a chain of these cells around their hearts and mid-body (trunk) region.

With RNA sequencing, they confirmed that these sympathetic nerve cells are the same type seen in other vertebrates. And using an injected dye, they found these precursor cells come from the neural crest.

The sympathetic nervous system they discovered is “very simplified compared to what it would be in mammals,” Marianne Bronner, a co-author of the study and developmental biologist at Caltech, tells Science News.

Other researchers might have previously missed this evidence, because they were looking at lampreys too early in their development, Bronner says in a statement from Caltech. While the sympathetic nervous system develops very early in many vertebrates, the study authors found that in lampreys, the cells derived from the neural crest do not fully develop into sympathetic neurons until four months after fertilization, during the larval stage.

“Over a hundred years of literature has suggested that lamprey lack a sympathetic nervous system,” Bronner says in the statement. “Surprisingly, we found that sympathetic neurons do, in fact, exist in lamprey but arise at a much later time in lamprey development than expected.”

“Now it looks like the only thing that lampreys don’t have is a jaw,” Bronner tells Live Science.

The results challenge the idea that the sympathetic nervous system evolved in jawed vertebrates, the study authors write, pushing its origins farther back than expected.

“As you look deeper, it becomes clear that the basic building blocks of these complex systems present in humans are, in fact, very old,” Shreyas Suryanarayana, a neuroscientist at Duke University who did not contribute to the findings, tells Science News.

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