How Our Outside World Turned Inward

The nervous system that sprouts from the brain may seem like an incomprehensible tangle. But anatomists can divide it pretty cleanly into two parts. One part is directed to the outside world, while the other is turned inward.

The somatic nerves take in sensory information from the outside world from our eyes, nose, ears, and skin. They also relay commands to move muscles. They are essential for our responding to the external world. Visceral nerves, on the other hand, detect information about our internal state. They sense blood pressure, the queasiness in our guts, even the level of oxygen in our bodies. And they also send signals to those organs, causing racing hearts, gasping lungs, and puking stomachs.

Recently, Marc Nomaksteinsky of Institut de Biologie de l’École Normale Supérieure in Paris and his colleagues explored the evolution of this divide. They discovered evidence suggesting that it’s a profoundly ancient one.

Tracking the evolution our nervous system is especially hard. Teeth and bones leave behind sturdy fossils that contain clues to how earlier forms gave rise to later ones–a fish’s fin becoming a foot, for example. Neurons dissolve away after death. The brain, with the consistency of custard, cannot withstand the elements and leaves behind a hollow cavity. That cavity can tell scientists about the size and shape of a brain, but not much about the function of the brain within. The holes through which neurons pass through the skull and other bones offer the skimpiest of hints about what signals they relayed in life.

Scientists can add to those skimpy hints from fossils by comparing living animals. Humans and other living primates share certain features in their brains not found in other animals. The emergence of our primate ancestors 60 million years ago was marked by a massive expansion of the visual cortex, for example.

Nomaksteinsky and his colleagues used a different method to explore the evolution of our two nervous systems. The somatic and visceral nerves in our bodies have distinctive molecular profiles. Each type makes its own combination of proteins to carry out its own particular task. Almost all the visceral nerves, for example, make a protein called Phox2b. The somatic nerves that relay sensory information, on the other hand, all make a protein called Brn3.

The scientists wondered if they could find neurons with these molecular profiles in distantly related animals. They chose to look at a snail and a related species called Aplysia. Both species are mollusks, which sit on a branch of the animal evolutionary tree far from our own. The common ancestor of mollusks and us lived about 600 million years ago, at an early stage in animal evolution.

Nomaksteinsky and his colleagues found versions of Phox2b, Brn3, and other markers of somatic and visceral nerves in the mollusks. What’s more, they found the two kinds of markers in two distinct sets of neurons. This is pretty remarkable when you consider how different our nervous systems are. We humans and other vertebrates have one big brain in our head, out of which sprouts a system of neurons. Molluscs have a cluster of neurons in their head, but they also have clusters in other parts of their body, all connected in what looks like a complex snarl.

But when you consider what the two kinds of neurons do in mollusks, some similarities emerge. Some of mollusk neurons with a “somatic” profile are sensitive to touch and pain–just like some of our own somatic neurons are. Some of mollusk neurons with a “visceral” profile control a siphon they use to suck in water in order to filter food. That’s the sort of function our own visceral nerves carry out with our lungs and digestive system.

These results suggest that a snail’s nervous system is split between the outer and inner worlds much like ours is. The molecular profile of their neurons suggests the split didn’t evolve indepedently, once in molluscs and vertebrates. It arose instead in our common ancestor–a small, worm-shaped creature crawling on the ocean floor.

In a commentary on the paper, Paola Bertucci and Detlev Arendt of European Molecular Biology Laboratory speculate on how these two parts of the nervous system may have arisen. In us, the visceral system senses the inner chemistry of our bodies. But for an ocean worm 600 million years ago, this kind of information was important to sense in its external environment, too–the pH of the sea water, its saltiness, its oxygen levels, and so on. Perhaps the entire nervous system started pointing outward. Only later did it evolve to tell us something about our inner world.