Over thousands of years of eating and evolving, humans have developed the ability to learn to avoid the foods that make them sick.

This capacity, to know better than to eat another piece of that cheese that kept you awake last night with a stomach ache, is not a uniquely human trait though. In fact, according to a recent Harvard study, even one-millimeter-long worms can also develop food aversions.

Assistant Professor of Biology Yun Zhang and her team of researchers have studied the neural network of C. elegans, a small transparent roundworm, to understand how organisms use their sense of smell to learn to avoid dangers.

“The central question we’re seeking to answer is,” Zhang said, “How does the nervous system process information and adjust so that behavior can be changed?”

The team mapped the roundworms’ nervous systems, and then observed the organisms’ neural pathways while they were in action.

The research, which was featured in the December 22 issue of Neuron magazine, showed the neural circuitry by which the roundworms process information in the environment—say, the smell of a harmful bacteria—and learn to change their behavior and move away.

“They remember the smell that makes them sick,” Zhang said.

Two different neural circuits are at play here, said Zhang. The first circuit is active when the roundworms experience the pathogenic bacteria for the first time.

After they get sick, Zhang said, another circuit kicks in, effectively regulating the roundworm’s motor response so that it doesn’t go back to the harmful bacteria.

“We look at their behavior to understand the nervous system on the scale of individual neurons,” Zhang said.

While she said she hopes that this research will help illuminate the mechanisms behind human learning, Zhang stressed that the human nervous system is vastly more complex and difficult to disentangle than that of the roundworm.

The C. elegans have only around 300 neurons and are, to date, the only organism for which we have a complete connectome—a comprehensive map of an organism’s nervous system.

This feature, as well as the organism’s rapid growth rate and its transparent body, makes the C. elegans an attractive subject of study for researchers like Zhang.

Compare this simplicity to the overwhelming number of connections within a human brain. With near one hundred billion neurons and some seven hundred trillion synaptic connections, the prospect of a fully mapped human nervous system is still a while away.

“We’re still at the very beginning stages of understanding how the human nervous system is organized,” Zhang said.