Did Human Brains Evolve from Neurons in Our Colon?

The researchers studied brain cells in the colon of lab mice. A scientists holds a lab mouse not affiliated with this project. Peter Parks/AFP/Getty

We have brain cells in a very unexpected place in our bodies, our digestive tract. Known as the enteric nervous system, these neurons are commonly referred to as our 'second brain', but a new study from Australia suggests these neurons may actually be the very 'first brain' that our mammalian ancestors evolved.

For the new study, published online in the Journal of Neuroscience in late May, the researchers detail their discovery of the millions of neurons in the gastrointestinal system of mice.

The research team, led by Nick J. Spencer, a neurophysiology professor at Flinders University, conducted a test that measured the electrical activity of the heart to record activity in the small intestine of lab mice. The tests, which used neuroimaging in combination with electrophysiology, revealed that neurons in the intestines fired at the same time in repetitive bursts to help activate muscle cells. This behavior created the muscle activity necessary to move feces through the mice's colons and eventually out of their body.

"It is really a brain of its own," Spencer told ScienceAlert. "The unique feature of the GI tract is that it is the only internal organ with its own complete nervous system that can operate totally independently of the brain and/or spinal cord."

The cells are part of the enteric nervous system, which operates independently of the brain and the spinal cord. The enteric nervous system is commonly referred to as the "second brain," and has been previously noted in past research. However, in the new study researchers present a pattern of rhythmic and coordinated neural firings of the enteric nervous system that have not been previously noted. In addition, the team suggests 'second brain' is a misnomer and the system was actually the 'first brain' and evolved before the central nervous system.

The enteric nervous system remains much of a mystery to scientists and doctors. It has as many neurons as the spinal cord, and is involved in digestion from the esophagus to final expulsion, Colorado State University reported. Although the system acts independently from other nervous systems, scientists note that problems with the enteric nervous system, may also play a role in neurological disorders such as autism spectrum disorder, amyotrophic lateral sclerosis, Parkinson disease and Alzheimer disease. For example, animal research has shown an increase in gastrointestinal problems among individuals with these conditions, but the true nature of the link remains unclear, a 2016 study reported.

The team plans to use this finding to further explore the connection between the 'second brain' and human disease.

"Now that we know how the ENS is activated under healthy conditions... we can use this as a blueprint to understand how dysfunctional neurogenic motor patterns may arise along the colon," Spencer told Science Alert.