For centuries, scientists have thought of the brain as a single, unified organ. But new research led by Stanford Medicine reveals that what we call the brain is two distinct organs that evolved independently over hundreds of millions of years.

The discovery overturns a prevailing model of brain development. For decades researchers have subscribed to the theory that there is a single progenitor cell early in development that gives rise to the entire brain. This model suggested all parts of the brain shared a common developmental origin.

The new research finding shows that the human brain consists of two ancient nervous systems cleverly packaged together — a more primitive part that regulates our hearts’ beating, our breathing and other functions, and another that makes us distinctly human, capable of poetry, mathematics and wondering about our own origins.

The discovery could help explain why scientists have struggled for decades to grow certain types of brain cells in the laboratory — and it opens new avenues for studying devastating diseases that affect the brain stem, such as spinal muscular atrophy (also known as SMA) and amyotrophic lateral sclerosis (also known as ALS or Lou Gehrig’s disease).

“We’ve shown for the first time that the front of the brain arises from a totally different progenitor cell than the back of the brain,” said Kyle Loh, PhD, associate professor of developmental biology. “Our discovery means that we can now grow neurons from the back of the brain, the hindbrain, in a petri dish and study their functions.”

Two brains
The adult brain has three main regions: the forebrain, midbrain and hindbrain. The forebrain handles higher-level thinking — language, consciousness and abstract reasoning. In contrast, the hindbrain, located at the back of the skull and often called the brain stem, controls essential, automatic functions that keep us alive: breathing, sleeping, and regulating our heartbeat and hunger urges. The hindbrain neurons also control the muscles of the face, tongue and throat, which affect speech and swallowing.

Despite the critical importance of the hindbrain, scientists have struggled for decades to generate human hindbrain neurons in the laboratory. This gap has hampered research into devastating diseases affecting the brain stem, including spinal muscular atrophy and amyotrophic lateral sclerosis...