Stanford-Led Study Finds the Human Brain Is Two Separately Evolved Organs

According to Stanford Medicine's description of the work, the human brain consists of two ancient nervous systems packaged together: a more primitive part that regulates the beating of the heart, breathing and other functions, and another that the announcement describes as making humans distinctly human, capable of poetry, mathematics and wondering about their own origins.

The practical problem the work addresses is a long-standing one. Scientists have struggled for decades to generate human hindbrain neurons in the laboratory, a gap that has hampered research into devastating diseases affecting the brain stem, including spinal muscular atrophy and amyotrophic lateral sclerosis.

The team's breakthrough came from studying the earliest moments of embryonic development. Study co-authors Carolyn Dundes and Rayyan Jokhai found that the hindbrain follows a separate developmental path, running in parallel to, rather than branching off from, the pathway that creates the forebrain and midbrain. That result, according to the report, explained decades of frustration in the field, because scientists had been trying to turn one type of progenitor cell into another that it is fundamentally incapable of becoming.

With that knowledge, the researchers for the first time successfully coaxed human pluripotent stem cells, a kind of cell able to create any cell in the human body, to become functional hindbrain motor neurons in the laboratory.

The researchers then looked back over roughly 550 million years of evolutionary time and found the same two-origin brain pattern in chickens, in zebrafish and, in what the report calls a remarkable result, in acorn worms, small creatures living on the ocean floor that share a distant common ancestor with humans. Jellyfish, which diverged from humans about 600 to 700 million years ago, have two nervous systems at different ends of their body.

"Our research suggests that evolution took two existing neural systems and pushed them together spatially," Loh said in the announcement. "Having the brain as one organ would probably be more efficient, but we rely on this primordial way to make the brain as two separate pieces."

The account of the research was circulated by Slashdot, which credited its reader Beeftopia with sharing the article. The details above rest on that single published account of the Stanford Medicine announcement.

Editor's Summary

A Stanford Medicine-led team concludes that the human brain comprises two nervous systems that developed along separate paths and were later brought together in one structure. The work attributes decades of difficulty in growing human hindbrain neurons in the laboratory to that split, and reports the first laboratory production of functional hindbrain motor neurons from human pluripotent stem cells. The same two-origin pattern was observed in chickens, zebrafish and acorn worms, suggesting the arrangement is very old in evolutionary terms.