Smithsonian Magazine · People & society

Human Brain May Have Evolved from Two Merged Nervous Systems

New research suggests the human brain might be composed of two distinct nervous systems that combined during evolution. This theory stems from observations of embryonic development and genetic markers.

Researchers found that the forebrain and hindbrain form independently in embryonic development, indicating they may have originated from two separate neural systems that became integrated over millions of years for greater efficiency.

The forebrain handles higher cognitive functions like language and reasoning, while the hindbrain (brainstem) controls essential automatic functions such as heartbeat regulation and sleep.

Scientists have faced challenges in studying hindbrain nerve cells using human pluripotent stem cells. The new findings suggest this difficulty may be due to attempts to generate them from forebrain or midbrain progenitors.

The study observed distinct brain progenitor cells in mouse embryos, one expressing Otx2 (forming forebrain/midbrain) and another expressing Gbx2 (forming hindbrain). These cell populations package their DNA differently, suggesting independence.

This evolutionary trait of distinct progenitor cells appears conserved across species like chickens, zebrafish, and acorn worms. The timing suggests the merging of dual nervous systems occurred in the lineage leading to humans.

Using this new understanding, the researchers successfully generated working hindbrain motor neurons from human pluripotent stem cells in the lab, a significant step for studying brainstem-related conditions.

Some scientists remain unconvinced, questioning whether the distinct cell populations truly originate from separate systems or if they could arise from a single progenitor type. Others note that cell fates can sometimes be transiently exchanged.

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