On the origin and evolution of the tripartite brain

Area of Science:

• Evolutionary biology
• Developmental genetics
• Neuroscience

Background:

• The diverse nervous systems in bilaterally symmetric animals suggest multiple origins for the tripartite brain.
• Conserved molecular mechanisms orchestrate tripartite brain development across various taxa, including arthropods, annelids, urochordates, and vertebrates.

Purpose of the Study:

• To investigate the evolutionary origin of the tripartite brain and central nervous system (CNS) in Bilateria.
• To evaluate the evidence for a monophyletic origin versus multiple independent evolutions of the tripartite brain.

Main Methods:

• Comparative developmental genetics across diverse animal groups.
• Analysis of structural homology, character identity networks, and functional equivalence of genes.
• Estimation of secondary brain loss based on metazoan divergence times and molecular clock data.

Main Results:

• Three lines of evidence support a monophyletic origin for the tripartite brain and potentially an elongated CNS.
• The findings imply that the brain has been secondarily reduced and lost multiple times in evolutionary history.
• Brainless bilaterians may have had hundreds of millions of years for secondary modification and loss of an ancestral brain and CNS.

Conclusions:

• The tripartite brain likely originated from a single common ancestor (monophyly).
• Secondary loss of the brain is a plausible evolutionary event, explaining the existence of extant brainless bilaterians.
• Future research should test ancestral genes from brainless bilaterians for their role in brain development.