The corpus callosum as an evolutionary innovation

Area of Science:

• Neuroscience
• Evolutionary Biology
• Developmental Biology

Background:

• The corpus callosum (CC) is a key interhemispheric fiber bundle unique to eutherian (placental) mammals.
• Existing commissures like the anterior commissure (AC) and hippocampal commissure (HC) are present in nonplacental mammals.

Purpose of the Study:

• To review functional, developmental, and genetic factors influencing the evolutionary origin of the corpus callosum.
• To propose a hypothesis for the evolutionary development of the CC.

Main Methods:

• Literature review of functional, developmental, and genetic concepts related to the CC.
• Formulation of a hypothesis based on developmental mechanisms and gene expression patterns.

Main Results:

• The CC may have evolved for more effective interhemispheric integration of sensory cortices compared to AC and HC.
• The evolution of motor cortices in placental mammals might have also driven CC fixation.
• A hypothesis suggests altered gene expression patterns facilitated initial axonal crossing via HC pathways.

Conclusions:

• The evolutionary origin of the CC likely involved changes in regulatory gene expression during development.
• Early callosal axons may have utilized existing pathways (HC) to cross the midline.
• Subsequent evolution involved fasciculation and increasingly complex axonal guidance systems, leading to the modern CC.