The acquisition of myelin: An evolutionary perspective

Area of Science:

• Evolutionary Biology
• Neuroscience
• Paleontology

Background:

• Vertebrate evolution is linked to neural crest cells and complex head structures.
• The peripheral myelin sheath, a neural crest derivative, is vital for vertebrate existence.
• Understanding myelin's evolutionary origins addresses key questions about its appearance and cellular basis.

Purpose of the Study:

• To determine when myelin first appeared in vertebrate evolution.
• To ascertain whether myelin originated in the central nervous system (CNS) or peripheral nervous system (PNS).
• To investigate if a novel cell type was required for myelin formation.

Main Methods:

• Analysis of fossil records to date the emergence of myelin.
• Comparative examination of CNS and PNS structures in early vertebrates.
• Tracing the lineage of myelin-forming cells through evolutionary history.

Main Results:

• Myelin first appeared approximately 425 million years ago in placoderms, the earliest jawed fishes.
• Myelin acquisition is proposed to have occurred simultaneously in both the PNS and CNS.
• Myelin-forming cells are likely evolved from pre-existing invertebrate ensheathing glia, adapting membrane synthesis capabilities.

Conclusions:

• The evolution of myelin was a critical prerequisite for vertebrate success and the development of gigantism in gnathostome species.
• Myelin formation did not necessitate a new cell type but rather an adaptation of existing glial cells.
• The simultaneous appearance in CNS and PNS suggests a coordinated evolutionary event crucial for early vertebrate diversification.