Evolution of Myelinated Nervous Systems

It is probably not surprising that the myelinated nervous system and jaws are two characters that originated in the common gnathostome ancestors. For myelination would both allow rapid wielding of powerful jaws and rapid swimming responses as a means to escape. Clearly the structural features of oligodendrocyte (OL)-based central nervous system (CNS) myelin and Schwann cell-based peripheral nervous system (PNS) originated in the common gnathostome ancestor, as, without exception, among species examined, OLs myelinate all large-caliber CNS axons and Schwann cells myelinate all large-caliber PNS axons. In spite of the fact that most of our current understanding of myelination comes from studies with common laboratory animals, mainly rodents, and that many current techniques are designed for working with these animals, a substantial and growing literature exists on myelination in nonmammalian vertebrate species and, to a lesser extent, myelination in invertebrates. The need for more diversification has recently been stressed, and investigators who consider their research from an evolutionary perspective, e.g., use GenBank data to suggest evolutionary features of molecular interactions, as was done for ion channel–ankyrin interactions, will give their research wider scope.