The evolution of the vertebrate CXC chemokines and receptors
has not been resolved. Based on the evolution of the immune system
and central nervous system, Huising et al. proposed that primordial
CXC chemokines and receptors developed first as
regulators of neuron chemotaxis (Huising et al., 2003a) and have
identified CXCL12/CXCR4 as the possible primordial molecules.
This theory has been disputed by others because the CXCL12/
CXCR4 orthologues are not found in the primitive chordate species
such as amphioxus (Branchiostoma floridae) and sea squirt (Ciona
intestinalis) which possess a central nerve system