Evolution of ionic channels of biological membranes

This paper presents a view of the evolution and phylogenetic distribution of ionic channels of biological membranes. The view is based on the assumptions that ionic channels (1) appeared very early in the history of life, (2) have evolved from a common ancestor, and (3) have been subjected to evolutionary pressure to reach precision and high speed of signaling. We propose that Ca2+ was the intracellular messenger and modulator of the most primitive biological systems, which implies that the first channel to appear may have been a calcium channel. Then, very soon the entire group of potassium channels evolved from the calcium channel to improve the shape of signals and to restore initial conditions. Sodium channels probably appeared relatively late, diversifying from calcium channels in the early metazoan groups. Mainly because Na+ ions do not interfere with cellular metabolism (thus allowing the inward current--and, consequently, the speed of conduction--to be greatly increased), sodium channels probably proved advantageous in the generation of the action potential, and selection replaced calcium channels with sodium channels in this function. Finally, with the acquisition of multicellularity, channels responsible for synaptic transmission appeared. The case of the acetylcholine receptor channel is briefly discussed.