How potassium came to be the dominant biological cation: of metabolism, chemiosmosis, and cation selectivity since the beginnings of life

In the cytoplasm of practically all living cells, potassium is the major cation while sodium dominates in the media (seawater, extracellular fluids). Both prokaryotes and eukaryotes have elaborate mechanisms and spend significant energy to maintain this asymmetric K+/Na+ distribution. This essay proposes an original line of evidence to explain how bacteria selected potassium at the very beginning of the evolutionary process and why it remains essential for eukaryotes. A minor affinity for K+ over Na+ of the peptide carbonyl oxygen atom can explain the dominance of the K+ ions inside all bacteria. Eukaryotes keep K+ as a main cation of the cytoplasm because this ion is better than Na+ for DNA processing (replication and transcription) on the chromatin template.