In which taxa did H1 linker histones appear in the course of evolution? Detailed comparative analysis of the histone H1 and histone H1-related sequences available to date suggests that the origin of histone H1 can be traced to bacteria. The data also reveal that the sequence corresponding to the 'winged helix' motif of the globular structural domain, a domain characteristic of all metazoan histone H1 molecules, is evolutionarily conserved and appears separately in several divergent lines of protists. Some protists, however, appear to have only a lysine-rich basic protein, which has compositional similarity to some of the histone H1-like proteins from eubacteria and to the carboxy-terminal domain of the H1 linker histones from animals and plants. No lysine-rich basic proteins have been described in archaebacteria. The data presented in this review provide the surprising conclusion that whereas DNA-condensing H1-related histones may have arisen early in evolution in eubacteria, the appearance of the sequence motif corresponding to the globular domain of metazoan H1s occurred much later in the protists, after and independently of the appearance of the chromosomal core histones in archaebacteria.