Among eukaryotes, the four core histones show an extremely high conservation of 
their structure and form nucleosomes that compact, protect, and regulate access 
to genetic information. Nevertheless, in multicellular eukaryotes the two 
families, histone H2A and histone H3, have diversified significantly in key 
residues. We present a phylogenetic analysis across the green plant lineage that 
reveals an early diversification of the H2A family in unicellular green algae 
and remarkable expansions of H2A variants in flowering plants. We define motifs 
and domains that differentiate plant H2A proteins into distinct variant classes. 
In non-flowering land plants, we identify a new class of H2A variants and 
propose their possible role in the emergence of the H2A.W variant class in 
flowering plants.