The evolution of land plant hemoglobins

► Land plant non-symbiotic hemoglobins originated from a precursor shared with green algae. ► Plant 2/2 truncated hemoglobins originated and vertically evolved from a bacterial 2/2 hemoglobin. ► Hemoglobins were highly variable during the colonization of land by plants. ► Diversification of angiosperms is a likely date for the emergence of non-symbiotic hemoglobins type 1 and 2. ► Symbiotic hemoglobins originated from non-symbiotic hemoglobins and spread to function in nodulating flowering plants.