Figure 2. Evolution of SRs including GR. A: Appearance of the SR member receptors through evolution of the chordate lineage. The first ancestral SR, which is close to the current ER, appeared ~540 million years ago. At lamprey, 3 receptors, ER, PR and CR, emerged. From the ray-finned fishes, all SR members, ER, PR, AR, GR and MR, appeared. Modified from (14). B: Phylogeny of the SR family genes. Current human SRs including GR were generated through several gene duplications (shown as orange squares). Appearance of the ancestral (Anc) SR1, SR2 and CR are shown with arrows in the phylogeny tree. Blue lines indicate the lamprey-gnathostome divergence. Modified from (10).