Fig. 1: The evolutionary age of the OT signaling pathway.

The emergence of genes supporting the OT signaling pathway is outlined on a simplified phylogenetic tree tailored to human evolution, thus starting with cellular organisms and ending with modern humans. The clade name and an example of a representative extant species of the respective lineage ancestor are shown for each branch. The absolute gene counts per phylostratum (PS, 1 = oldest, 20 = newest) are given in the smaller circles at the ends of each branch. Branches with ancient genes are highlighted in white, branches with medium-aged genes in light gray and modern genes in gray. Time estimates for lineage splits (e.g., 650, 310, 60) are given in mya (“million years ago”). For example, three genes supporting the OT signaling pathway had their earliest homolog emerge in the PS 8 ancestor (~580 mya), for which the extant echinoderm purple sea urchin (S. purpuratus) is the proxy species, which suggests these three genes in the pathway emerged in the deuterostomia ancestor.