Absolute differences in the variation between then and now are established by the "molecular clock" concept. https://www.nature.com/scitable/topicpage/the-mole

Absolute differences in the variation between then and now are established by the "molecular clock" concept. https://www.nature.com/scitable/topicpage/the-molecular-clock-and-estimating-species-divergence-41971/ https://www.nature.com/scitable/topicpage/the-molecular-cloc...