The robust timetree could be constructed using a calibration function of BEAST v1. X released in 2018 simply by applying times of the most recent (= the latest) common ancestors (tMRCAs) for specific monophyletic species groups (clades). The present research is probably the first trial to fully use the calibration function in BEAST X. The specific node age (child tMRCA) in BEAST X = “minimum age” in conventional MCMCTree, but the “maximum age” in MCMCTree can be equivalent to, e.g., the parent node age (parent tMRCA) in BEAST X. We applied 19 mammalian fossil calibration ages considering Benton et al. (2015; solely their minimum ages), including those of fossil Gorilla and Pan + one geologic event calibration age for otters (= Quaternary isolation time of the Ryukyu islands and start of vicariant speciation), and we estimated our targeted splitting age of Homo and Pan at 5.69 Ma (calibration dates by Benton et al., 2015 were incorrect). After the initial rifting at 120 Ma, the Atlantic Ocean spread over 500 km on Chron 34 (84 Ma), and Afrotheria (Africa) and Xenarthra (South America) started vicariant speciation at this time (~ 70 Ma), reflecting the progressed continental isolation. Ordinal-level differentiations started just after the K-Pg boundary (66.0 Ma), and this timing reconfirmed that mammalian radiation occurred by rapidly filling the niches left vacant by the non-avian dinosaurs. In addition, we made a base substitution rate vs age diagram using the BEAST X function and showed that the rate exponentially increased and accelerated toward the Holocene, other than having the 55 Ma mild peak reflecting the post K-Pg mammalian explosion. The increased rate might have consequently increased the biodiversity, and extensive adaptive radiation might have ultimately birthed Homo sapiens. The basic factor of radiation might be the generation and spreading of C4 grasses since 20 Ma, which has been linked to increasing carbon fixation, decreasing atmospheric CO2 concentrations, cooling Earth, and triggering the Quaternary (2.58 Ma ~) glacier-inter glacier cycle and severe climatic change. Note that Perissodactyla and Cetartiodactyla (Laurasiatheria) feed on C4 grasses (savanna), and Carnivora (also Laurasiatheria) is the predator, also suggesting coevolutions since 20 Ma.