Figure 6

Proposed evolutionary progression of interpretation in the Tetrahymena ribozyme system. At or near the origins of life on the Earth 4 Ga, there were no catalytic RNAs with any interpretive ability. Selection for interpretation in a relatively calcium-rich but magnesium poor environment (Table 3) drove the advent of RNAs akin to the PV, which used Ca²⁺ as a sign for the preferred Mg²⁺ ion. As the abiotic and biotic environments evolved to sequester Mg²⁺ + ions in cells, the advantage to interpretation waned, and in contemporary cells relatively enriched in Mg²⁺ + RNAs such as the CG evolved under pressures to maximize their use of Mg²⁺.