Origin of the isotopic dichotomy. Left: Scheme of the accretion of material of changing isotopic composition in the inner part of the disk, as the disk is radially spreading, from Nanne et al. (2019). This established an isotopic gradient throughout the disk which is then turned into a dichotomy by the formation of early planetesimals at two distinct sites (Morbidelli et al. 2022) and the establishment of a barrier against radial drift of dust, here symbolized by Jupiter (see Sect. 4.2). Right: Isotopic composition in Δ⁹⁵Mo of CAIs (Brennecka et al. 2020), showing that the shift from the original isotopic composition toward the NC composition started to occur while condensation of calcium-aluminum inclusions was still ongoing.