It is now known from study of the Sun and primitive meteorites that the solar system was initially almost homogeneous in isotopic composition. Deviations from the (evolving) galactic average, locally sampled around the time that the Sun's nuclear burning began, can generally be accounted for by mass fractionation (see the article on mass-independent fractionation) plus a limited number of nuclear decay and transmutation processes. There is also evidence for injection of short-lived (now-extinct) isotopes from a nearby supernova explosion that may have triggered solar nebula collapse. Hence deviations from natural abundance on Earth are often measured in parts per thousand (per mille or ‰) because they are less than one percent (%).
An exception to this lies with the presolar grains found in primitive meteorites. These small grains condensed in the outflows of evolved ("dying") stars and escaped the mixing and homogenization processes in the interstellar medium and the solar accretion disk (also known as the solar nebula or protoplanetary disk). As stellar condensates ("stardust"), these grains carry the isotopic signatures of specific nucleosynthesis processes in which their elements were made. In these materials, deviations from "natural abundance" are sometimes measured in factors of 100.