The sweet orange is not a wild fruit, having arisen in domestication from a cross between a non-pure mandarin orange and a hybrid pomelo that had a substantial mandarin component. Since its chloroplast DNA is that of pomelo, it was likely the hybrid pomelo, perhaps a BC1 pomelo backcross, that was the maternal parent of the first orange. Based on genomic analysis, the relative proportions of the ancestral species in the sweet orange is approximately 42% pomelo and 58% mandarin. All varieties of the sweet orange descend from this original cross, differing only by mutations selected for during agricultural propagation. Sweet oranges have a distinct origin from the bitter orange, which arose independently, perhaps in the wild, from a cross between pure mandarin and pomelo parents. The earliest mention of the sweet orange in Chinese literature dates from 314 B.C.