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The origin of Ovis aries, the modern domestic sheep, is still uncertain. Mitochondrial DNA (mtDNA) from European, African and Asian sheep breeds was assayed for restriction fragment length polymorphisms (RFLP) to investigate the origins and phylogenetic relationships of domestic sheep. Two major domestic sheep lineages were apparent in the branches containing mouflon and domestic sheep. Asian and European domestic sheep lineages were found to differ by 4.43%, with the mouflon sequence diverging by 4.52% from the Asian type, but just 1.36% from the European type. This suggests that some modern domestic sheep and European mouflon have a common ancestor.

The complete mitochondrial DNA (mtDNA) molecule of the domestic sheep, Ovis aries, was sequenced, together with part of the mtDNA of a specimen representing the other major O. aries haplotype group. The length of the complete ovine mtDNA presented is 16,616 nucleotides (nt). This length is not absolute, however, due to heteroplasmy caused by the occurrence of different numbers of a 75-nt-long tandem repeat in the control region. The sequence data were included in analyses of intraspecific ovine molecular differences, molecular comparisons with bovine mtDNAs, and phylogenetic analyses based on complete mtDNAs. The comparisons with bovine mtDNAs were based on the central domains of the ovine control regions, representing both major ovine haplotype groups, and the corresponding domains of Bos taurus and B. indicus. The comparisons showed that the difference between the bovids was 1.4 times greater than the intraspecific ovine difference. These findings suggest that the strains of wild sheep from which domestic sheep originated were more closely related than were the B. primigenius subspecies which gave rise to B. indicus and B. taurus cattle. Datings based on complete mtDNAs suggest that the bovine and ovine lineages diverged about 30 million years before present. This dating is considerably earlier than that proposed previously.