See also The origin of European cattle, and Neolithic Near Eastern Origin of domesticated cattle. Some introductory information about the aurochs via the BBC and Wikipedia and the Extinction Website.
Curr Biol. 2008 Feb 26;18(4):R157-8.
Mitochondrial genomes of extinct aurochs survive in domestic cattle.
Achilli A, Olivieri A, Pellecchia M, Uboldi C, Colli L, Al-Zahery N, Accetturo M, Pala M, Kashani BH, Perego UA, Battaglia V, Fornarino S, Kalamati J, Houshmand M, Negrini R, Semino O, Richards M, Macaulay V, Ferretti L, Bandelt HJ, Ajmone-Marsan P, Torroni A.
Archaeological and genetic evidence suggest that modern cattle might result from two domestication events of aurochs (Bos primigenius) in southwest Asia, which gave rise to taurine (Bos taurus) and zebuine (Bos indicus) cattle, respectively [1-3]. However, independent domestication in Africa [4,5] and East Asia  has also been postulated and ancient DNA data raise the possibility of local introgression from wild aurochs [7-9]. Here, we show by sequencing entire mitochondrial genomes from modern cattle that extinct wild aurochsen from Europe occasionally transmitted their mitochondrial DNA (mtDNA) to domesticated taurine breeds. However, the vast majority of mtDNAs belong either to haplogroup I (B. indicus) or T (B. taurus). The sequence divergence within haplogroup T is extremely low (eight-fold less than in the human mtDNA phylogeny ), indicating a narrow bottleneck in the recent evolutionary history of B. taurus. MtDNAs of haplotype T fall into subclades whose ages support a single Neolithic domestication event for B. taurus in the Near East, 9-11 thousand years ago (kya).