Yes, of course, leavingwt. While the specific theological language of transubstantiation is a later development (the way homoousious was, for example), the belief that the wine and bread are Jesus is the ancient teaching of Christianity. We find it quite explitly stated in the early 2nd century by guys like Sts. Justin Martyr and Ignatius of Antioch, both writing prior to 135 A.D., I believe. For that matter, it is pretty explicit in the Gospel of John. In any case, the Orthodox and Catholics both teach it (or something arbitrarily close to it) and always have.
Probably the best place to get a detailed explanation of the theological basis of the teaching is Aquinas, who leverages the philosophical concepts of essences and accidents to describe what he thinks is going on. It is an interesting treatment, but from my perspective, the more important aspect is simply that it was the original teaching of the Christian community.