To explain better:
the origins - or "tradition" - of portrait, are in the pre-artistic representation of anthropogenic forms. Those representations of idols, gods or dead, was often with few humans elements; sometimes a trace of eyes, of mouth, of nose, or sometimes without those details and only a kind of oval for head. The rest of body was, according cultures, a simply parallelepiped, more or few lenght, or totally without it, and only with neck. For exemple; the Xoana in the celtic culture or in the apulian pre-roman culture. For celtic Xoana is, in most cases, impossible to establish a short arc of time for its realization; cause its typology are persisted for several centuries. In the early Rome, before every suspicion of nationalism (don't worry), it was the cult of ancestors, that was represented with a wax kind of portrait, very involuted in their forms, only with head (and neck), that begun to improve during the time. And this is the origin of roman portrait. So says like inside origins of roman portraits was present the developmental germ of personal portraits of followings centuries. Until become (I jump a lot of stages) to our... oh, sorry, to "your" portraits. In a lot of ancients cultures, included Egipt, the personal portraits was not a tipical expression of status; also Pharaoh was represented more with ideals tracts than realist. And also greeks don't had a tradition of personal portrait. The realism in the roman personal portrait was, instead, inborn in its original ideology. This is not "nationalism", sorry, but history. It most to study it without turbids and idiots and barbaric prejudices.

Reply