Is the beginning of coming into birth described in the twelve nidanas?
If we look at Robert and rewind time backwards to the time when Robert
  has not yet come into his first birth, can't we see how Robert came
  into his first birth?


There is no first birth. Samsara is beginningless. But you can end it. Robert is a concept. There's no Robert existing in reality even from this moment to the next, let alone from life to life.