 The primary characteristic differentiates a human and a shapeshifter is the large quantity of mercury in their blood, and the presence of a metallic data disc at the base of their spine.William Bell believes the shapeshifters were designed this way to withstand travel between the universes.The first shapeshifter witnessed by the Fringe division is one that follows Olivia after her meeting with William from the parallel world in the second season premiere, "A New Day in the Old Town".