 The distinction Bickerton draws between these categories is one of displacement, with the indexical signs of animal communication systems having no capacity for displacement, and the symbolic signs of human language requiring it.Iconic signs, however, "may or may not have it depending on how they’re used ... iconicity, therefore, is the most probable road that our ancestors took into language".Using a niche-construction view of human evolution, Bickerton has hypothesized that human ancestors used iconic signs as recruitment signals in the scavenging of dead megafauna.