Language is often pointed to as the hallmark of the human species. This assertion has had many proponents into the twentieth century and it is widely held that humans differ markedly from animals in their use of language. In the past thirty years, this assertion has been the subject of many debates as scientists have researched capacity of sign language acquisition by apes. Incredible claims have been made by some researchers about the linguistic capabilities of their subjects, mostly chimpanzees. These claims have been refuted and counter-refuted many times, and the literature on the subject is extensive. Currently, there is no general consensus on the ability of non-human primates to acquire language.