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. This process "would have created new words and deployed old words in new contexts, further weakening the uncoupling of words from situations, from current occurrence—even from fitness", and thus allowing for the creation of symbolic language. In The Symbolic Species, Terrence Deacon argues that the emanation of symbolic capacities unique to language was a critical factor in the evolution of the human brain, and that these symbolic capacities are vital to differentiating animal from human forms of communication, processes of learning, and brain anatomy. "The doorway into this virtual world was opened to us alone by the evolution of language, because language is not merely a mode of communication, it is also the outward expression of an unusual mode of thought—symbolic representation." Calls and gestures Iconic calls and gestures mimic the forms of the things they stand for (such as outlining shapes or moving your hands back and forth multiple times to show repetition.) Iconic calls and gestures are not formally considered language, or language-like communication in that they do not contrast or possess arbitrary characteristics. Noises that imitate sounds of the surrounding environment (ideophones) are also iconic. Though humans possess a repertoire of iconic calls and gestures, other mammals produce few iconic signals. Despite this, a few captive chimpanzees have shown the beginning stages of iconicity. Burling et al. states: "Chimpanzees in the wild do not point, and rarely do so in captivity, however there is a documented case of one named Kanzi, described by Savage-Rumbaugh et al., who could indicate direction of travel by "extending his hand". Another chimpanzee, Viki (Hayes and Nissen 1971:107) made motions of kneading or ironing when she wanted to knead dough or iron napkins. Bee dances are another example of iconicity in animal communication systems. Sign languages Iconicity is often argued to play a large role in the production and perception of gesture. Proposed ways in which iconicity is achieved is through Hands that Act, Embody, Model, and Draw. In sign languages iconicity was often argued to be largely confined to sign formation (comparable to onomatopoeia). Some proponents believe that iconicity does not play an actual role in perception and production of signs once they have undergone phonological reduction and become part of the