Final answer:
Researchers switched from trying to teach chimpanzees spoken language to using sign language because chimps lack the vocal tract for speech and have a natural use of gestures, which are better suited for sign language with its structured rules similar to spoken language.
Step-by-step explanation:
Researchers switched from attempting to teach chimpanzees to speak to teaching them sign language primarily because chimpanzees lack the specialized vocal tract that humans possess, making it physically impossible for them to produce the same range of sounds required for spoken language.
Moreover, chimpanzees already utilize a form of gestural communication in the wild.
Adopting sign language aligns with their natural communication tendencies and capitalizes on the structural similarity between sign language and spoken language, which both have rules and structure that govern the combination of signs or sounds to convey meaning.
Studies with individuals like Koko, the gorilla, have demonstrated that great apes can learn sign language to a certain extent, although they do not reach the complexity and flexibility of human language.
Yet, these studies indicate that some apes have the capacity for a form of protolanguage, a precursor to full-fledged human language, which suggests that they have certain biological features that enable limited learning of human language.
While some researchers remain skeptical about the extent to which apes like Koko truly understand and use sign language, the efforts to teach them have provided insights into the biological underpinnings of language and its evolutionary history.