Speech is the only universal medium of language. That is, each and every human population exhibits, at least, one spoken language. Spoken language is also the basis for virtually all major writing systems, wherein symbols encode sound (typically of phonemes or syllables). For these reasons, the vocal capacities of great apes have historically been a key component in the study of possible precursors and processes for language evolution within the human clade.
Once we're reaching the end we see that we focus more on human evolution and how vocal capacities have changed. Moving onto studies from the 1960s we start seeing the current evolution and the skills humans have when it comes to vocal language.