Take the word ‘cat’. Although the three sounds ‘c’ [k], ‘a’ [æ] and ‘t’ [t] are individually meaningless, combined into the word ‘cat’ they take on a conventionalised meaning. The sequence of meaningless sounds in a word can be recognised qua sequence. Once this is done, we can substitute sounds, such as ‘p’ for ‘c’ to get ‘pat’, or ‘d’ for ‘t’ to get ‘cad’, or ‘augh’ [ᴐ] for [æ] to get ‘caught,’ and so on. This relies upon an understanding or discovery of ‘slots’ (the positions for ‘c’, ‘a’ and ‘t’ in the word and syllable) and ‘fillers’, the individual sounds of the language. From this simple slot-filler principle, in conjunction with meaning and culture, we can build grammars of varying levels of complexity.

Source: Tools and voyages suggest that Homo erectus invented language | Aeon Essays