A language needs ‘rules’. Communication would break down if there were no agreed ways of combining linguistic units.
In theory there are nearly endless possibilities to combine words and sounds. However, any full language has narrowed down the range of possibilities to a few allowable combinations
=> the grammar.
How could grammar have arisen in the evolution of language?
Nouns and verbs together constituted the first ‘grammar’ => humans could mentally distinguish things and people from actions and events for a long time before establishing them as nouns and verbs.
Nouns and verbs are regarded as the universal categories of all languages.