Language has long been considered one of the things that separates humans from animals. Descartes used the language test to determine that animals completely lack a mind, which he equated with an immortal soul. To him, they have no thoughts, only chemo-mechanical reactions. The language test is becoming less and less reliable, as we are finding that most animals have some form of communication.
Understanding language is not very hard. It requires a tiny portion of our brains to understand abstract concepts. But to create original, complex abstractions is extremely complicated. We act as though our language is fundamentally logical or characteristic of ‘higher thinking’. But we can see the basis of our language is quintessentially a social invention.
Language allows hierarchy to be established without anybody getting hurt or killed. Or a person can convey a lack of hierarchy (as a request may not be in the form of a command). Language provides a basis for cooperation and bartering. Language circumvents the need for co-evolution, creating autonomous social structures within the species. Social animals have an unmistakable advantage over solitary ones.
We have not yet found a species which can write, which is the advent of history, which some scientists argue is a better defining characteristic. In any case, science is progressively blurring the lines of ‘intelligence’ which Homo sapiens have traditionally prided themselves on.