There are only two species that have language — humans, and honeybees. Other animals communicate, but its only us two that have language. And language means grammar; some abstract structure which conveys meaning according to the arrangement of symbols in that structure.
Our language is vastly more sophisticated than the honeybees. Their language is something called a waggle-dance, which conveys information about a food source between a individual who has returned to the hive from foraging and between her fellow workers. She peforms her waggle dance and the length and orientation of different parts of the dance indicates the quality of the food source and the direction in relation the the current position of the sun. The dance has structure, and that structure conveys the meaning of the components within it — its a language, a primitive language, but still the only thing that looks close to ours in the animal kingdom.
Why is that? Why is the only other grammar not found in a fellow primate, nor even a fellow mammal but found in an insect?
Here’s my theory — language is a system with unparalleled power to communicate information. But this means that it also has unparalleled ability to deceive, which is what one of the basic properties of communication systems. If you can use language to convey a very specific message, you can also use it to make very specific deceptions, for example tricking someone the food is in one direction, while you go and enjoy it in another. Because of the unprecedented capacity of language to deceive, it exists at the top of a steep evolutionary mountain. Any species which is evolving language must have some protection against the threat of deception, otherwise the only defense is to ignore language-based communications all-together (in which case you don’t get any benefit, and so language-evolution never gets off the ground).
The human defense against deception-using-language is based on our other cognitive abilities — the ability to reason about who to trust and when to trust them. Co-evolving these capacities with language is one strategy which allows the evolution of language. The honeybees have used another, circumventing the threat of deception by making deception evolutionarily pointless: honeybees in a hive are all genetically identical, so although language inherantly contains the capacity to deceive, in honeybees there is no reason why deception itself would evolve to diminish the benefits of communicating through language.
Update: I was wrong about honey bees being genetically identical, but I don’t think it demolishes the argument (quoting N.) “Honey bees are not genetically identical. Worker sisters share 3/4 of the genes with each other, but would only share 1/2 their genes with any offspring they might produce, so they can better propagate their genes by helping the queen produce more sisters. Wikipedia link here