Language is the fundamental trait that sets apart homo sapiens from other animals. The physical ability to generate complex sounds has given us the ability to go beyond instinct.
Culture could not exist without language, and not only because we wouldn’t be able to share it with one another. Animals, lacking full languages, are only able to communicate simple pieces of information: there is food over there; a predator is approaching; I am ready to mate. Contrary to popular belief, they do not chit-chat with one another; they are simply relaying basic information. A cat might indicate to one of its kind that it is happy, but will not seek a full conversation with its fellow. They lack the body parts to do so, and are therefore unable to conceive any higher form of communication. This is not unlinke people who are blind from birth: not having ever seen colors, they simply don’t know what color is. It’s an entirely abstract concept to them, much like every human fails to grasp the concept of a fourth dimension. (I strongly recommend reading Edwin A. Abbott’s “Flatland” to get a better idea of the problem.)
Without language, we wouldn’t have any literature, or math, or philosophy, or religions, or politics, or engineering. If we weren’t able to fully communicate and share our thoughts with others, we would be forever stuck in an environment without being able to improve it. Chimpanzees are the most intelligent animals. They are very similar to us, and do have remarkable communication skills compared to most animals, and they are able to use tools. Yet they are not able to do more than what they do. They have no push for innovation. For instance, there are no reports of wild chimpanzees ever attempting to build anything that wasn’t directly targeted at obtaining food. If a chimpanzee had a “human-like” idea, it wouldn’t be able to share it with its fellows, and for all intents and purposes, it would be as if it had not had it. Moreover, we wouldn’t even know, because non-verbal communication has strict limits. Sign language works in humans because we have the innate ability to communicate extensively, and it could be said that muteness is a statistical error: the instinct to communicate is still there. On the other hand, chimpanzees who were taught the sign language were only able to express comparatively simple thoughts.
It would therefore be naïve to limit the idea of language to spoken word, or to a set of grammar rules, or even to a writing system. Language, in itself, is a much broader concept: it’s a foundation upon which we build everything. We are humans not because we have opposable thumbs, or because we walk while standing up. Our very humanity exists because we are able to think about abstract concepts, and we can do that because we have the potential for language.