Language is everywhere. No culture has been documented to not have a language. Wherever we go in the world, if we find two people, they will surely be talking in a spoken language, or in a signed one.
As we observe this omnipresence of language in our species, we also observe what looks like a great linguistic diversity at first sight. If we examine the languages in the world, the resulting picture appears to show a very high degree of variation.
Nevertheless, the languages in the world share among them more features than meets the eye. These similitudes, which often go unnoticed on the surface, become evident when we review the pieces of data provided by neuroscience, linguistics, psychology, or biology.