The warning signs can be difficult enough to recognize; but the problem of identifying major depression in teens is further complicated by the fact that even these ambiguous signs are not always present.
They are using what we know as signs, to communicate with each other about some situation that is present at that moment: gathering the group, alarm, hunting, mating.
Is it possible to imagine something as highly refined and subtle as the cave paintings of 15,000 years ago without language to enable manipulation and transference of ideas?
Possession of language is a defining mark of our humanity, and it has been faithfully accompanying us ever since our branch first emerged as a distinct species.