Fully fledged language is among the hardest human traits to date, because it is a capacity that leaves no direct trace. Speech does not fossilize; researchers rely on indirect evidence: anatomical changes (the vocal tract and breath control), traces of symbolic behavior (use of pigment, ornamental beads, deliberate burials), and the shared linguistic capacity of all human populations. This evidence is scattered and its interpretation contested; some argue the capacity developed gradually over hundreds of thousands of years, others for a more abrupt cognitive shift.

What sets language apart from earlier communication is that it generates an unbounded number of new meanings from finite elements (sounds, words) by rules. This made it possible to speak about what is not here and now — the past, the future, hypotheticals, another mind.

The consequences were decisive: knowledge could now pass between generations without each individual rediscovering it. Cumulative culture, long-range planning, and large-scale cooperation rest on this capacity. The whole of subsequent history is, in a sense, built on transmissible language.