The first known human languages were already very complex. Languages do
not show the evolutionary progression we would expect if humans evolved
gradually.
The first known languages were written languages (else they would not
be known). Since most cultures in the world have had no written
language, and most people have been illiterate even where written
language existed, written language is a poor metric to use to measure
language in general. Language had been developing for an unknown
period of time before written language evolved.
The earliest known writing is simpler than written languages today.
There are very simple, nonlinguistic precursors (no grammar) to
cuneiform writing (Coulmas 1989).
References:
Coulmas, F., 1989. The Writing Systems of the World. Malden, MA: B.
Blackwell.
