Fascinating! Thanks for the posting; I wish I had the time to dig deeper. I am fascinated with 'prehistoric' societies. The old myth that Sumer began civilized societies - a complex urban structure that just mysteriously exploded out of the Mesopotamian desert - is dissolving, opening completely new vistas.
Sumeria gets the nod because it produced the earliest writing system that can be read (more later if desired). Samuel Noah Kramer noted that the names for the major rivers and even the cities in Sumer didn’t have Sumerian names, but thanks to the characteristics of cuneiform (which the Sumerians did invent) and a cultural quirk (they believed that cities were never founded by humans, but by the gods), they preserved tiny bits in transliterations of otherwise vanished languages.
By their own account, the Sumerians arrived by sea. They called themselves “the black headed people” (I guess, complexion problems ;^) and their accounting systems and writing systems endured a long time (it was in use in multiple cultures, languages, eras, for at least as long as we’ve been using the alphabetic system). Sumerian literature survived in translation and adaptation among the Akkadians, a Semitic people who lived alongside and gradually became much like the Sumerians. The Sumerian language continued to be written and probably spoken among the learned class for a long while after the Sumerians themselves ceased to be a discernable people.
The Elamites were the big dog in the area of Iran long before the Persians. They too had a written language, but it’s still obscure (I don’t think a large preserved archive has been discovered). The Harappan script is also pretty old, but hasn’t been cracked to the satisfaction of anyone but the individual translators — there’s been no bilingual texts discovered, which may be a permanent problem and stumbling block.