The pre-Columbian distribution of languages in the Americas is rather different from what we see in the Old World. In Eurasia, Africa, and Australia, we mostly see large areas occupied by families of clearly related languages – such as Indo-European, Afro-Asiatic, Niger-Congo, Malayo-Polynesian, and Pama-Nyungan . Much splotchier and localized in the Americas.
It seems that each of these large language families is the product of a massive, relatively recent population expansion. Not without admixture, usually, but some group developed some sort of advantage and rolled over its neighbors. This means in most of the world where we have information, the current inhabitants are quite different genetically from the people who lived there 10,000 years ago.
Europe experienced two expansions (first Middle Eastern farmers and then sort-of Siberian Indo-Europeans). Guys from Western Asia (and eventually some Indo-European speakers) flooded into an India originally occupied by people distantly related to the Andaman islanders. Bantu farmers swamped most of Africa’s hunter-gatherers. Somehow some genes and ideas traveled from India to Australia (about five thousand years ago, triggering an expansion that covered most of the continent. Chinese-looking people largely replaced the original inhabitants of Southeast Asia – the Cambodians seem to have a significant admixture from the previous tenants. Similar story for the Malayo-Polynesians in the Philippines and Indonesia. Chinesian farmers moved up in the world and mixed with altitude-hardy proto-Sherpas to give us Tibetans. Levantines carried languages and farming into Ethiopia (undoubtedly picking up the local altitude adaptations).
Although the Americas were settled more recently than any other continent, they have an older-looking language distribution – because there there just hasn’t been as much population turnover there in the Holocene. You don’t see the same dramatic population expansions. Until Columbus, of course. We have good reason to think this, because they’ve just sequenced a child’s skeleton from the Clovis culture, 11,000 years ago, and the kid is genetically similar to contemporary Amerindians. Genetic continuity over such a long period is not seen in most of the Old World – probably only in a few places.
Which is not to say there has been no expansions or population turnover in the Americas- just considerably less so than in most places of the Old World. The Na-Dene were late arrivals who had a decent expansion. There have been fair-sized expansions that originated among the descendants of the original settlers, like the Algonquians. But languages have had longer to differentiate in the Americas, because the slate hasn’t recently been wiped clean here. All the Amerindian language families other than Na-Dene and Eskimo-Aleut must form an ancient family that only smart guys like Greenberg, Ruhlen and Sapir could see.
Linguists usually assert that language and genes are entirely separate, but they had it backwards. Genetic expansions play a key role in language expansions. From current data, every time, in prehistory.
Why have there been fewer and less powerful population expansions in the Americas? I don’t know, but I can guess. Agriculture came later: that may matter, but it can’t be a necessary cause, because both Pama-Nyungan and Na-Dene managed decent expansions before agriculture. Maybe acquiring a winning advantage against other groups was difficult in the Americas because the population was so genetically uniform -much more so than in any other continent. Cultural innovations aren’t as good as genes at conferring a long-lasting advantage – you can copy them.
What else? Before the advent of the Indo-Europeans, almost all of Europe must have spoken related languages, originating somewhere in the Middle East or Balkans, carried by the LBK and Impressed Ware culture. Basque must be the last survivor of that family, rather like the tuatara. Theo Vennemann, can you hear me?