Out of Africa (OOA) has been the main theory of the origin of modern humans since the mid-20th century. Strictly speaking this is only a theory of anthropology. Since language is a human phenomenon,  it has however also sprouted a “linguistic Out of Africa” theory alongside.
According to what could be called “the evolutionary theory of language”, we observe that new languages only come about by the spreading and splintering of earlier languages. (Or, perhaps a better biological analogue still is the third tenet of cell theory: that cells only come about from other cells.) This alone already suffices to imply that there exists a family tree of languages, tracing back to the ancient era of glottogenesis. Connected with a relatively late (“recent”) expansion of modern humans out of Africa, we can then in particular infer the highly likely existence of a language that could be called “Proto-Exo-African” (PEA) — the language of the humans who first set on this exodus, which must also have been a common ancestor of all languages spoken in Eurasia, Oceania and the Americas. This is an idea that is in principle sound, even if, in my impression, underappreciated among historical linguists. The smaller number of so-called evolutionary linguists out there do understand it well, at least.
This argument though says nothing about if this common descent would be in any way identifiable from the linguistic data itself. Language does not have a strict analogue of DNA (or any other similarly transferred major biochemical machinery), and is not strictly speaking “transmitted” as much as “constructed” over and over again every generation. No child is born knowing a language, only with the ability to acquire a language. This could add up to the result that every linguistic feature of PEA has been by now either lost or diluted to undetectability. And it happens to be the case that all language families with general approval so far are still at least an order of magnitude younger than the assumed recent OOA spill-out starting some 70,000 years ago. Even the more ambitious proposals like Amerind or Nostratic (that actually have some legitimate comparative evidence backing them, unlike attempts to scrape together things like Proto-World) are only proposed to reach at most some 20-30 millennia of age, i.e. barely a third of the way back.
If PEA might be undetectable by direct means, how error-proof is the indirect demonstration of its existence then? As long as we do not question the underlying OOA theory, there are really only four possibilities under which the assumption of PEA might fail:
- humans leaving Africa did not yet have language, and it has come about only later, possibly several times independently;
- at some point in prehistory, new languages were created from scratch to replace earlier natural languages, and some or all modern languages rather descend from these “new” languages;
- the African exodus population spoke more than one language;
- at some point in prehistory, other (possibly entirely unrelated) language families have secondarily spread out of Africa, to replace some or all descendants of PEA.
Of these, #1 is difficult to directly refute. Spoken language does not fossilize, and hence the study of the biological evolution of language is to a large extent an issue of speculation. The most common opinion around however is that language would have existed at least by the transition to anatomically modern humans (AMHs), as distinct from Neanderthals and the newly found Denisovans, so at least a couple hundreds of millennia ago. In this post series I will continue to follow this assumption as well.
All the others, however, would not make major dents in the hypothesis of monogenesis of non-African languages.
#2 is a priori improbable, and hence not actually a major objection. If we take seriously the rarity of languages being freshly invented (i.e. stick to the principle of uniformitarianism), then even recent glottogenesis events will actually only leave a slightly weakened OOA theory of language, one where we can allow for e.g. Basque or Turkic to have been created from scratch, but all other non-African languages can be still assumed to be descendants of PEA. Similarly #3 would only split the family tree into a small copse of unrelated-at-OOA-time trees, probably at most no more than 3-4. These would have good chances of being still related to one another at some pre-OOA date, so that there is a PEA in the last common ancestor sense, even if confusingly enough it was not itself exo-African (and could be therefore also the ancestor of various African languages).
#4 actually has good chances of being true: maybe the best contender for non-PEA languages spoken in Eurasia today are the Semitic languages in the Levant and Mesopotamia, grouped in the larger Afrasian family, whose homeland is often (though not always) placed somewhere in northeastern Africa. But we can also see that this too would only very slightly push back the boundary of African vs. exo-African languages. Languages spread only step by step. Perhaps some other lineages in the vicinity of Africa, say Sumerian or Dravidian, could be also of yet more recent African origin, but once modern humans had first colonized places like Siberia and Southeast Asia, new intrusions all the way from Africa are unlikely to happen.
Altogether even a buffer zone of maybes doesn’t seem to shake the conclusion that the 100+ exo-African language families known today to linguistics (including isolates) must be only a few top boughs of at most a handful of much larger underlying language families, dating back to the time of the OOA expansion.
But has there really been a recent Out-of-Africa expansion?
There definitely has been at least one OOA event, since also the Neanderthalians, Denisovans and Homo erectus are thought to descend from African hominins. For recent modern human OOA in particular though, the main line of hard evidence has originally rather come from the fossil record. 200k years old Homo sapiens remains from Omo Kibish in Ethiopia, the oldest known throughout the late 20th century, have been some of the best supporting evidence. This is however but a single archeological datapoint — and one that has been overturned even. Since last year, the oldest known remains of AMHs now come instead from Jebel Irhoud in Morocco, dated around 300k years old. Note that this is quite a gap, both chronologically and geographically! The data doesn’t get especially dense going forward either. Only a handful of any modern human remains under 100k years of age are still known. More importantly this slightly extended selection already includes locations also outside Africa, in modern Israel and Oman (discovered not too long ago as well). These have been suggested to represent “failed migrations that died out”, but this strikes me as special pleading. I dout that anyone looking at this scattered early record without the weight of research history (and with understanding of the Signor–Lipps effect) would place the origin of anatomically modern humans within Africa with great confidence. At minimum a Near Eastern origin seems to be entirely within the question as well. Paleoecology could be able to suggest other likely locations still.
Several posts by anthropology blogger Dienekes have moreover drawn my attention to a few interesting additional arguments to consider recent OOA on very shaky ground by now. For one, recall that modern humans’ closest known relatives are the Neanderthalians and the Denisovans — two Eurasian species, with Denisovans branching off first, which would suggest that the common ancestor of the three, and even the Neanderthal-AMH last common ancestor, lived in Eurasia as well. (This does not need to coincide with the LCA of crown group AMHs, however.)
For two, genetics has for long pointed out that the modern human populations of sub-Saharan Africa  show altogether greater genetic diversity than those of Eurasia (+ with even further rarification in Oceania and America). However with the rapid development of archaeogenetics in the last few years, we have by now first lines of evidence that this could be due to admixture with archaic Homo sapiens groups.
Maybe the Neanderthal and Denisovan OOA event was then also the main modern human OOA event after all?
This would also imply at least two inverse “into Africa” expansions (one leading to archaic African substratal groups, the other for crown group AMHs). This does not seem to be a very costly assumption though, since the distribution of several archaic haplogroups already demands multiple major population movements across the continent. E.g. the archaic mtDNA haplogroup L0 of South Africa is both first-to-branch-off and present in only fractional proportion in the populations that do carry it, clearly requiring multiple admixture events along the way (instead of, say, a Great San Migration that starts 100k years ago followed by them hanging out in South Africa mostly intact after that). There are also haplogroups with primarily Eurasian distribution but some inroads even into sub-Saharan Africa, requiring their own more recent but still prehistoric into Africa or out of Africa movements or gene flows (e.g. Y-haplogroup T, mtDNA haplogroup U).
An option to be also kept in mind is haplogroup extirpation: various today-African haplogroups may have once existed in Eurasia too, but eventually died out, e.g. under later population movements. The same could be the case anywhere of course, but to me it seems that Eurasia is a priori the most likely location for this. For one already due to size, for two due to the historically well-documented extensive population movements, particularly across and around the major “crossroads” that is the Near East.  A third and maybe the most powerful candidate for wiping out genetic diversity from Eurasia would be the latest glaciation period. Still, the human Y-chromosomal and mtDNA haplogroup trees both start off with a remarkably large series of exclusively African early groups. Any theory of AMH origins outside of Africa would have to explain most of these through archaic admixture, with haplogroup extirpation probably only granting some wiggle room around those points in the two family trees where the branches start to turn Eurasian-centric instead.
I’m aware I’m outside my zone of expertise here, so in case this sounds like I am suddendly a few moomins short of a valley, I do want to note that overturning recent OOA is still not looking cut and dry exactly (and if any of the arguments above have big glaring holes in them, I would appreciate readers pointing them out). If you bear with me for now, though, I will develop in the next post some potentially very interesting corollaries this possibility would have.
 It may be at times useful to think of linguistics, likewise most other humanities subjects, as a subdiscipline of anthropology (in a somewhat similar way as how biology could be considered a subdiscipline of chemistry).
 Important as a human genetic and cultural area, and to some extent also as a more general biogeographical region. North Africa by contrast on many marks aligns with Eurasia instead. I have wondered if sub-Saharan Africa would deserve its own underived term, along the lines of “Maghreb” for North Africa. There are a few historical candidates, but nothing that really stands out as immediately usable: Ethiopia and Sudan have been already claimed by states (much as also Libya), and Zanj also has been kind of claimed by Tanzania. The analogy of Australia could suggest e.g. “Meridionalia” or “Equatoria”, but outright coinages have quite a few orders of freedom to them.
 Later also the “highway” that are the steppes, but my impression is that pseudo-periodic nomad invasions of Eastern Europe / Persia / China have only really been a thing after the domestication of the horse, and related inventions such as chariots, saddles, stirrups etc., all fairly recent in the big scheme of things.