The Puzzle of Human Origins

African population structure and the origins of the Boas-Chomsky universal

The biological capacity of our species for language is closely-tied to our capacity for creativity and innovation. Fundamental to this capacity is Chomsky’s Universal Grammar. At the heart of Chomsky’s idea about the language capacity is our unique ability to form an infinite number and variety of structured sentences from a finite lexicon. Other species have calls and songs to communicate by sound. But only humans can form an infinite variety of mutually-intelligible, completely-novel sentences. Since there is an irreducible gap between the finite and countable infinity, this is a zero-one character — you either have it or you don’t. As far as we know, this capacity is exclusive to our species and uniform across it.

The uniformity of this capacity across our species is evident from the fact that if you take a child from any linguistic population, say Bengali, and put them in any other linguistic population, say, Japanese, at an early age, they will grow up speaking Japanese like a native. Who was the first scholar to make this observation?—I asked Chomsky. He told me that he believed it was Franz Boas, a physical anthropologist at the turn of the century, and the first antiracist.

Boas had observed that all anthropological populations observed since the ethnographic present, c. 1492, had language, dreamworlds, stories, traditions, song and dance — culture in the thick sense. What Chomsky did is trace Boas’ insight into the universality of culture in the thick sense to our biological capacity for generative grammar. We find language, dreamworlds, stories, traditions, song and dance across anthropological populations because, Chomsky argued, of the uniformity of the language capacity across our species. Because Boas was the first to observe anthropological universality of culture in the thick sense and Chomsky the first to explain it by the uniformity of the language capacity in our species, I have called it the Boas-Chomsky universal.

The Boas-Chomsky universal is technically an autapomorphy or diagnostic character of our species, meaning that it is derived and exclusive to the species: observation of this biological character is sufficient to classify any specimen in the wild as a member of our species in the proper sense of phylogenetic systematics sensu Hennig (1966). So far so good.

The problem is that the Boas-Chomsky universal can only be documented for populations during the historical era. So, it can be observed in contemporary anthropological populations; it was observed and reported by European explorers in the early modern era, 1492-1870; and by writers in Antiquity when they put on their anthropological hats. History, in the modern disciplinary sense, begins with the written word because discipline on the truth claims of historians is imposed by means of testing them against verifiable archives of written historical documents. But direct observation of the Boas-Chomsky universal is unavailable where it is most needed — in our fossil record.

Paleoanthropologists distinguish between anatomically modern humans and behaviorally modern humans. Homo sapiens or anatomically modern humans are defined by their skeletal autapomorphies — derived and unique cranial, craniodental and postcranial traits. In particular, H. sapiens is not defined genetically, although, of course, the genomes of all H. sapiens, including those of the most derived geographic isolates, cluster together and far from other hominin taxa such as Neanderthals. In fact, Denisovans are the only taxon to have been identified and generally recognized exclusively through DNA — all other hominin taxa in our genus are defined in terms of skeletal morphology.

What is modern behavior? It is everything that Boas observed across anthropological populations at the turn of the century. Since the Boas-Chomsky universal itself is not directly recoverable from the archaeological record, paleoanthropologists look for archaeologically-visible proxies of this diagnostic character of our species. A lot of proxies have been proposed over time, some of them more compelling than others. A general consensus has emerged among paleoanthropologists that evidence of symbolic storage is diagnostic of the fossil population being behaviorally modern. Symbolic storage is the embedding of formal, idealistic, cultural, social, more generally, symbolic ideas into material artifacts. Cave paintings, sculpture, musical instruments and ornamentation are obvious instances on which there is general agreement. There used to be agreement that sea-faring was evidence of the Boas-Chomsky universal. But that line of argument weakened dramatically after the discovery of ‘the Hobbit’—H. floresiensis on the island redoubt of Flores in southeast Asia.

Our genus emerged soon after the glacial-interglacial cycle of the Pleistocene began 2.58ma (million years ago). For at least the first two million years, the archaeological assemblages associated with hominins show minor to non-existent variation in stone tools — no regional or temporal change in lithic styles is clearly visible. For instance, they were making the same damn Acheulean hand-axes for a million years. All this would later change dramatically. Lithic assemblages begin to display regional and temporal styles towards the end of the Middle Pleistocene, 780-130ka (thousands of years ago). Many workers consider lithic styles to be evidence of symbolic storage. Conversely, the extraordinary stability of lithic techniques suggests that the hominin populations associated with them were not fully modern, not fully human — Recency is the name that Proctor (2003) gave to this idea.

This idea of Recency is contested by many workers. They have argued that lithic assemblages associated with many fossil populations of anatomically modern humans do not exhibit regional and temporal styles, whereas those associated with late Neanderthals do. In general, the idea that Sapiens were behaviorally modern while Neanderthals were not, has been contested in what I have called the Twenty Years’ War. As one of the fiercest critics of Recency notes,

Many have attempted to define a specifically “modern human behavior” as opposed to a specifically “Neandertal behavior,” and all have met with a similar result: No such definition exists that does not end up defining some modern humans as behaviorally Neandertal and some Neandertal groups as behaviorally modern.

Zilhão (2011) 

Following Binford (1998)’s notion of modular use of space, Lyn Wadley (1996, 2000, 2001, 2006) has made a compelling argument that symbolic storage can be inferred from the formal use of space. Specifically, spatial differentiation of campsites into area reserved for specialized activities may be diagnostic of the Boas-Chomsky universal because ‘people who do not deliberately use space for symbolic purposes may, like the great apes, have simple, repetitive spatial patterning in their campsites’ (Wadley, 2006). This undifferentiated use of space at any archaeological site is replaced at some point by a formal, symbolic use of space, with the characteristic pattern of reserving certain areas for special purposes:

Modern people use space symbolically to demarcate their cultural environment and to codify their relationships with strangers and with kin. Almost all modern people order space in their dwelling places (Kolen 1999:141). Seating arrangements at a formal dinner provide a good example in complex western societies.

Wadley (2006)

I have argued that the symbolic use of the front yard as a lawn is a diagnostic character of the suburban middle class in the United States and elsewhere as the material culture of this class—“the American way of life”—has globalized. Another instance can be seen from the archaeological record of the Gravettian mammoth hunters who segregated mammoth bones from the bones of other animals, suggesting religious or spiritual, at any rate, symbolic significance attached to mammoth bones.

In general, wherever the deliberate encoding of symbolic information in material artifacts is clear from the archaeological record, we can conclude that the associated population had the Boas-Chomsky universal.

---

In the mid-1980s, paleoanthropologists were jubilant. They were celebrating what academic conferences called ‘The Human Revolution’. Workers working on European sites, who were then a majority of workers in the discipline, came to believe that some time shortly before the dispersal of Homo sapiens from Africa 50ka, the Boas-Chomsky universal became a universal character in our species; one that was not shared by Neanderthals — and this was ultimately the reason why H. sapiens replaced Neanderthals and other archaic hominins. In general, the Out-of-Africa hypothesis prevailed over regional continuity/Multiregional hypothesis which had posited that, what were until recently called continental races, had been separated from each other for a very, very long time. In fact, between 1859, when ‘the theory of the antiquity of man burst upon the scientific world with an irresistible force’ (Thompson, 1877) and 1987, when workers at Berkeley established the Out-of-Africa hypothesis, it had been believed that the continental races had been separated from each other by at least a million years. The discovery of the recent emergence of our species from Africa warmed the hearts of antiracists. The Out-of-Africa consensus relegated old ideas about radical racial particularism to the realm of discredited theories — we’re all Africans! went the cry.

At the heart of the new consensus was the idea that the Boas-Chomsky universal had emerged in Africa, as Chomsky and Berwick (2016) put it, ‘some time before 80,000 years ago if we can judge from associated symbolic proxies,’ and was responsible for both the radiation of our species from Africa and the fact that we replaced other hominin species still extant everywhere. Central to the schema was a certain smug self-congratulation: we replaced them because they weren’t fully human—we had the Boas-Chomsky universal, they did not! The new consensus was not far from the one imagined by H.G. Wells a century ago. In his classic 1921 short story,The Grisly Folk, Wells had monstrous Neanderthals hunted down and exterminated by ‘True Men’.

I have described the resistance mounted by scholars working in Europe who were more sympathetic to Neanderthals. Paleoanthropologists working in Africa and Asia also resisted the new orthodoxy. Non-European workers explained that they could not detect the discontinuities implied by the arrival of H. sapiens and the replacement of earlier occupants in their regions. Middle Stone age lithic technologies in many places were evident until the Holocene, tens of thousands of years before and after the arrival of anatomically modern humans. No paintings or other symbolic storage could be discerned around the time of the arrival of H. sapiens. Asian scholars kept insisting that they could not even detect the arrival of H. sapiens around 50ka—they insisted on continuity in cranial forms in China. Conferences over ‘the global human revolution’, also called ‘the global Middle Paleolithic-to-Upper Paleolithic transition’, quickly devolved into bitter fights.

Genetics had already revolutionized paleoanthropology in 1987. It would soon be in the driver’s seat. Before long, molecular anthropologists would colonize physical anthropology, destroying some cherished beliefs and assumptions on the one hand, and vindicating long-suppressed ideas on the other. One such idea, hegemonic since the 1960s antiracist turn, was that ‘pots are not people’. The antiracists had challenged the high racialist orthodoxy that ‘the races of Europe’ could be placed in a natural hierarchy: physical anthropology allowed one to track the movement of peoples in deep history and associate them with cultural developments in the archaeological record—thus establishing that culturally-superior (Germanic) Iron Age races had replaced (Aryan but inferior) Bronze Age races, who had in turn replaced the “most primitive” autochthonous Stone Age races of Europe. ‘Pots are not people’, the war cry of the antiracists, meant to severe the connection between physical anthropology and archaeology, biology and culture. It would not fare well at the hands of the molecular anthropologists led by David Reich.

As whole genome mapping became cheap and easy, molecular anthropologists began documenting a deep history of sex-biased migrations, invasions, population replacement and marginalization. The DNA of contemporary populations, they showed, bore the tell-tale signatures of this violent population history. What we find everywhere is considerable population stratification, with multiple layers of Neolithic populations having annihilated, absorbed, or pushed into marginal environments, autochthonous populations who had occupied the area during the Pleistocene. This is what happened across Eurasia, Africa, and Asia during the Holocene; and, of course, this is what happened in the ethnographic present in Australia, New Zealand, and the Americas. 

But mapping the genomes of contemporary populations did not tell the full story. In particular, the populations of different regions that had been driven away or exterminated without leaving surviving lineages could not be detected from contemporary DNA. As I wrote last year,

There is, in fact, an easy way to see the vanished world; the populations that were erased from the face of the earth by the relentless expansion of Neolithic peoples. We simply look at the worldwide distribution of languages that belong to families with less than a million speakers. See next figure. The great erasure is visible in the blankness of most of Eurasia and Africa. Most of the smaller families survive in Sahul (Australia and Papua New Guinea), the Americas, and the wilderness of the Kalahari desert in Africa. Otherwise, we find one isolate in Europe (Basque), one in Tibet, one on the Andaman Islands, and a small number in the circumpolar region.

What truly revolutionized paleoanthropology was the ancient DNA revolution at the end of the 2000s. By looking at the genomes of fossil humans, molecular anthropologists could start to discern the population history of different regions at a much finer resolution. The old certainties especially did not fare well after ancient DNA sequencing arrived at scale. The picture that emerged even overturned many of the claims established by the molecular anthropologists not ten years earlier. The idea that Europeans and Asians had split soon after the Out-of-Africa dispersal around 45ka, did not stand up to scrutiny. It turned out that a basal or ancestral population roughly equally distant from Europeans and Asians had occupied much of Europe and Asia before the Last Glacial Maximum (LGM) around 20ka.

Moreover, it turned out that what had been thought of as the continental races of man (in particular, the big three in Europe, Asia and Africa) that were assumed, in 1859-1987, to have occupied their continents in genetic insulation from each other for a million years, were all recent populations that had emerged from the LGM and expanded during the Holocene. They cannot be said to have existed before 20ka; they were, in fact, created by the shock of the LGM. Even later, they were confined to their tiny homelands (southern Europe, northeast Asia, and central west Africa), until well into the Holocene. The Bantu expansion in Africa, the Indo-European expansion in western Eurasia, the expansion of East Asians (Turner’s Sinodonts), and Austronesians from Taiwan all date to later than 10ka.

Astonishingly, it turns out that the Americans had split from the northern Siberians considerably earlier, ~30ka, got trapped in ‘the Beringia Standstill’ for nearly fifteen thousand years, before populating the Americans as the ice sheets retreated shortly after the LGM around 14ka. Other geographic isolates turned out have been split much earlier, around the time of the Out-of-Africa radiation. The Andaman Islanders, the Melanesians and the Australian Aborigines were the oldest lineages of H. sapiens outside Africa. They had split from the rest at the time of the dispersal, ~50ka. It is not clear whether this corresponds to a slightly earlier dispersal out of Africa or not. The balance of scholarly opinion among molecular anthropologists is leaning towards the latter—a single Out-of-Africa dispersal.

But the true shock of ancient DNA was felt within Africa. Out-of-Africa theory had simply assumed that H. sapiens was endemic throughout Africa, or at any rate, had quickly overrun other populations in Africa (since it was assumed to exclusively sport the Boas-Chomsky universal) before the dispersal. This turned out to have been badly wrong. What truly destroyed the hopes of a tidy story of human origins is the discovery of deep population structure within Africa.

It would’ve been really tidy if Homo sapiens had emerged in, say, East Africa and spread out across Africa around the time it was venturing into Asia. But that’s not what we find at all. The deepest splits in the human lineage are all within Africa. As you can see from the following phylogenetic tree obtained from a study published recently by Hollfelder et al. (2021), the oldest branch of humanity is that of the San hunter-gatherers of the Kalahari, who split from the rest ~300ka. The second oldest split is that of the Pygmy population of the central African rainforest, who split ~200ka. The researchers document that the fossil individual recovered from Shum Lake and dated to 8ka, was from a deeply divergent population that shared a common ancestor with west Africans and Pygmies — a population that has now vanished. Other workers have established that archaic populations had survived within Africa until well into the Holocene and interbred at different points with the ancestors of contemporary populations.

In light of the discovery of deep population structure within Africa, the origins of our species is now hopelessly shrouded in mystery. When and where did our species originate? East Africa had been assumed to be the original home of Sapiens for many decades. More recently, the southern Cape has been suggested as original home of Sapiens. But the fossil record is inconsistent with a population from a single region within Africa replacing other regional populations. It has been established without a shred of doubt that population structure, an order of magnitude deeper than found on any other continent, persisted within Africa all the way down to the present since before the posited origins of our species. In fact, the earliest Homo sapiens skulls are no older than 130ka, although they lack some modern features. Fully modern morphology do not appear until well after 100ka. Meanwhile, the San split from the rest long before, ~300ka. So, when and where did H. sapiens evolve?

Perhaps we can bracket the question of the origin of our species predating anatomically modern skulls that can be identified as belonging to H. sapiens. Maybe skulls give us a poor handle on biological species within our genus (a suggestion that will make any card-carrying physical anthropologist bristle). But even leaving anatomy to one side hardly eases the explanatory burden. Simply declaring the genetic cluster of ancestral populations of contemporary populations as Sapiens leaves us with a massive explanatory gap: for if our speciation predates 300ka, then they must have lacked the Boas-Chomsky universal themselves. And there goes your explanatory schema for why Sapiens replaced other hominin species.

Some workers have proposed an ‘African multiregional’ hypothesis, which seems to be the only one consistent with the genetic data but lacks definition. In this scheme, genetically distinct regional populations evolved together by exchanging genes. In a recent paper published in Nature, Rito et al. (2019) document gene flow from southern Africa to eastern African in the period preceding the Out-of-Africa radiation—‘a small stream of migrants who moved from southern to eastern Africa around 70–60ka’. This is an intriguing paper because the earliest evidence of symbolic storage is from Pinnacle Point and Blombos Cave at the southern Cape of South Africa. As the authors note, ‘the most impressive evidence for symbolic activities and technological complexity is seen in southern Africa before ~70ka, and appears in eastern Africa only after 70ka.’ That is, they’re suggesting that the Boas-Chomsky universal emerged in southern Africa and was transmitted to East Africa around ~70-60ka by gene flow:

A cluster of Homo sapiens populations, living in a glacial refuge area on the southern coast of Africa, developed a sophisticated repertoire of complex technological and symbolic behaviours over many tens of thousands of years. … Thus the use of beads, incised ochre, heat treatment and possibly microlithic technologies, [all proxies for the Boas-Chomsky universal] might then potentially have been transmitted to eastern Africa after 70ka. … By 60,000 years ago, however, a fully “syntactic” language akin to those used today must have been widespread across both southern and eastern Africa, and have been carried out of Africa.

Rito et al. (2019). “A dispersal of Homo sapiens from southern to eastern Africa immediately preceded the out-of-Africa migration.”

The problem, of course, is that this is a just-so story. The evidence is hardly dispositive that the Boas-Chomsky universal arose in the Cape, that it was carried north by the gene-flow they document, or that it was universal in East Africa at the time of the dispersal from there in Eurasia. Indeed, as the authors themselves note, ‘there has been growing scepticism amongst archaeologists in recent years against the possibility that human symbolic behaviour radiated outwards from a single source in southern Africa at such a recent date’.

What is absolutely clear is that the emergence of the Boas-Chomsky universal did not coincide with the speciation of H. sapiens. What is also clear is that it must have emerged in one, or more likely, multiple paleo-demes (breeding populations situated in space and time) and spread out from there by gene-flow. Once you separate the two events—the emergence of the Boas-Chomsky universal and the speciation of H. sapiens—as you have to on account of deep population structure in Africa, it is not at all obvious that the Boas-Chomsky universal arose at a single time and space.

The archaeological record shows evidence for symbolic storage appearing and disappearing for long periods of time both within and outside Africa (Blombos Cave, ~90ka; Sulawesi, ~44ka). In most of the world, symbolic storage becomes ubiquitous and universal by the onset of the LGM, 30ka. (The Americans clearly had the Boas-Chomsky universal when they were stranded in Beringia from ~30-15ka). But this is not true of all places, although taphonomy and poor preservation or coverage may be responsible for the patchy record. For instance, a plausible reason for cave paintings being hard to find between Sulawesi in the southeast Asia archipelago and Europe is that they were destroyed by bats. In the most extreme geographic isolates, symbolic storage does not appear until well into the Holocene (Australia, ~4ka), suggesting a much more complex process of the spread of the Boas-Chomsky universal. This complex and polarized pattern of the appearance of symbolic storage, proxies for the Boas-Chomsky universal, can no longer be evaded.

In my work with Noam Chomsky, I want to build a parsimonious model of the emergence and universalization by gene-flow of the Boas-Chomsky universal. Instead of pretending that the Boas-Chomsky universal arose before the last common ancestor of contemporary genetic populations (which is totally incongruent with the archaeological record) or positing just-so stories connecting the Cape to the peopling of the world, we should pay attention to how the archaeological record can be deployed to constrain different scenarios of the origins and spread of the Boas-Chomsky universal. For then we may get a better handle on the origins of the capacity that makes us truly human.