First Peoples: it’s complicated

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First Peoples is PBS’ attempts to update the story of human evolution for 2015, particularly informed by the ancient DNA revolution of the past 5 years. It is worth watching, but not without its faults. The first two episodes are online, and I’ve watched them. Kristina Kilgrove at Forbes has a review of the whole series up. Below are some impressions of the first two episodes.

300px-Namibian_Bushmen_GirlsOne immediate problem is that modern actors have to play the “First Peoples” in reenactments. This imposes limitations. The Africa episode begins with Omo 1, one of the first anatomically modern humans in the fossil record. When dramatically depicting his life and death the actors used looked vaguely East African to me. This stands to reason because Omo 1 died in southwest Ethiopia. But this individual died between 100 and 200 thousand years ago! There’s been a lot of population movement, and evolutionary change, between then and now. There’s no reason to assume that modern Africans are a good representation of ancient hominins who were resident within Africa. The Khoisan people of southern Africa, whose ancestry diverged first from the rest of modern humanity 150 to 200 thousand years ago, look notably different from their Bantu speaking neighbors. The Nilotic Luo of Kenya look different from their Bantu speaking Kikuyu neighbors. And so forth.

To be fair, there is a vaguely similar phenotype among some non-African people in relationship African groups. Melanesians and Andaman Islanders come to mind. Remember, these “African-looking” Asian and Oceanian groups are genetically no closer to Africans than a Swede or Native American (Oceanians are actually somewhat further because of more Denisovan admixture). The best modern genetic data point to common descent for non-Africans from one ancestral population, so that their descendants are symmetrically distinct from all Africans without recent Eurasian admixture.

The important point to remember with these phenotypic racial categories is that these groups are only vaguely similar. Even a geneticist like me can tell that Papuan Highlanders look quite distinct from any African group. Phenotypic variation among Sub-Saharan Africans, Oceanians, and Negrito Asian peoples, should tell us that the human populations of African 100 to 200 thousand years ago were probably also somewhat diverse, even if their expected range of variation is not arbitrary (e.g., it is likely that their skin was on the dark side; loci for pigmentation are somewhat functionally constrained around the tropics). But the expectation should not be overwhelming that modern Africans more accurately reflect the ancestral phenotype of tropical adapted modern humans than, say, Bogainville Islanders.*

My nit with this issue is that it feeds into a larger narrative of “ancient people” and “living fossils.” In the Africa episode the narrator states that Pygmies are closer to the earliest anatomically modern humans. This is wrong. All modern human populations are about equal close to the earliest anatomically modern humans. The main qualification here is that likely all of us have different proportions of ancestry form distinct “archaic” lineages. That is, hominin groups which are outside of the main branch that contributed between 90 to 99 percent of our ancestry. It does seem that the ancestors of the Pygmy groups of Central Africa are the second most basal group of humans in comparison to non-Africans (the most basal being Khoisan). But the generations between early humans and modern Pygmies is about the same as that between early humans and Europeans or Asians or Oceanians. Though we aren’t totally sure from what I know, it also seems likely that the earliest modern humans were residents of the open woodland ecology, and not the deep forest. The presence of Pygmy-like groups across tropical forest biomes from Gabon to the Philippines suggests that in fact this is a derived phenotype. An evolutionary change from the ancestral state.

519gldjJoAL._SY344_BO1,204,203,200_In general First Peoples does a rather good job by the standards of the media not recycling older models which have embedded an implicit “Great Change of Being.” Though I would find some minor fault in their depiction of the phylogenetic relationship of Pygmies and the first humans and humans more generally, their treatment of hybridization was mixed as best, and misleading at worst. In particular, the attitude toward the idea of species was confused and incoherent. The biological species concept (BSC) comes closest to colloquial understandings of what species are, but First Peoples seems to crystallize this framework as if it was an indubitable iron law of nature. With that in mind, rather frequent instances of hybridization between divergent hominin lineages seem more startling, adding dramatic twists to the narrative. The problem I have with this is that most biologists I know view the BSC as just one of many species concepts. It’s not written in stone, but rather, an instrumental device in getting science done. Obviously the biological species concept, predicated on sexual reproduction, is irrelevant for asexual organisms. Additionally, there are wide swaths of the tree of life where hybridization is ubiquitous. In particular, plants. It is in mammals, with our peculiar system of reproduction which involves rather complex mechanisms of gestation, that hybridization barriers are particularly high. But even among mammals there is variation in obstacles to hybridization conditional on placenta type.

With all that in mind it’s pretty unsurprising, with hindsight, that there was gene flow across diverged hominin lineages. First Peoples itself acknowledges this likelihood by referencing research on baboon hybrid zones, and asserting that this phenomenon might be totally par for the course among primates. There’s nothing special about humans biological makeup then that would preclude hybridization.

Not only does hybridization play a role in the emergence of new human traits (one researcher seems to imply that human faces are subject to a liger-like effect), but John Hawks seems to posit a multi-regionalist like model for the origin of modern human within Africa, whereby hybridization between lineages eventually produced a complex suite of characters which define modern humans. More specifically Hawks points to the archaeological record that suggests contacts all across the continent, which would also likely mediate gene flow from different African hominin groups. This is set as a counterpoint to the classic “African Eve” narrative, informed by mtDNA, in the 1980s. This model is illustrated by an explosion from a small East African group 50 to 100 thousand years ago, as it swept across the continent, and then the world. Today, we know the reality was more complex, and Hawks presents a reasonable contrast in perspective.

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The most recent evidence, informed by genomics, indicates that the migration out of Africa was the major bottleneck. Within Africa the situation is less clear. Hawks thesis of a sort of multi-regionalist network of hominin lineages may be correct. In fact much of human evolutionary may be characterized by a series of alternating phases of multi-regional evolution mediated by gene flow and admixture, along with rapid demographic expansions whereby one lineage overwhelms the others (much of this may be due to cultural changes and inter-group dynamics). This is a complicated model, and hard to render in TV-friendly units of consumption. I feel for the producers of First Peoples.

Which brings me to a quibble about the America episode. Overall I really enjoyed how they framed the slow erosion of the Clovis overkill hypothesis. I’ll be honest that the “unveiling” of the Kennewick Man results, which suggest that he was the ancestor of modern groups in the region, was overly dramatic for me. But the biggest issue I had is that it oversimplifies the peopling of the Americas. It does seem that the New World was predominantly populated by the first wave out of Berengia, but there is also genetic evidence that later waves were significant, even in the Kennewick results themselves!

First Peoples is worth watching. Definitely a step forward, as they put a lot of new concepts in front of the general public. But everyone is still figuring things out, the science, and how to present it to people. I’d say that you just have to remember this is an alpha version, and if you want to know the whole story you had better do your follow up.

* Let me be clear here that I understand that some prejudice should be given to the African phenotypes as being more likely, as that continent was the ancestral home with particular environmental conditions which have not varied. And, Oceanians have admixture from other hominin lineages. But to me the priors are not such that the case is overwhelming. Ancient modern humans may have looked nothing like any extant population!

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