The culture of reasoning: the Ummah shall not agree upon error


Because I watch Screen Junkies‘ “Honest Trailers” I get recommendations like the above from Looper, The Real Reason Why Valerian Flopped At The Box Office. Of course no one knows the ‘real reason’ Valerian flopped, aside from “it didn’t seem like a good movie.” The reality is that Valerian and the City of a Thousand Planets based on a French comic book and cast a 31 year old actor who looks like a haunted 15 year old. That’s all there is to say definitively. All the various failure points are overdetermined. But the video above gives you a lot of “reasons” if you want them in a list format in a British accent. All in the service of infotainment.

Hugo Mercier and Dan Sperber’s The Enigma of Reason offers up an explanation for you have things like “top 10 reasons” for pop culture artifacts of an ephemeral nature (for a preview, The Function of Reason at Edge).

I’ve mentioned this book a few times. I finished while in the Persian Gulf (I’ll blog that at some point soon), and have been ruminating on its implications, and whether to mention it further. The issue I’m having is that I am very familiar with Sperber’s work, and those who he has influenced, and research domains complementary to his. Even if I didn’t know all the details of the argument in The Enigma of Reason, in the broad sketches I knew where they were going, and frankly I could anticipate it. I suppose somewhat ironically I managed to infer and reason ahead of the narrative since I had so many axioms from earlier publications.

The Enigma of Reason comes out of a particular tradition in cognitive anthropology. What Dan Sperber terms the “naturalistic paradigm” in anthropology. This is in contrast to the more interpretative framework that you are probably familiar with in the United States. No one would deny that the naturalistic paradigm has scientific aspirations. That is, it draws from natural science (in particular cognitive anthropology as well as the field of cultural evolution), and conceives of itself as the study of natural phenomenon.

Scott Atran’s In Gods We Trust: The Evolutionary Landscape of Religion comes out of this tradition, and some of the experimental literature in The Enigma of Reason seem very familiar from the earlier book (as well as Pascal Boyer’s Religion Explained). This is due to the fact that Atran goes to great lengths to show the ultimate nature of religion does not have to do with rational inferences as we understand them. That is, theological is a superstructure overlain atop a complex phenomenon which is not about philosophical reflection at all.

Of course the flip side can be true as well. When I was a teenager and younger adult I explored the literature on the existence of God a bit, from old classics like Thomas Aquinas’ arguments in Summa Theologica, to more recent and contrasting proofs of Norman Malcolm and Richard Swinburne (Michael Martin’s Atheism: A Philosophical Justification was actually a good sourcebook for high level arguments to theism).

When I read In Gods We Trust I realized that my earlier explorations were primarily intellectual justifications, and had little relationship why most people around me believed in God. And yet how did I become an atheist? For me this is a flashbulb memory. I was eight years old, in the public library. I was thumbing through the science books in the children’s section (particular, books on biology and medicine). The third row from the front of the stacks. And all of a sudden I had the insight that there wasn’t a necessary reason for the existence of God.

It all happened over the course of a minute or so. Mind you, I was never really religious in any deep sense. Something I’ve confirmed when talking to religious friends about their beliefs and how it impacts them. Though I nominally adhered to my parents’ religion when I was a small child, I was fascinated much more by science, and that really engaged most of my thoughts and guided my actions (contrastingly, going to the mosque was one of the most horribly boring things I recall doing as a small child).

My point here is that many of our beliefs are arrived at in an intuitive manner, and we find reasons to justify those beliefs. One of the core insights you’ll get from The Enigma of Reason is that rationalization isn’t that big of a misfire or abuse of our capacities. It’s probably just a natural outcome for what and how we use reason in our natural ecology.

Mercier and Sperber contrast their “interactionist” model of what reason is for with an “intellectualist: model. The intellecutalist model is rather straightforward. It is one where individual reasoning capacities exist so that one may make correct inferences about the world around us, often using methods that mimic those in abstract elucidated systems such as formal logic or Bayesian reasoning. When reasoning doesn’t work right, it’s because people aren’t using it for it’s right reasons. It can be entirely solitary because the tools don’t rely on social input or opinion.

The interactionist model holds that reasoning exists because it is a method of persuasion within social contexts. It is important here to note that the authors do not believe that reasoning is simply a tool for winning debates. That is, increasing your status in a social game. Rather, their overall thesis seems to be in alignment with the idea that cognition of reasoning properly understood is a social process. In this vein they offer evidence of how juries may be superior to judges, and the general examples you find in the “wisdom of the crowds” literature. Overall the authors make a strong case for the importance of diversity of good-faith viewpoints, because they believe that the truth on the whole tends to win out in dialogic formats (that is, if there is a truth; they are rather unclear and muddy about normative disagreements and how those can be resolved).

The major issues tend to crop up when reasoning is used outside of its proper context. One of the literature examples, which you are surely familiar with, in The Enigma of Reason is a psychological experiment where there are two conditions, and the researchers vary the conditions and note wide differences in behavior. In particular, the experiment where psychologists put subjects into a room where someone out of view is screaming for help. When they are alone, they quite often go to see what is wrong immediately. In contrast, when there is a confederate of the psychologists in the room who ignores the screaming, people also tend to ignore the screaming.

The researchers know the cause of the change in behavior. It’s the introduction of the confederate and that person’s behavior. But the subjects when interviewed give a wide range of plausible and possible answers. In other words, they are rationalizing their behavior when called to justify it in some way. This is entirely unexpected, we all know that people are very good at coming up with answers to explain their behavior (often in the best light possible). But that doesn’t mean they truly understanding their internal reasons, which seem to be more about intuition.

But much of The Enigma of Reason also recounts how bad people are at coming up with coherent and well thought out rationalizations. That is, their “reasons” tend to be ad hoc and weak. We’re not very good at formal logic or even simple syllogistic reasoning. The explanation for this seems to be two-fold.

First, reason is itself an intuitive process.

For the past few weeks we’ve had an intern at the office. I’ve given them a project using Python…a language they barely know. One of the things that is immediately obvious when going through pitfalls is that a lot of the debugging process relies on intuition one accrues over time, through trial and error. When someone is learning a programming language they don’t have this intuition, so bugs can be extremely difficult to overcome since they don’t have a good sense of the likely distribution of probabilities of the errors they’d introduce into the system (or, to be concrete, a novice programmer might not even recognize that there’s an unclosed loop, when that is one of the most obvious errors to anyone).

Second, reason is an iterative process which operates optimally in a social context. While  The Enigma of Reason reviews all the data which suggests that humans are poor at formal logic and lazy in relation to production of reasons, the authors also assert that we are skeptical of alternative models. This rings true. I recall an evangelical Protestant friend who once told me how ridiculous the idea of Hindu divine incarnations were. He was less than pleased with I simply switched his logic to a Christian context. But Mercier and Sperber suggest that these two features of loose positive production of reasons and tighter negative skepticism of those reasons come together in a social context to converge upon important truths which might increase our reproductive fitness.

The framework above is fundamentally predicated on methodological individualism, focusing in natural selection at that level. The encephalization of humans over the past two million years was driven by increased social complexity, and this social complexity was enabled by the powerful ability to reason and relate by individual humans. In some ways  The Enigma of Reason co-opts some of the same arguments presented by Robin Dunbar over ten years ago in Grooming, Gossip, and the Evolution of Language, except putting the emphasis on persuasion and reasoning.

At this point we need to address the elephant in the room: some humans seem extremely good at reasoning in a classical sense. I’m talking about individuals such as Blaise Pascal, Carl Friedrich Gauss, and John von Neumann. Early on in The Enigma of Reason the authors point out the power of reason by alluding to Eratosthenes’s calculation of the circumference of the earth, which was only off by one percent. Myself, I would have mentioned Archimedes, who I suspect was a genius on the same level as the ones mentioned above.

Mercier and Sperber state near the end of the book that math in particular is special and a powerful way to reason. We all know this. In math the axioms are clear, and agreed upon. And one can inspect the chain of propositions in a very transparent manner. Mathematics has guard-rails for any human who attempts to engage in reasoning. By reducing the ability of humans to enter into unforced errors math is the ideal avenue for solitary individual reasoning. But it is exceptional.

Second, though it is not discussed in The Enigma of Reason there does seem to be variation in general and domain specific intelligence within the human population. People who flourish in mathematics usually have high general intelligences, but they also often exhibit a tendency to be able to engage in high levels of visual-spatial conceptualization.

One the whole the more intelligent you are the better you are able to reason. But that does not mean that those with high intelligence are immune from the traps of motivated reasoning or faulty logic. Mercier and Sperber give many examples. There are two. Linus Pauling was indisputably brilliant, but by the end of his life he was consistently pushing Vitamin C quackery (in part through a very selective interpretation of the scientific literature).* They also point out that much of Isaac Newton’s prodigious intellectual output turns out to have been focused on alchemy and esoteric exegesis which is totally impenetrable. Newton undoubtedly had a first class mind, but if the domain it was applied to was garbage, then the output was also garbage.

A final issue, which is implicit in the emergence of genius is that it exists in can only manifest in a particular social context. Complex societies with some economic surplus and specialization are necessary for cognitive or creative genius to truly shine. In a hunter-gatherer egalitarian society having general skills to subsist on the Malthusian margin is more critical than being an exceptional mind.**

Overall, the take-homes are:

  • Reasoning exists to persuade in a group context through dialogue, not individual ratiocination.
  • Reasoning can give rise to storytelling when prompted, even if the reasons have no relationship to the underlying causality.
  • Motivated reasoning emerges because we are not skeptical of the reasons we proffer, but highly skeptical of reasons which refute our own.
  • The “wisdom of the crowds” is not just a curious phenomenon, but one of the primary reasons that humans have become more socially complex and our brains have larger.

Ultimately, if you want to argue someone out of their beliefs…well, good luck with that. But you should read The Enigma of Reason to understand the best strategies (many of them are common sense, and I’ve come to them independently simply through 15 years of having to engage with people of diverse viewpoints).

* R. A. Fisher, who was one of the pioneers of both evolutionary genetics and statistics, famously did not believe there was a connection between smoking and cancer. He himself smoked a pipe regularly.

** From what we know about Blaise Pascal and Isaac Newton, their personalities were such that they’d probably be killed or expelled from a hunter-gatherer band.

The search for Eden opens up new vistas

The end of Eden

A particular conception of the “Out of Africa” model of human origins died in this decade. This model hooked into preexistence narratives about “Adam” and “Eve”, utilizing Y and mitochondrial DNA lineages passed down through direct male and female lines respectively. Its most extreme manifestation could be exemplified by Richard Klein’s ideas in the early 2000s outlined in his book The Dawn of Human Culture.

For Klein the chasm between Homo sapiens sapiens, humans, and other hominins was vast. A physical anthropologist who surveyed with skill the rapid expansion and proliferation of modern human cultures over the past ~50,000 years, Klein relied on a particular evolutionary model to explain how this occurred. He posited that humankind emerged in East Africa as a punctuated speciation event, triggered by a mutation which allowed for the development of fully elaborated recursive language.

The difference between our own lineage and our relatives in this framework was huge. To not put too fine a point on it, Neanderthals and other archaic humans were animals. We, Homo sapiens sapiens, were humans qua humans.

Though Klein was a paleoanthropologist, he gained great support from a school of molecular evolution which arose in the 1970s and 1980s under Allan Wilson. Wilson’s initial fame arose because he utilized a “molecular clock” analysis of primates to contend that the divergence of our human lineage from great apes was much more recent than paleontologists had believed. Eventually new fossil finds confirmed the molecular phylogeny. After this event Richard Leakey has stated paleoanthropologists were reluctant to challenge molecular results.

Wilson later focused on recent on human origins, utilizing mitochondrial DNA, which is passed down directly through the maternal lineage. In this way they found that African mtDNA lineages were very diverse, and that non-African lineages were nested within the broader tree of African lineages.

The conclusion from this finding was that modern humans arose in Africa and spread to other parts of the world. This conclusion in general has been confirmed.

But over the years more and more evidence has accumulated that the story is more complicated than the original narrative that all modern humans descend from a small bad of East Africans who populated the whole world ~50,000 years ago.

Dissenters from Eden

There were always geneticists who were skeptical of the neat Out of Africa with total replacement model. In Origins Reconsidered Richard Leakey recounts a conference in 1992 where he was pigeon-holed by geneticists who thought there was no reason to accept without dispute the mitochondrial Eve narrative. Over the years talking to some older geneticists I can say that Leakey was reporting a real undercurrent of irritation with the confidence that Allan Wilson’s group and their fellow travelers projected in relation to their model. Nordborg 1998 On the probability of Neanderthal ancestry reflects some of the technical objections to inferring too much from one locus when it came to the possibility of other components of ancestry.

When genome-wide analyses in the middle 2000s became feasible, a visible counter-culture within genetics argued that total replacement was not supported by the data. In 2006 Wall & Hammer published Archaic admixture in the human genome. They concluded that “Recent work suggests that Neanderthals and an as yet unidentified archaic African population contributed to at least 5% of the modern European and West African gene pools, respectively.” They were not that far off with European populations. As far as Africa goes, that is a question that will be explored in detail in the next few years.

That analysis though has only 44 citations. I have had debates on Twitter with how exotic and marginal these ideas were. In general it is safe to say that they were not exotic and marginal in the community of human evolutionary population geneticists. But that’s not a large set. Both John Hawks and Milford Wolpoff have indicated a lot of marginalization for models outside of the narrow window of Out of Africa with total replacement. From everything I’ve heard about the run up to the 2010 publication of the Neanderthal genome many of the principal researchers, including Svante Paabo, were totally surprised by the evidence of admixture into modern lineages. Wolopff even emailed me after I reviewed the paper to suggest that it felt so good to come out of the wilderness and have some of his views accepted.

Anagenesis and punctuation?

But were Wolpoff’s views accepted? The revised model actually kept much of the Out of Africa framework in place, except it added the wrinkle of assimilation of some archaic lineages. The dominant signal in the non-African genomes seems to have come from an African lineage which left around ~50,000 years ago.

The classical multi-regional model that Wolpoff was associated with, whereby modern humans evolved across the whole world from local archaic lineages, but maintained species cohesion through gene flow, was not supported. Rather, the archaic admixture of Neanderthals and Denisovans into Oceanians pointed to local continuities, which was a broader position of multi-regionalism. But this is not speciation without branching, anagenesis.

Nevertheless, there was another aspect of Out of Africa with replacement that needed revision. Though not explicitly outlined in many framings, one aspect implicit is that the dynamics that Africa and Eurasia were subject to during the emergence of modern humans were the same.

But that doesn’t seem to be the case. The ancestors of all non-Africans went through a major population bottleneck. On the order of ~1,000 individuals (this is a very large bottleneck actually, and I’ve seen numbers as low as 100, though that seems on the small side; calculating effective population size ~50,000 years ago can be tricky). The same is not true of African populations. Though many of them show signals of population declines during the Pleistocene, the extreme uniform bottleneck which characterizes all non-Africans, from Iberia to Australia to Patagonia is just not evident in Sub-Saharan African populations.

In other words, the Out of Africa event did not apply within Africa. Here’s an excerpt of an email I sent to Carl Zimmer in December of 2010 (he was updating the second edition of The Tangled Bank):

…it may be that there was no rapid antique population expansion in Africa which was analogous to [the] out of Africa migration. IOW, non-Africans are just a branch of Northeast Africans, and the Bushmen and other groups were already differentiated by that point. So you could theoretically remove the arrows within Africa! I think this is a subtle and tendentious point, so probably best to leave that as it is. But remember how deep the basal branching of the Bushmen was in the Denisova paper? It WAY predates any possible out of Africa migration by multiples.

Which brings me to the current year and the present time. The recent paper which utilized an ancient genome from South Africa to push back the date of the diversification of African lineages to about ~250,000 years before the present was not entirely surprising to me. Every time I talked to people who had access to African whole genomes their dates kept getting pushed back further and further into the past.

And of course we now have fossil confirmation that human populations which seemed to be anatomically modern (or close) were already present ~300,000 years ago in Morocco. The New York Times has a good overview of the work, Oldest Fossils of Homo Sapiens Found in Morocco, Altering History of Our Species. I read the papers and the commentaries and don’t have much to add, nor do they add much for non-specialists in my opinion (since we can’t really judge the morphology too well, nor do we have a detailed understanding of the fossil record). In one of the Nature letters the authors conclude in the abstract that “The emergence of our species and of the Middle Stone Age appear to be close in time, and these data suggest a larger scale, potentially pan-African, origin for both.”

This suggest to me anagenesis. Has multi-regionalism come back, but no within Africa?

Parameters, not paradigms

John Hawks has put in his two cents, and it’s always worth paying attention. My major take home is that we don’t know a lot even though we know more, and we need to be careful here. The genome blogger from the 2000s who has been relatively quiet over the last five years, Dienekes, resurfaced, dismissing the idea of pan-African anagenesis and asserting an Out of North Africa viewpoint. He’s been talking about this model since 2011, so there’s nothing new here. In January of 2011 he asserted that “Africa was home to a structured population.” That is what we are seeing today.

The publication of the Nature letters triggered a lot of discussion on Twitter. When I was involved it mostly consisted of Aylwyn Scally and Pontus Skoglund, with John Hawks, Chris Stringer, and others jumping into the stream. Here are some points which are of note:

1) Most people now suspect that large scale population structure within Africa over the past few hundred thousand years is a major story.

2) But there is an assumption that collapsing of that structure through gene flow was not reciprocal. That is, some populations likely expanded at the expense of others. The arguments are whether the assimilation of the secondary groups is on the order of a few percent, as seems to be the case in Eurasia, or a much higher fraction.

3) Because the phylogenetic distance between within African lineages is likely smaller than between Neanderthals and modern humans, as well as the likely similar census sizes and technological toolkits, I contended that it is not unreasonable to guess that as much as 20% of the ancestry of a daughter population of an expanding group could be from the local substrate. There was no great objection to this guess.

4) Remember, even simple mtDNA phylogenies as far back as the 1980s, as well as paleontological analyses of fossils, indicated that an Out of Africa movement into Eurasia. This was such a strong signal in the data that it was clear with even relatively little to go on. The situation for within Africa is not analogous, suggesting to me that an extreme model of replacement or gene flow across persistent demes in local regions is not tenable.

5) Ultimately, the issue will resolve on parameters of admixture and the nature of demographic expansion in the details. Instead of a tree, we will conceive of this as a graph, a trellis with lengths of different thicknesses.

6) Hawks brought up the fact that one reason classic multi-regionalism did not work is that the Fisher wave of expansion of favored genes is slower than the migration of humans. When I suggested it does not seem that the genetics of gene flow in plants, which do resemble classical multi-regionalism, were a good analogy for humans, Skoglund contrasted the sessile nature of the taxon in contrast to mobile humans. I did point out though that after favored alleles moved through migration into a population, there was often in situ selection. He agreed.

7) A key issue that both Hawks and Dienekes emphasize is that we don’t know the role that extremely diverged lineages from our own ancestors play in our story. That is, were there many modern human populations across Africa, interspersed with other human species? Or was there one modern human population that mixed with other species? We don’t really know the details of all of this.

8) I expressed skepticism of the idea of “behavioral modernity.” My reason for being skeptical is that the origin of modern humans is not as neat as we like to think, and the origin of “behavioral modernity” is also not as neat as we like to think. When the consensus was that humans emerged as a punctuated de novo event, ensouled by the Lord God on High 50,000 years ago (or, coming down from the skies as in Battlestar Galactica or in Larry Niven’s Ringworld), the idea of behavioral modernity kind of made sense. But it’s all more confused now (in any case, in Clive Finlayson’s The Humans Who Went Extinct he seems to be arguing that much of modern culture was invented by Gravettians, well after the Out of Africa event).

The consensus seems to be that rather than focusing on a set of human universals as behaviorally modern, we should look at the demographic patterns of the past to infer when our own lineage came into its distinctive being. Those of you who have read me for a while know this is already congenial to me, Luke Jostins’ plot of the encephalization of all hominin lineages over the past million years was suggestive to me long ago that our own lineage is not so special. Rather, something like us was probably inevitable so long as an asteroid didn’t wipe out large mammals once Homo erectus spread across the globe. Humanity is a destiny, not a lineage.

A reticulation + pulse expansion of modern human genetic variation

In response to a little bit of fatigue at the constant stream of ancient DNA, John Hawks digs the knife in a bit deeper. There’s more. Since Hawks is co-author with Lee Berger of Almost Human: The Astonishing Tale of Homo Naledi and the Discovery That Changed Our Human Story, I’m hoping for something related to naledi. But barring that, there is still lots to come down the pipeline (Alexander Kim has a Twitter account worth following to keep up on this line of research).

But I thought I’d enter something else into the record about what we’re starting to understand about the origins of modern human genetic variation.

  1. The “Out of Africa” movement is a real thing. That is, ~50,000 years ago a population seems to have diverged from a broader pan-African group, and replaced archaic hominins across Eurasia (with some admixture at low levels), and then pushed the boundaries of our genus to Oceania and the New World. This does not mean that there were not earlier “Out of Africas.” Just that the dominant signal of variation is due to the pulse that swept out ~50,000 years ago.
  2. Within Africa the story is different and more complicated. The expansion of anatomically modern humans out of Africa occurred relatively rapidly from a small founder population (within an order of magnitude of ~1,000 individuals seems correct). The archaeology and genetics are in pretty good alignment. But within Africa it looks like the lineages which led to modern humans are much deeper, and preserve structure that may be hundreds of thousands of years old. Just as we see admixture events giving rise to new lineages outside of Africa over the last 50,000 years, the same dynamic probably applied to within Africa far earlier.

One way to think about it is that the old “Africa Eve” model is very useful for the 10,000 year period around 50,000 years ago. And, it applies to non-Africans.

The story within Africa though may be more like the old multi-regionalist model, though with stronger biases of gene flow between populations, so that at one time one lineage may be preponderant. Over the last 10,000 years the expansion of certain populations within Africa though (in particular Bantus) has collapsed a lot of the deep structure, and now Africa resembles Eurasia much more (with some Eurasian back-migration too).

Note: I am talking about “modern human genetic variation” because I am starting to think talking about “modern humans” obscures far more than it illuminates.

Beyond “Out of Africa” and multiregionalism: a new synthesis?

For several decades before the present era there have been debates between proponents of the recent African origin of modern humans, and the multiregionalist model. Though molecular methods in a genetic framework have come of the fore of late these were originally paleontological theories, with Chris Stringer and Milford Wolpoff being the two most prominent public exponents of the respective paradigms.

Oftentimes the debate got quite heated. If you read books from the 1990s, when multiregionalism in particular was on the defensive, there were arguments that the recent out of Africa model was more inspirational in regards to our common humanity. As a riposte the multiregionalists asserted that those suggesting recent African origins with total replacement was saying that our species came into being through genocide.

Though some had long warned against this, the dominant perception outside of population genetics was that results such the “mitochondrial Eve” had given strong support to the recent African origin of modern humans, to the exclusion of other ancestry. 2002’s Dawn of Human Culture took it for granted that the recent African origin of modern humans to the total exclusion of other hominin lineages was established fact.

In 2008 I went to a talk where Svante Paabo presented some recent Neanderthal ancient mtDNA work. It was rather ho-hum, as Paabo showed that the Neanderthal lineages were highly diverged from modern ones, and did not leave any descendants. Though of course most modern human lineages did not leave any descendants from that period, Paabo took this evidence supporting the proposition that Neanderthals did not contribute to the modern human gene pool.

When his lab reported autosomal Neanderthal admixture in 2010, it was after initial skepticism and shock internally. I know Milford Wolpoff felt vindicated, while Chris Stringer began to emphasize that the recent African origin of modern humanity also was defined by regional assimilation of other lineages. The data have ultimately converged to a position somewhere between the extreme models of total replacement or balanced and symmetrical gene flow.

This is not surprising. Extreme positions are often rhetorically useful and popular when there’s no data. But reality does not usually conform to our prejudices, so ultimately one has to come down at some point.

The data for non-Africans is rather unequivocal. The vast majority of (>90%) of the ancestry of non-Africans seems to go back to a small number of common ancestors ~60,000 years ago. Perhaps in the range of ~1,000 individuals. These individuals seem to be a node within a phylogenetic tree where all the other branches are occupied by African populations. Between this period and ~15,000 years ago these non-Africans underwent a massive range expansion, until modern humans were present on all continents except Antarctica. Additionally, after the Holocene some of these non-African groups also experienced huge population growth due to intensive agricultural practice.

To give a sense of what I’m getting at, the bottleneck and common ancestry of non-Africans goes back ~60,000 years, but the shared ancestry of Khoisan peoples and non-Khoisan peoples goes back ~150,000-200,000 years. A major lacunae of the current discussion is that often the dynamics which characterize non-Africans are assumed to be applicable to Africans. But they are not.

A 2014 paper illustrates one major difference by inferring effective population from whole genomes: African populations have not gone through the major bottleneck which is imprinted on the genomes of all non-African populations. The Khoisan peoples, the most famous of which are the Bushmen of the Kalahari, have the largest long term effective populations of any human group. The Yoruba people of Nigeria have a history where they were subject to some population decline, but not to the same extent as non-Africans.

What do we take away from this?

One thing is that we have to consider that the assimilationist model which seems to be necessary for non-Africans, also applies to Africans. For years some geneticists have been arguing that some proportion of African ancestry as well is derived from lineages outside of the main line leading up to anatomically modern humans. Without the smoking gun of ancient genomes this will probably remain a speculative hypothesis. I hope that Lee Berger’s recent assertion that they’ve now dated Homo naledi to ~250,000 years before the present may offer up the possibility that ancient DNA will help resolve the question of African archaic admixture (i.e., if naledi is related to the “ghost population”?).

The second dynamic is that the bottleneck-then-range-expansion which is so important in defining the recent prehistory of non-Africans is not as relevant to Africans during the Pleistocene. The very deep split dates being inferred from whole genome analysis of African populations makes me wonder if multiregional evolution is actually much more important within Africa in the development of modern humans in the last few hundred thousand years. Basically, the deep split dates may highlight that there was recurrent gene flow over hundreds of thousands of years between different closely related hominin populations in Africa.

Ultimately, it doesn’t seem entirely surprising that the “Out of Africa” model does not quite apply within Africa.

Addendum: Over the past ~5,000 years we have seen the massive expansion of agricultural populations within the continent. The “deep structure” therefore may have been erased to a great extent, with Pygmies, Khoisan, and Hadza, being the tip of the iceberg in terms of the genetic variation which had characterized the Africa during the Pleistocene.

The logic of human destiny was inevitable 1 million years ago

Robert Wright’s best book, Nonzero: The Logic of Human Destiny, was published nearly 20 years ago. At the time I was moderately skeptical of his thesis. It was too teleological for my tastes. And, it does pander to a bias in human psychology whereby we look to find meaning in the universe.

But this is 2017, and I have somewhat different views.

In the year 2000 I broadly accepted the thesis outlined a few years later in The Dawn of Human Culture. That our species, our humanity, evolved and emerged in rapid sequence, likely due to biological changes of a radical kind, ~50,000 years ago. This is the thesis of the “great leap forward” of behavioral modernity.

Today I have come closer to models proposed by Michael Tomasello in The Cultural Origins of Human Cognition and Terrence Deacon in The Symbolic Species: The Co-evolution of Language and the Brain. Rather than a punctuated event, an instance in geological time, humanity as we understand it was a gradual process, driven by general dynamics and evolutionary feedback loops.

The conceit at the heart of Robert J. Sawyer’s often overly preachy Neanderthal Parallax series, that if our own lineage went extinct but theirs did not they would have created a technological civilization, is I think in the main correct. It may not be entirely coincidental that the hyper-drive cultural flexibility of African modern humans evolved in African modern humans first. There may have been sufficient biological differences to enable this to be likely. But I believe that if African modern humans were removed from the picture Neanderthals would have “caught up” and been positioned to begin the trajectory we find ourselves in during the current Holocene inter-glacial.

Luke Jostins’ figure showing across board encephalization

The data indicate that all human lineages were subject to increased encephalization. That process trailed off ~200,000 years ago, but it illustrates the general evolutionary pressures, ratchets, or evolutionary “logic”, that applied to all of them. Overall there were some general trends in the hominin lineage that began to characterized us about a million years ago. We pushed into new territory. Our rate of cultural change seems to gradually increased across our whole range.

One of the major holy grails I see now and then in human evolutionary genetics is to find “the gene that made us human.” The scramble is definitely on now that more and more whole genome sequences from ancient hominins are coming online. But I don’t think there will be such gene ever found. There isn’t “a gene,” but a broad set of genes which were gradually selected upon in the process of making us human.

In the lingo, it wasn’t just a hard sweep from a de novo mutation. It was as much, or even more, soft sweeps from standing variation.