The vines around the tree trunks


A lot of the understanding of scientific theories and models in the public domain is communicated by evocative metaphors and turns of phrase. For example, Charles Darwin famously wrote:

It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us….

When it comes to understanding the origin of our own species and the broader human lineage over the past two million years, I’ve started to come to a mental model of a weighted-graph with edges. Some of the edges traverse time and have strong weights. These are analogous to the normal phylogenetic tree model, representing phyletic gradualism and anagenesis along each branch before some bifurcation event. But, some of the edges move horizontally between others. These represent migration and/or gene flow between the primary lineages.

I’m not sure though that a graph theory derived mental model helps many people, so I’ll use another one: imagine large trunks defining the primary lineages, and vines tying them together representing gene flow events. The above figure is from a new preprint, Mapping gene flow between ancient hominins through demography-aware inference of the ancestral recombination graph. This is a methods-heavy preprint. It utilizes an “ancestral recombination graph” (so a model of the genealogy of genes in the genome) and MCMC generate Bayesian probabilities of particular events (e.g., introgression of a lineage that diverged x years ago at fraction y).

The abstract presents some specific findings:

…While much attention has been paid to the relatively recent gene flow from Neanderthals and Denisovans into modern humans, other instances of introgression leave more subtle genomic evidence and have received less attention. Here, we present an extended version of the ARGweaver algorithm, ARGweaver-D, which can infer local genetic relationships under a user-defined demographic model that includes population splits and migration events. This Bayesian algorithm probabilistically samples ancestral recombination graphs (ARGs) that specify not only tree topology and branch lengths along the genome, but also indicate migrant lineages…We show that this method is well powered to detect the archaic migration into modern humans, even with only a few samples…We apply it to human, Neanderthal, and Denisovan genomes, looking for signatures of older proposed migration events, including ancient humans into Neanderthal, and unknown archaic hominins into Denisovans. We identify 3% of the Neanderthal genome that is putatively introgressed from ancient humans, and estimate that the gene flow occurred between 200-300kya. We find no convincing evidence that negative selection acted against these regions. We also identify 1% of the Denisovan genome which was likely introgressed from an unsequenced hominin ancestor, and note that 15% of these regions have been passed on to modern humans through subsequent gene flow.

ARGweaver-D is gnarly. Not in a bad way. But you should never really trust computational wizard of this sort unless you’ve taken it for a test drive, or it’s been around decades and people have validated it. A “play with the parameters” phase is necessary for these packages to become more than magic.

That being said, for about half a decade people have been detecting evidence of a “super-archaic” lineage within Denisovans. This is just another confirmation with another method. The super-archaic hypothesis seems plausible as an explanation of the patterns in the data (there may be other explanations). Second, there’s a lot of circumstantial evidence for gene flow into Neanderthals from moderns. E.g., mtDNA replacement in Neanderthals. Though not in the abstract, the preprint mentions the likelihood of “super-archaic” introgression into Neanderthals as well. From a recent ancient DNA paper on Nuclear DNA from two early Neandertals reveals 80,000 years of genetic continuity in Europe:

We find that population split times between HST and other Neandertals of less than 150 ka ago make the occurrence of a mitochondrial time to the most recent common ancestor (TMRCA) of 270 ka ago unlikely (1.2% of all simulated loci have such a deep TMRCA; note S11). We note that this result is robust to uncertainties in the estimates of the Neandertal population size and of the mitochondrial TMRCA (note S11). The presence of this deeply divergent mtDNA in HST thus suggests a more complex scenario in which HST carries some ancestry from a genetically distant population.

It seems entirely likely that we’re going to see “shadows of forgotten ancestors” in our genomes. But wait, there’s more!

…ARGweaver-D only detected a small amount of Sup→Afr introgression, which was somewhat lower than our estimated false positive rate. One aspect to note here is that the power to identify introgression from an unsequenced population is highly dependent on the population size of the recipient population. The larger the population, the deeper the coalescences are within that population, making it more difficult to discern which long branches might be explained by super-archaic introgression…If we had used a smaller population size, ARGweaver-D would have produced more Sup→Afr predictions, but most of these would be false positives unless that smaller population size is closer to the truth. Overall, we caution that the problem of detecting super-archaic introgression into a large and structured population such as Africas is very difficult and that claims of such introgression need to be robust to the demographic model used in analysis. It may not be possible to address the question of ancient introgression into Africans without directly sequencing fossils from the introgressing population.

In northern Eurasia, in particular, one might imagine a scenario with large fluctuations in population size, and patchy landscapes. This would reduce gene flow between populations, and also foster drift to produce distinct lineages. Simple stylized models of gene flow at particular times across disparate lineages makes a great deal of sense in this context. But if Africa had larger populations of humans, with more interconnected networks with continuous, if variable, levels of gene flow then the stylized models will mislead in important features.

This preprint is likely reporting some true robust results that will hold up. But I think the bigger picture is that it will lead us toward moving beyond the extremely simple models in vogue a generation ago, to a more subtle understanding of complex emergence and collapse of human population structure over the last two million years.

The natural history of song – a human universal


“I am human, and I think nothing human is alien to me.”
– Terrence

One of the bizarre things about modern cultural anthropology is that its tendency toward extreme relativism means that it engages in so much “thick description” that generalities of humanity disappear in the avalanche of prose. A deep sense of ontological incommensurability creeps into the discussions of cross-cultural patterns. The prestige, what there is, of academic anthropology, then infects normal people, so that some can say that religion, as understood in the West, is qualitatively different from religion understand in the East. With a straight face.

I think this is wrong and leads us down a path to intellectual nihilism (well, actually, we’re at the end of that path today aren’t we!).

This sort of thing applies to other cultural phenomena as well. Consider music and in particular song. A new preprint uses the Human Area Relation Files (an ethnographic database) to statistically analyze patterns in songs across many societies, A natural history of song:

What is universal about music across human societies, and what varies? We built a corpus of ethnographic text on musical behavior from a representative sample of the world’s societies and a discography of audio recordings of the music itself. The ethnographic corpus reveals that music appears in every society observed; that variation in musical behavior is well characterized by three dimensions, which capture the formality, arousal, and religiosity of song events; that musical behavior varies more within societies than across societies on these dimensions; and that music is regularly associated with behavioral contexts such as infant care, healing, dance, and love. The discography, analyzed through four representations (machine summaries, listener ratings, expert annotations, expert transcriptions), revealed that identifiable acoustic features of songs predict their primary behavioral function worldwide, and that these features fall along two dimensions, melodic and rhythmic complexity. These analyses show how applying the tools of computational social science to rich bodies of humanistic data can reveal both universal features and patterns of variability in culture, addressing longstanding debates about each.

The figure at the top reports the three largest components of variation, formality, arousal, and religiosity. Not surprisingly, some types of songs are more weighted toward one feature than another. Lullabies are not particularly religious, arousing, or formal.

Interestingly, the vast majority of variation in songs is found within societies, not between them. There is some difference, with some societies lacking formal songs, at least in the ethnographic record. But, this illustrates that the basic repertoire for this cultural feature was probably present by the late Pleistocene in our species.

Songs seem to be aspects of human behavior which are both consumption and production goods. That is, on the individual and social level we consume songs for pleasure. On the individual level songs are essential parts of the parental toolkit to soothe the infant beast. They also serve a purpose in society to generate cohesion and produce fellow feeling. This is clear in a confessional religious context, but consider that drummers were important elements of the Ottoman war machine. Human cultural phenomena are so often multivalent that they need to be inspected and examined from a variety of dimensions.

I’m not a very musical person myself, so there is obviously individual variation in the ability to appreciate or produce music. But the basic cognitive toolkit seems to emerge out of a concert of neurological processes, somewhat distinct from common language (as evident by aphasics who can sing but can not speak).

It’s a Denisovan world, and paleoanthropology just lives in it!


Unless you’ve been sleeping under a rock, you may have seen a new paper, A late Middle Pleistocene Denisovan mandible from the Tibetan Plateau. The reason it is a big deal is that except for a fragment of a skull reported on at a conference, this is the first remains outside of Denisova cave identified as “Denisovan.” Part of the identification was morphological. Both this find and those in Denisova cave, are characterized by very large teeth.

But the really interesting aspect is that they used analysis of proteins to place this sample phylogenetically. You can see the results above. Proteins don’t degrade as fast as DNA, from what I know, so this isn’t surprising. This individual, from high altitude Tibet, dated to at least 160,000 years ago, is in the same clade as the Denisovan that has been sequenced in the broader context of hominin evolution. This is not a rock-solid inference…there wasn’t that much informative variation (I believe Janet Kelso said on Twitter that one particular position where the Denisovan were derived compared to all other hominins in particular matched this paleo-Tibetan sample). But, if you had to guess, it does seem likely that this was an individual related to the Denisovans that we’ve come to know and love.

Finally, there is an important twist that the high altitude adaptation in Tibetans due to EPAS1 seems to have arrived from an introgressed haplotype from Denisovans. Perhaps then the introgression occurred 40 to 50 thousand years ago, as modern humans replaced Denisovans. The majority of the ancestry of Tibetans though seems to share rather recent Holocene origins with groups such as the Han Chinese. Therefore, rather than absorption of an old substrate in Tibet, it could be that you are looking at a variant widely found in the northern Denisovans.

I’ve been talking a lot about Denisovans recently. Why? It seems that the investigations prompted by the original surprise sequencing of 2010 are finally yielding results. But one thing that is clear is that our understanding of the origin of our lineage, and how various hominins interacted with each other, and who they were, is much sketchier than we might like to think. Though the Tibetan and Denisova cave Denisovans were both robust, if the lineage began to diversify ~400,000 years ago, that’s certainly enough time for various morphological types to have emerged in different parts of Asia.

It could simply be we’ll never be able to specifically understand a lot of the detailed processes that occurred in terms of how different hominin groups related to each other. But, we will probably be able to get a better general picture in the near future. As Spencer mentioned in our podcast last week, the Neanderthals in some ways may have been atypical for ancient hominins, and not a good guide to the long term trajectory of the Denisovans.

African genomics tells us about deep structure and history


Two interesting papers in Genome Biology that are open access, Whole-genome sequence analysis of a Pan African set of samples reveals archaic gene flow from an extinct basal population of modern humans into sub-Saharan populations and African evolutionary history inferred from whole genome sequence data of 44 indigenous African populations. Since they are open access you should just read both of them.

I believe they are the first in a series of papers over the next few years using whole-genome analysis to understand the population structure within Africa, and how it relations to the people who branched off from Africans. Eventually, this will also lead to research focused on medical and population genomics, looking at characteristics and forces beyond phylogeny.

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The expansion of modern humans ~50,000 as part of a regular Poisson process

Last of the giants: What killed off Madagascar’s megafauna a thousand years ago?:

The first job is to understand exactly when the megafauna died out.

Radiocarbon dating of over 400 recent fossils demonstrates that animals under 22 pounds lived on Madagascar throughout the last 10,000 years. For animals over 22 pounds, there are abundant fossils up to 1,000 years ago, but relatively few since. The biggest decline in number of large animals occurred rapidly between A.D. 700 and 1000 – practically instantaneous given the long history of their existence on the island.

According to new dates on fossil bones with cut marks on them, humans arrived on Madagascar 10,500 years ago, much earlier than previously believed. But whoever these early people were, there’s no genetic evidence of them left on the island. New analysis of the human genetic diversity in modern Madagascar suggests the current population derives primarily from two waves of migration: first from Indonesia 3,000 to 2,000 years ago, and later from mainland Africa 1,500 years ago.

So it seems that people lived alongside the megafauna for thousands of years. How did the humans interact with the large animals?

Our new study found dozens of fossils with butchery marks. Cut and chop marks provide compelling evidence as to which species people were hunting and eating. Evidence of butchery of animals that are now extinct continues right up to the time of the megafaunal crash. Some people on Madagascar hunted and ate the megafauna for millennia without a population crash.

The abrupt land use change might hold some clues. The transition from a forest-dominated ecosystem to a grassland-dominated ecosystem appears to be widespread….

This research about Madagascar is important. If it turns out correct, I think it gives us deep insights about the expansion of modern humans outside of Africa ~50,000 years ago, and why their arrival resulted in the extinction of so many other human lineages. A generation ago we might have posited that some massive bio-behavioral change is what triggered this, but I am coming closer to the idea that cultural changes are punctuated enough that that may actually explain things. The culture changes first, then genes follow the culture.

Perhaps one might posit a model with massive turnovers in the hominin lineage due to this cultural dynamic occurs periodically, as if it’s a Poisson process.

Reevaluating “multiple origins” for modern humans

Following up on the post below, The Deep Origins Of East African Hunter-Gatherers, as well as some discussions on Twitter, I think I want to do some clarification about where I think we are now. My thoughts shouldn’t be a surprise if you have read everything I’ve said, but I may not have put them all together in one place.

Around the turn of the century, nearly twenty years ago, the consensus had definitively turned against a “multiregional” origin of modern humans, toward one where an “out of Africa” migration ~50,000 years ago was paramount. Many people took the “paramount” part and simply asserted that we are all Africans descended from a population that flourished in the east of the continent about 50,000 years ago. There was a lot of circumstantial evidence to support this, at least spottily, from both archaeology and genetics. There were also problems and lacunae in both fields. But the data was spotty enough that the extreme position was defensible.

We now have a lot more information and need to update our model. First, most people agree that indigenous Eurasian hominins, Neanderthals and Denisovans, contributed to the ancestry of people outside of Sub-Saharan Africa. Additionally, it’s been evident for a long time now that the massive population bottleneck that is present in all non-African populations dating to ~50,000 years ago is far less evident in Sub-Saharan African genomes.

Finally, it’s pretty clear that humans with modern morphology were present within Africa for hundreds of thousands of years before the movement out of Africa.

Therefore, a new reevaluation of the old model that is converging is a possibility is that multi-regionalism was operative within Africa for hundreds of thousands of years, followed by a massive expansion on the northeast edge of Africa that resulted in most of the ancestry of other human groups outside of the continent, with some assimilation (e.g., Neanderthal). This is a far more complicated model than the older one, but sometimes the truth is more complicated than simplicity.

But I think we’ll probably need to make further modifications, and that’s because gene flow is not always unidirectional. Specifically, the Y chromosomal work, in particular, is strongly indicative of migration of lineages more typical of Eurasians expanding within Africa within the last 50,000 years. And, as a commenter on this weblog has pointed out, even the “deep lineages” within Africa, Y haplogroups A and B, show signs of massive expansion within the last 50,000 years.

This may mean that a population liminal to Africa and Southwest Asia underwent a very rapid expansion ~50,000 years ago. The replacement of indigenous lineages was far more thorough outside of Africa, with 5% or less assimilation in most places. But, it probably impacted Africa as well. Though a larger fraction of diverged modern ancestry persisted in Africans than Eurasian hominin ancestry in non-Africans. In other words, the high genetic diversity of Africans today, and particular groups like the Khoisan, is due to the mixture between an ancient migration from the same population that was the source of “out of Africa” in Eurasia and Oceania, and disparate deeply structured lineages within Africa, that date back 200-400 thousand years ago.

Additionally, I think some earlier “modern” lineages were assimilated in eastern Asia with the latest migration out of Africa. And, some of the ancestry within Africa probably predates the origin of anatomically modern humans, analogous to the case of Neanderthals and Denisovans.

Note: This is not that different of a model from Dienekes Pontikos’ ideas in the 2000s, More support for the Afrasian/Palaeoafrican hypothesis, at the high level. Basically the more evidence has come in, the less crazy his model has gotten.

The state origins of “modern humans” in 2018 (it’s a flux)

After reading the supplements to the new Siberian paper I have a few general thoughts that I want to layout.

First, the clines vs. clusters considerations seem to be one we need to revisit. Like the expansion of Native American peoples ~15,000 years ago, it seems that the “Out of Africa” migration pulse happened so quickly that a lot of different groups emerged at the same time. In the new paper the earliest proto-“Ancient North Eurasians” can be modeled as most similar to the West Eurasian branch of humanity (sans Basal Eurasian), but with some minor component affinity to East Eurasians. It could be that this is a function of admixture between the distinct lineages. Or, it could be that there was a fair amount of substructure within the post-Basal Eurasian “Out of Africa” meta-population.

The problem with the idea of lots of structure within this population that I see is that it might depend on the plausible effective population sizes. I’d need to know more ethnography than I do, but it seems not impossible for ~10,000 humans to be highly structured in Paleolithic social contexts, even if they were close geographically. But, this would entail a great deal of xenophobia and likely inter-group conflict.

Second, I am convinced that there were earlier “Out of Africa” migrations. Many of them. As John Hawks pointed out at ASHG the Neanderthals and Denisovans seem to be descended from a migration of African hominins that dates to somewhere after 1 million years ago. This means they replaced hominins that were present in Eurasia for ~1 million years already. Geneticists and paleontologists have both also discovered suggestive clues to likely “proto-modern” human populations that were present and admixing before the rapid expansion of Eurasians and Australasians ~40-50,000 years ago. With more ancient DNA and subtle analysis, I think we’ll find that modern human absorbed some layers between that of Denisovans and Neanderthals and the most recent expansion.

Finally, I think multi-regionalism within Africa is between plausible and likely, and that major back-to-Africa migrations that modify/challenge “Out of Africa” are possible. We are learning a lot. But that means simple elegant models are falling by the wayside.

The new African “multi-regionalism” & pan-Neanderthalism

We live in times when our understanding of the origin and diversification of modern humans is undergoing great change. More concretely, our understanding of what it means to be human is transforming. The terms are overused, but perhaps it could be called a “revolution” or “paradigm shift” between the year 2000 and today.

At the end of 2010 ancient DNA made it highly likely that people outside of Sub-Saharan Africa had non-trivial Neanderthal ancestry. That is, enough ancestry that it is detectable genomically. I should also add that I think it is highly probable that the good majority of people within Sub-Saharan Africa have Neanderthal ancestry. Some of this is due to recent attenuated Eurasian back-migration (e.g., many West Africans, Nilotic people, and KhoeSan have Holocene gene-flow signals which derive from the agricultural expansions of the past 10,000 years). But, I think once deep Pleistocene genomes of African humans are sequenced we will see evidence of some Eurasian back-migration at a very ancient date (there is already some suggestive inferential evidence of this).*

Talking with a few friends this week, I realized that the famous “We are all Africans” t-shirts, which have turned into recognizable memes, should be supplemented with “We are all Neanderthals” t-shirts. So yeah, now selling them on DNA Geeks. If the Richard Dawkins Foundation can make quid on it, why not the Razib Khan et al. Foundation?

This has all been on my mind due to a review paper in Trends in Ecology and Evolution, Did Our Species Evolve in Subdivided Populations across Africa, and Why Does It Matter? (OA). If you read this blog closely you’ll see there’s not much new in it. But, it is a signpost, a marker, of the times we live in. Here’s the important bit:

Together with recent archaeological and genetic lines of evidence, these data are consistent with the view that our species originated and diversified within strongly subdivided (i.e., structured) populations, probably living across Africa, that were connected by sporadic gene flow…This concept of ‘African multiregionalism’…may also include hybridization between H. sapiens and more divergent hominins (see Glossary) living in different regions…Crucially, such population subdivisions may have been shaped and sustained by shifts in ecological boundaries…challenging the view that our species was endemic to a single region or habitat, and implying an often underacknowledged complexity to our African origins.

The first person who explicitly used the term “African multi-regionalism” that I recall was Alwyn Scally, though the general framework was shaping up years before. Frankly, I was waiting for someone to use that word. If Richard Klein’s The Dawn of Human Culture, published in 2002, was the apogee of the old model, often inchoate and more crisp in popularization than within the scientific community that we are all descended from a single East African tribe, this review paper heralds the emergence of a more complex and pluralistic framework. The emergence of modern humans within Africa then may have been a polycentric gradual and interactive process; not a singular explosion against the firmament of the antique savanna landscape.

By the late 2000s, even before the 2010 Neanderthal draft genome paper, it was starting to be evident due to genome-wide analyses of contemporary populations, that the extreme bottleneck clear in non-African populations was much more modest within Africa. That opened the possibility for the existence of deep structure within the continent that pre-dated the “Out of Africa” event. A deeper look at African hunter-gatherers indicated to many researchers that these groups diverged from other modern humans in the range of ~200,000 years before the well. Recent paleontological work has confirmed this genetic insight.

Where we are today is that some people are now arguing for the overthrow of the “Out-of-Africa” idea, whether by replacing it with an “Into-Africa” model of some sort, or resurrecting a more polycentric classical multi-regionalism (“some people” as evident in the increased frequency of emails and Twitter messages I get in this vein). I don’t think we’re there yet, not by any measure. But, it is now in the realm of very unlikely, not extremely unlikely (at least the “Into-Africa” model; it is clear that strong overwhelming demographic pulses from somewhere singular dominate the genome of most modern humans).

* I don’t think it is all that implausible that some Neanderthal back-migration into Africa occurred at some point in the last ~500,000.

Beyond “Out of Africa” within Africa

It looks as if the vast majority (95% or more depending on the population) of the ancestry of non-African humans derives from a population expansion which began around ~60,000 years ago. Before this period some researchers argue there was a non-trivial period of isolation. The “long bottleneck” (David Reich alludes to this in Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past). For the vast majority of humans then the last 60,000 years is characterized by a branching process, some reticulation (e.g., South Asians merge West and East Eurasian lineages) between these branches from a common ancestor, as well as introgression from archaic lineages like Neanderthals and Denisovans.

Though I do accept that it seems that modern humans probably migrated out of Africa before 60,000 years ago, mostly due to the results from archaeology, I think the genetic evidence is strong that these groups contributed very little genetically to contemporary populations.

The situation within Africa is very different. Being conservative it seems likely that the Khoisan ancestral lineage diverged from some other Africans ~200,000 years ago. I say conservative because there are researchers who want to push the divergence much further back. Additionally, several different research groups are now converging in a result that West Africans are a mixture between eastern Sub-Saharan Africans (think the population ancestral to Mota in Ethiopia) and a lineage basal to all other humans. That means that the Khoisan are not the most basal, so even assuming the conservative 200,000 year divergence point for Khoisan, modern humans share a common ancestor earlier than 200,000 years ago.

The upshot here is that around 75 percent of the history of modern humans is within (greater)* Africa. The distinctive “Out of Africa” bottleneck and expansion defines most humans only in the last 25 percent of the history of our species. And, within Africa, the dynamics were very different. The biggest difference is that African populations are not defined by a large number of lineages emerging and diverging around the same period, because there wasn’t a massive and singular expansion within Africa analogous to what occurred outside of Africa (at least until the recent past, with the Bantu expansion). That’s why there’s deep structure within Africa today between groups as divergent as the Bantu, Mbuti, Hadza, and Khoisan.

The term “Basal Eurasian” kind of makes sense in the non-African context because of the singular importance of divergence between lineages in the first 10,000 years or so after the “Out of Africa” event. I’m not sure “Basal human” makes as much sense because there wasn’t a singular event within Africa that allowed for the emergence of modern humans. Rather, it was a process, and probably quite resembles something like multiregionalism.

* Some wiggle room here for the likelihood that modern humans were long present in the liminal Near East.