Razib Khan One-stop-shopping for all of my content

June 6, 2017

Origin of modern humanity pushed back 260,000 years BP (?)

Filed under: Ancient DNA,Genetics,Khosian,South Africa — Razib Khan @ 12:45 am


The above figure is from a preprint, Ancient genomes from southern Africa pushes modern human divergence beyond 260,000 years ago. The title and abstract are pretty clear:

Southern Africa is consistently placed as one of the potential regions for the evolution of Homo sapiens. To examine the region’s human prehistory prior to the arrival of migrants from East and West Africa or Eurasia in the last 1,700 years, we generated and analyzed genome sequence data from seven ancient individuals from KwaZulu-Natal, South Africa. Three Stone Age hunter-gatherers date to ~2,000 years ago, and we show that they were related to current-day southern San groups such as the Karretjie People. Four Iron Age farmers (300-500 years old) have genetic signatures similar to present day Bantu-speakers. The genome sequence (13x coverage) of a juvenile boy from Ballito Bay, who lived ~2,000 years ago, demonstrates that southern African Stone Age hunter-gatherers were not impacted by recent admixture; however, we estimate that all modern-day Khoekhoe and San groups have been influenced by 9-22% genetic admixture from East African/Eurasian pastoralist groups arriving >1,000 years ago, including the Ju|’hoansi San, previously thought to have very low levels of admixture. Using traditional and new approaches, we estimate the population divergence time between the Ballito Bay boy and other groups to beyond 260,000 years ago. These estimates dramatically increases the deepest divergence amongst modern humans, coincide with the onset of the Middle Stone Age in sub-Saharan Africa, and coincide with anatomical developments of archaic humans into modern humans as represented in the local fossil record. Cumulatively, cross-disciplinary records increasingly point to southern Africa as a potential (not necessarily exclusive) ‘hot spot’ for the evolution of our species.

These results in the outlines were actually presented at a conference. I saw it on Twitter and don’t remember which conference anymore. But this is not entirely surprising.

First, much respect to Mattias Jakobsson’s group for breaking through the Reich-Willerslev duopoly. Hopefully this presages some democratization of the ancient DNA field as expenses are going down.

Second, notice how in most cases ancient DNA shows that modern reference populations turn out to be admixed. This was the problem with much of Eurasia, and why using modern genetic variation to make inferences about the past totally failed.

I am entirely convinced that the genome from Ballito Bay dating to ~2,000 years does not carry the Eurasian inflected East African admixture. The Mota genome implies that Eurasian admixture did not come to eastern Africa much before 4,500 years ago. There needs to be a much deeper big picture analysis of the archaeology of Africa and the genetic information we have to get a sense of what happened back then…but, it seems likely that the Bantu migration has over-written much of the earlier genetic variation.

The fact that ancient genomes always show that our current populations are admixed makes me wonder if the Ballito Bay sample itself is admixed from more ancient populations. That is, if we found a genome from 20,000 years ago, would it be very different from the Ballito Bay samples? The relatively thick time transect from Europe indicates that turnover happens every 10,000 years or so. Australian Aborigines seem to have been resident in their current locations for ~50,000 years, but this seems the exception, not the rule. Do we really think that the ancestors of the Bushmen were living in southern Africa for five times as long as Australian Aborigines?

Another curious aspect of this paper is that it suggests the effective population size of Bushmen is smaller than we might have thought, and they’re somewhat less diverse than we’d thought. That’s because East African (with Eurasian ancestry) gene flow increased heterozygosity, as well as inferred effective population sizes. I’ve mentioned this effect on statistics before. Unless you have a true model of population history (or close to it) your assumptions might distort the numbers you get.

There is another aspect to this preprint mentioned glancingly in the text, and a bit more in the supplements: they seem to only be able to model Yoruba well if you assume that they themselves are a mix of “Basal Humans” (BH) and other African population which gave rise to East Africans and “Out of Africa” populations. Note that the BH seem to diverge from other human populations before the ancestors of Southern Africans like the Ballito Bay sample. That is, BH could push the diversification of the ancestors of modern humans considerably before 260,000 years before the present.

The possibility of deep structure in the Yoruba is pretty notable because they’ve been the gold standard in many human population genetic data sets as a reference population. But this is not result of deep structure is not entirely surprising. For years researchers have been hinting at confusing results in relation to the possibility of Eurasian back-migration. Perhaps the deep structure was confounding inferences?

The authors themselves are quite cautious about their dating of the divergence. It’s sensitive to many assumptions, and in particular the mutation rate being known and constant over time. But I think it’s hard to deny that this is pushing back the emergence of modern humans beyond what we know today. The earliest anatomically modern humans are found in Ethiopia 195,000 years ago from what I know. As I said, I’m convinced that the ancient genome has shown that modern “pristine” populations have some serious admixture. But I’m not as convinced about any specific point estimate, because that’s sensitive to a lot of assumptions which might not hold.

Finally, first a quick shout out to the blogger Dienekes. As early as ten years ago he anticipated the basic outlines of these sorts of results in the generality, if not the details. We really have come a long way from popular science declaring that all humans descend from a small group of East Africans who lived 50,000 to 100,000 years ago. The real picture was much more complex.

Also, I have to admit I considered titling this blogspot “Wolpoff’s revenge.” As in Milford Wolpoff. The reason being that we’re getting quite close to territory familiar to the much maligned multi-regionalist model of modern human origins.

Note: These findings should make us less surprised perhaps by a “modern” human migration before the primary one out of Africa.

May 26, 2017

The Canaanites walk among us: ancient DNA edition

Filed under: Ancient DNA,Bronze Age,History — Razib Khan @ 2:09 pm

Ancient DNA from here to there:

Ancient DNA has illuminated many things, but there is a logic as to what topics and questions it tackles. The focus on northern Eurasia is clearly a function of the probability of preservation, though techniques of extraction are getting better and better. I can’t imagine how we’d ever get a sample out of a moist tropical environment, but I won’t be surprised if something is obtained from a cave in southern Africa or high in the Tibesti in the near future.

But another parameter is time since the demographic events in question. Too ancient, and the probability of success is too low(ok, time is a parameter in much of science!). It seems plausible that in idealized circumstances we’re going to push beyond the one million year barrier. And yet too recent is also a problem (or not a problem!). For humans and even non-humans we have lots of corroboration about questions we might ask about the recent past. You could use “ancient DNA” to trace the migration of Mormons across the Intermontane West, but why would you?

So you see the earliest ancient DNA work on humans was biased toward testing models about gene flow and ancestry tens of thousands of years in the past, between modern humans and archaic lineages. Obviously we don’t have oral history or written texts from this period, and archaeology will only get us so far.

More recently the time depth has been getting shallower and shallower. Both David Reich and Eske Willerslev’s work on European prehistory is liminally historical. By this, I mean that what is prehistory in Europe is a historical period in the Near East. We may not have written records from the Corded Ware or Bell Beaker cultures, but we do have plenty of them from contemporaneous Near Eastern groups.

The Cauldron of Peoples:

There are still questions to be asked about European prehistory, but the gaps are getting narrower and narrower. Scholars are finally devoting resources to other regions of the world. Last year Iosif Lazaridis’ The genetic structure of the world’s first farmers finally opened up the box that was the prehistory of the Near East. This was important, because much of prehistory and history began in the Near East. Farmers from this region seem to have moved into Europe, South Asia, Central Eurasia, and Africa. To understand the population histories of these areas one needs to understand the population history of the Near East.

What Lazaridis et al. found this that there were at least two major groups of very genetically distinct Near Eastern farmers at the dawn of agriculture. Once group faced the eastern Mediterranean, while the other seems to have flourished on the slopes of the Zagros. Western and eastern farmers respectively. It is important to note that these two groups were very genetically distinct. If we sampled these two groups of farmers, who faced each other across northern Mesopotamia, in any modern population survey we’d assume that the genetic distance meant that they were sampled from different continents or very distant regions of Eurasia.

This finding suggest that the clinal patterns of variation in much of today’s world may be a consequence of massive population admixture between groups which had heretofore exhibited deep population structure. Why such deep structure existed and persisted is an interesting question, but at this point it is important to note descriptively that the past 10,000 years have seen a massive reduction of this structure due to gene flow between populations.

In the Near East Lazaridis et al. found that there was significant reciprocal gene flow between the western and eastern regions of the Near East after the emergence of farming, down to the historical period. This is one reason that estimates of “farmer” ancestry in modern Europeans always gave very low estimates: the reference populations no longer existed in unmixed form in the Near East. The peoples who brought agriculture to Southern Europe were related exclusively to the western farmers of the Near East, a population which no longer exists in unmixed form in that region of the world (ergo, among modern groups Sardinians are the closest proxies we have).

The Age of Bronze:

But there is much that occurred after prehistory in the Near East. We know this because we have extensive records going back 4,500 years, and even earlier. And though put into written form in the first millennium before Christ, the Hebrew Bible also records the deeds and names of people who have come and gone well before the Classical Age.

A new preprint on biorxiv sheds some light on a critical transitory period, Continuity and admixture in the last five millennia of Levantine history from ancient Canaanite and present-day Lebanese genome sequences:

The Canaanites inhabited the Levant region during the Bronze Age and established a culture which became influential in the Near East and beyond. However, the Canaanites, unlike most other ancient Near Easterners of this period, left few surviving textual records and thus their origin and relationship to ancient and present-day populations remain unclear. In this study, we sequenced five whole-genomes from ~3,700-year-old individuals from the city of Sidon, a major Canaanite city-state on the Eastern Mediterranean coast. We also sequenced the genomes of 99 individuals from present-day Lebanon to catalogue modern Levantine genetic diversity. We find that a Bronze Age Canaanite-related ancestry was widespread in the region, shared among urban populations inhabiting the coast (Sidon) and inland populations (Jordan) who likely lived in farming societies or were pastoral nomads. This Canaanite-related ancestry derived from mixture between local Neolithic populations and eastern migrants genetically related to Chalcolithic Iranians. We estimate, using linkage-disequilibrium decay patterns, that admixture occurred 6,600-3,550 years ago, coinciding with massive population movements in the mid-Holocene triggered by aridification ~4,200 years ago. We show that present-day Lebanese derive most of their ancestry from a Canaanite-related population, which therefore implies substantial genetic continuity in the Levant since at least the Bronze Age. In addition, we find Eurasian ancestry in the Lebanese not present in Bronze Age or earlier Levantines. We estimate this Eurasian ancestry arrived in the Levant around 3,750-2,170 years ago during a period of successive conquests by distant populations such as the Persians and Macedonians.

The period between 1700 and 1800 BCE in the Near East saw many changes and was a sort of nexus. Sumer had fallen, the Hittites had not emerged as a superpower, while Egypt was not heavily involve in the game of kings as of yet. The system of international relationships described in Brotherhood of Kings had not crystallized. That was for the late Bronze Age.

But some of the pieces we were to recognize were already in place. An Amorite Babylon under Hammurabi established the contours of the culture and polity we’d recognize down to the Persian conquest. In Egypt the Middle Kingdom was going into decline, and the Hyksos interregnum would give rise to the New Kingdom, which would become a major player in the Levant (and probably is the model for much of the Egypt we see described in the Bible).

The admixture plot above reflects the five individuals from Sidon dating to about ~1750 BCE. They are about a 50:50 mix of western and eastern farmer. Though they seem to be genetically rather similar to modern Lebanese (the authors sampled Lebanese Christians in particular), there have been some changes between the Bronze Age and the modern period. In particular, a genetic component that seems to be related to the Eurasian steppe is present in modern Lebanese. Explicit admixture estimates give a range of 5-10% mixing into a ~90-95% Bronze Age ancestral background.

This seems to establish basic continuity between the Bronze Age and the modern period. Totally unsurprising. Remember that Italy exhibits deep population structure that dates back to at least 2,000 years ago, and probably earlier. It is likely that much of the same applies to the Near East. Though looking at Muslim populations one can see minor and non-trivial contributions of populations which moved in after Islam (Sub-Saharan and East Asia segments are clear signs of slavery impacting Muslims that would not apply to ethno-religious minorities), most of the ancestry broadly is deeply rooted back to antiquity.

Because of sampling issues one can’t estimate admixture between eastern and western farmers just from looking at ancient DNA transects. We don’t have the density that we have in Europe (yet). So the authors used a more classic inference technique looking at decays of linkage disequilibrium in the genome. In short you can see how many generations that a pulse admixture between two populations occurred by looking at correlations of variants across the genome. The authors arrive at the intervals above, and in particular focus on the period that seems to overlap with the rise and fall of the empire of Sargon of Akkad and correlated with a climatic disruption.

I suspect they are wrong here. First, it seems pretty clear to me that LD based admixtures assuming a pulse event have a bias toward underestimating values. There are theoretical reasons for this. So usually I pad the mid-point value across the interval on these estimates.

One thing that ancient DNA has told us is that often the less complex the society, the more demographic turnover you have. All things equal then we would expect turnover to be an older event, as simple societies are succeeded by complex ones. The succession of complex societies by other complex societies is often less disruptive for the masses because this transformation is more a matter of elite replacement.

By ~2200 BCE the Near East was already quite complex. I believe that the massive western-eastern farmer admixture occurred between 3600 and 3100 BCE, during the Uruk Expansion. The evidence of lower Mesopotamian influence and demographic settlement in places as far afield as Anatolia, the Caucasus, and Syria, are well attested from the archaeology of this period. This was was a time when a very complex and sophisticated civilization emerged almost de novo across much of the Near East. I believe that a prehistoric expansion of Sumerian civilization mediated the merging of eastern and western farmers, though some of the mixing pre-dates and post-dates the Uruk Expansion and collapse (e.g., the movement of western farmer ancestry into Mesopotamia seems certain to have occurred through the arrival of groups like the Amorites).

Additionally, buried in this preprint is evidence of major Y chromosomal turnover. We’ve seen this  before. The prominence of haplogroup J in Bronze Age and modern Levantines seems to be due to eastern farmer migration. In fact, adding haplogroup J and R together we get the inference that more than half the paternal lineages of Lebanese today are not from western farmers native to the area.

Beyond the Bronze Age:

What about the second ancestral component? Drilling down on the Y chromosomes of the Levant, R1b seems to far outnumber R1a, though the R1a clades are all of the Asian/Scythian Z-93 branch which is dominant in Central Asia and the Levant. The R1a may have come with the Persians, but in region of the western Levant for several hundred years after the period of the Bronze Age Sidon samples there was a state, the Mitanni, which clearly had an Indo-Aryan ruling class.

An Aegean influence occurred multiple times. First, at the end of the Bronze Age many of the “Sea Peoples” were clearly of Aegean origin, and so may have brought steppe-like ancestry. Second, there was the long period under Hellenistic and Roman rule, when Greek and non-Greek ethnic identity existed side by side, and movement occurred in both directions. I think only ancient DNA will answer this question, and it may be that there were multiple post-Bronze Age inputs of genes which shaped modern Levantines.

After Babel:

The curious thing that many of these studies are telling us is two-fold:

  1. Most of the population genetic structure we see around us dates to the Bronze Age, on the borderlands between history and prehistory. I think we can start to set this as a strong prior. It holds true for the Near East, Africa, South Asia, Japan and Southeast Asia. We’ll see about core East Asia, but I think probably it is true there too.
  2. Selection has continued, so that alleles for lactose tolerance and lighter skin have changed in frequency even since that period. The derived allele for SLC45A2 is found at about 2/3 frequency in modern Lebanon, but was absent in these five Sidonians. Though the sample size is small, this was somewhat surprising, and suggests that they were a swarthier people than modern Lebanese.

Addendum: I have said little here about Afro-Asiatic languages, as I don’t know enough about this topic.

October 29, 2011

Ancient DNA in the near future

Filed under: Ancient DNA,Cheddar Man,Genomics,Human Genetics,Human Genomics — Razib Khan @ 12:27 pm

I recently inquired if anyone was sequencing Cheddar Man. In case you don’t know, this individual died ~9,000 years ago in Britain, but the remains were well preserved enough that mtDNA was retrieved from him. He was of haplogroup U5, which is still present in the local region. Cheddar Man is also particularly interesting because he is definitely a Paleolithic hunter-gatherer, predating the Neolithic in Britain by thousands of years.

It turns out that no one is looking at Cheddar Man now. But that’s probably because money and time are finite. I was told that there are plenty of other specimens which would also probably be good candidates for sequencing in the Museum’s collection (this doesn’t seem to be a case where curators are being stingy and overprotecting of their specimens). That’s not too surprising. We’ll probably answer a lot of questions about the roles of demographic diffusion vs. cultural diffusion when it comes to agriculture soon enough (as in, over the next 10 years as techniques for getting signal out of old degraded and contaminated samples get better).

September 14, 2011

Ötzi, first, but not last, farmer?

Filed under: Ancient DNA,Anthroplogy,Human Genetics,Iceman,Ötzi — Razib Khan @ 8:59 pm

Dienekes relays that Ötzi the Iceman carried the G2a4 male haplogroup. He goes on to observe:

We now have G2a3 from Neolithic Linearbandkeramik in Derenburg and G2a in Treilles in addition to Ötzi from the Alps. G2a folk got around. He joins Stalin and Louis XVI as a famous G2a.

It was already clear with the discovery of G2a in France and Central Europe, that this otherwise uncommon present-day haplogroup in Europe was more prominent during the Neolithic, and Ötzi’s data point seals the case.

In a sense, the triple G2a finds in Neolithic Europe confirm the origins of the European Neolithic population in West Asia, but renew the mystery as to how all the rest of the “players” of the European Y-DNA scene appeared on the scene, with everything except G and I first appearing in the ancient DNA record after the end of the Neolithic.

Yes, I believe that the Paleolithic-Neolithic dichotomy is more hindrance than help in understanding the European past (the Paleolithic itself may have exhibited more population turnover than we can appreciate). I suspect that the two most common European Y haplogroups, R1a and R1b, underwent rapid increase in frequency over the past ~5,000 years. I do not believe that this is necessarily representative of the rest of the genome. The spread of male lineages can be rather unrepresentative.

In other news, Ötzi’s genome is going to drop any day now. My prediction that it’s more West Asian than we might have expected seems more plausible, though less surprising and risky, at this point.

Image credit: 23andMe

August 9, 2011

The hunt for ancient DNA

Ewen Callaway has a good survey of what’s been going down in ancient human genomics over the past year in Nature, Ancient DNA reveals secrets of human history. It’s not paywalled, so read the whole thing. Most of it won’t be too surprising for close readers of this weblog, but this part is new:

By comparing individual DNA letters in multiple modern human genomes with those in the Neanderthal genome, the date of that interbreeding has now been pinned down to 65,000–90,000 years ago. Montgomery Slatkin and Anna-Sapfo Malaspinas, theoretical geneticists from the University of California, Berkeley, presented the finding at the Society for Molecular Biology and Evolution meeting in Kyoto, Japan, held on 26–30 July.

Slatkin says that their result agrees with another study presented at the meeting that came from the group of David Reich, a geneticist at Harvard Medical School in Boston, Massachusetts, who was involved in sequencing both the Neanderthal and Denisova genomes. The dates also mesh with archaeological finds bookending early human migrations out of Africa to between about 50,000 and 100,000 years ago. Reich’s team is now developing tools to find signs of more recent interbreeding that might have occurred after humans arrived in Asia ...

June 20, 2011

Hints of Ötzi’s genome

Filed under: Ancient DNA,Ancient Genomes,Genetics,Genomics,Ötzi — Razib Khan @ 7:39 pm

John Hawks points to a report in Science on some morsels of information about Ötzi-the-Iceman’s genetics, The Iceman’s Last Meal:

Also at the meeting, researchers led by geneticist Angela Graefen of the Institute for Mummies and the Iceman reported that they have succeeded in sequencing the Iceman’s whole genome, despite the highly fragmented nuclear DNA. The genome has already revealed some surprises. One preliminary finding shows that the Iceman probably had brown eyes rather than the blue eyes found in many facial reconstructions done by artists. Graefen and her colleagues are also examining the DNA to see if Ötzi possessed genetic predispositions to diseases such as arthritis, which other researchers have diagnosed based on radiological and other evidence.

I’m assuming we’ll know a whole lot more before the end of summer. So I’m going to go out on a limb and make a prediction based on what I suspect about the southern European genetic landscape ~5,000 years ago: Ötzi will be more like contemporary West Asian people, Georgians, Armenians, etc., than modern north Italians and south Germans are. Right or wrong, I hope the results will be interesting!

May 31, 2011

Mediterranean men on the move

ResearchBlogging.orgSeriously, sometimes history matches fiction a lot more than we’d have expected, or wished. In the early 2000s the Oxford geneticist Bryan Sykes observed a pattern of discordance between the spatial distribution of male mediated ancestry on the nonrecombinant Y chromosome (NRY) and female mediated ancestry in the mitochondrial DNA (mtDNA). To explains this he offered a somewhat sensationalist narrative to the press about possible repeated instances of male genocide against lineage groups who lost in conflicts.

Here is a portion of the book of Numbers in the Bible:

15 – And Moses said unto them, Have ye saved all the women alive?

16 – Behold, these caused the children of Israel, through the counsel of Balaam, to commit trespass against the LORD in the matter of Peor, and there was a plague among the congregation of the LORD.

17 – Now therefore kill every male among the little ones, and kill every woman that hath known man by lying with him.

18 – But all the women children, that have not known a man by lying with him, keep alive for yourselves.

Then there is the rape of the Sabine women. The ethnogenesis of the mestizo and mulatto populations of ...

January 26, 2011

Neandertal admixture, revisiting results after shaken priors

After 2010′s world-shaking revolutions in our understanding of modern human origins, the admixture of Eurasian hominins with neo-Africans, I assumed there was going to be a revisionist look at results which seemed to point to mixing between different human lineages over the past decade. Dienekes links to a case in point, a new paper in Molecular Biology and Evolution,  An X-linked haplotype of Neandertal origin is present among all non-African populations. The authors revisit a genetic locus where there have been earlier suggestions of hominin admixture dating back 15 years. In particular, they focus on an intronic segment spanning exon 44 of the dystrophin gene, termed dys44. Of the haplotypes in this they suggested one, B006, introgressed from a different genetic background than that of neo-Africans. The map of B006 shows the distribution of the putative “archaic” haplotype from a previous paper cited in the current one from 2003. As you can see there’s a pattern of non-African preponderance of this haplotype. So what’s dystrophin‘s deal? From Wikipedia:

Dystrophin is a rod-shaped cytoplasmic protein, and a vital part of a protein complex that connects the cytoskeleton ...

January 5, 2011

Hobbit DNA in 2011

Filed under: Ancient DNA,Genetics,Genomics,Hobbit,Human Evolution,Human Genetics — Razib Khan @ 9:03 am

I predicted earlier that Hobbit DNA would be extracted in 2011. It was pretty much an educated guess based on various omissions I sensed in papers in 2010. But it seems that an attempt is going to be made:

Scientists are planning an attempt to extract DNA from the ‘hobbit’ Homo floresiensis, the 1-metre-tall extinct distant relative of modern humans that was unearthed in Indonesia, following a study that suggests problems in standard sampling methods in ancient-DNA research could have thwarted previous efforts.

This year, geneticists at the Australian Centre for Ancient DNA (ACAD) at the University of Adelaide hope to recover DNA from a roughly 18,000-year-old H. floresiensis tooth, which was excavated in 2009 from the Liang Bua site on the Indonesian island of Flores.

No guarantees, but still exciting.

(via Dienekes)

November 9, 2010

European man of many faces: Cain vs. Abel

UE-1

When it comes to the synthesis of genetics and history we live an age of no definitive answers. L. L. Cavalli-Sforza’s Great Human Diasporas would come in for a major rewrite at this point. One of the areas which has been roiled the most within the past ten years has been the origin and propagation of the agricultural lifestyle across the European continent between 10,000-6,000 years before the present (starting in Europe’s southeast fringe a few thousand years after the origination of the Neolithic lifestyle in the Levant, and finally pushing into the southern Scandinavian peninsula only ~6,000 years ago). The reasons for this particular debate about the origin of the European are manifold. First, most scholars are of European ancestry, and some of the debates have roots going back a century. So a natural interest exists based on normal human biases. Second, when it comes to genetics the climate of Europe is ideal for the preservation and extraction of ancient DNA. Third, there are relatively clear and distinct theoretical models which can be tested by the data, whether to verify or refute.

ResearchBlogging.orgI have already reviewed earlier work in three previous posts, European man perhaps a Middle Eastern farmer, European man perhaps not a Middle Eastern farmer, and Völkerwanderung back with a vengeance. Instead of rehashing everything I’ll take it as a given that you’ve read or skimmed those posts. Rather, let’s move on to a new paper in PLoS Biology, Ancient DNA from European Early Neolithic Farmers Reveals Their Near Eastern Affinities:

In Europe, the Neolithic transition (8,000–4,000 B.C.) from hunting and gathering to agricultural communities was one of the most important demographic events since the initial peopling of Europe by anatomically modern humans in the Upper Paleolithic (40,000 B.C.). However, the nature and speed of this transition is a matter of continuing scientific debate in archaeology, anthropology, and human population genetics. To date, inferences about the genetic make up of past populations have mostly been drawn from studies of modern-day Eurasian populations, but increasingly ancient DNA studies offer a direct view of the genetic past. We genetically characterized a population of the earliest farming culture in Central Europe, the Linear Pottery Culture (LBK; 5,500–4,900 calibrated B.C.) and used comprehensive phylogeographic and population genetic analyses to locate its origins within the broader Eurasian region, and to trace potential dispersal routes into Europe. We cloned and sequenced the mitochondrial hypervariable segment I and designed two powerful SNP multiplex PCR systems to generate new mitochondrial and Y-chromosomal data from 21 individuals from a complete LBK graveyard at Derenburg Meerenstieg II in Germany. These results considerably extend the available genetic dataset for the LBK (n = 42) and permit the first detailed genetic analysis of the earliest Neolithic culture in Central Europe (5,500–4,900 calibrated B.C.). We characterized the Neolithic mitochondrial DNA sequence diversity and geographical affinities of the early farmers using a large database of extant Western Eurasian populations (n = 23,394) and a wide range of population genetic analyses including shared haplotype analyses, principal component analyses, multidimensional scaling, geographic mapping of genetic distances, and Bayesian Serial Simcoal analyses. The results reveal that the LBK population shared an affinity with the modern-day Near East and Anatolia, supporting a major genetic input from this area during the advent of farming in Europe. However, the LBK population also showed unique genetic features including a clearly distinct distribution of mitochondrial haplogroup frequencies, confirming that major demographic events continued to take place in Europe after the early Neolithic.

sommer-nadachowski-2006-figAs I’ve indicated before the archaeological jargon is rather mystifying to me. Some of this is due to translation, the Linear Pottery Culture is abbreviated “LBK” because in the German it is Linearbandkeramik. Rather, I try and focus on some basic concrete parameters: time and space. So we have the first agricultural society with a focus on Central Europe flourishing ~7,000 years before the present. Some now we have the time and space in mind around which we can bracket all the background variables. The big question being asked, and answered, is whether the practitioners of LBK were descendants of Ice Age Europeans who expanded from the “refugia” in the south of Europe during the Last Glacial Maximum (LGM) ~20,000 years ago. To speak intelligently about these issues you need some basic intuition, so you see the map which I found on John Hawks’ weblog showing the line of settlement during the LGM. As the ice retreated presumably the European hunter-gatherers would have rapidly pushed northward, following the species which they consumed.

I’ll pass over the methodological nuts & bolts; you can find them in the paper. Obviously this isn’t technically trivial; extracting, amplifying, and avoiding contamination, from DNA samples on the order of 7,000 years old is awesome. As usual they focused on mtDNA because this is found in much larger quantities than nuclear DNA. They did get a few Y chromosomal results though, though mtDNA is the star of the show here. The mtDNA is the maternal lineage, so it can tell you only so much. Additionally, there may be selection dynamics going on to change the frequencies of some of these variants. But with those caveats in hand I think mtDNA patterns can be very informative because if women are on the move that is a pointer to a classic folk-wandering, where a whole people transplant their culture via migration. Many more British women arrived in the New World than Spanish women, and therein lay one of the crucial factors in the difference between Anglo and Latin America.

The slide show below has all the major figures of interest. I’ve also replicated the full description, and made some minor edits. Please take in the table; much of the paper really presupposes an intuitive familiarity with mtDNA haplogroup frequencies.



mtDNA pairwise Fst by population
Hunter-Gatherers Near East LBK
Near East 4.46 * *
LBK 9.9 3.22 *
Central Europe 3.67 1 4.22

The authors also had an Fst table illustrating genetic distances using mtDNA of ancient and contemporary populations. I’ve cleaned up the table a bit, and standardized the values so that the smallest distance = 1. This is mostly so you can make immediate sense of it. What you clearly see is the enormous genetic distance between Central European hunter-gatherers and LBK, who were present in Germany right before the arrival of farmers. This comes very close to a falsification of the maximalist pots-not-people model, whereby farming spread from its point of origin in Anatolia and the Levant through a process of cultural diffusion, just like the alphabet or the potato. The relatively large distance between ancient and modern populations shouldn’t be too surprising, genetic distances operate across both time and space. There are interesting inferences one can make about the nature of gene flow over the past 10,000 years in Eurasia when viewing the relatively small distance between the two modern populations, but really the important point for the purposes of this paper is the high wall between the two cultures who practice differing modes of production.

In the paper the authors support, tentatively, a classic demic diffusion process. This is basically a very simple model whereby farmers with larger population growth rates expand into the “space” of hunter-gatherers. But as Dienekes Pontikos notes such a process would also be characterized by dilution of the original Middle Eastern “genetic signal” over time. Rather, what we see here seems to be a total transfer for a population across large distances. The authors themselves note that the LBK farmers seem to have followed the interior lines of rivers and flat-bottom plains. Farmers had discovered a new way to exploit nature, but in the end they were still ecologically constrained. The northern two-thirds of Scandinavia still had hunter-gatherer populations down to the period of the classical Greeks. This was not because of the powerful magic wielded by Väinämöinen. The Middle Eastern derived agricultural toolkit no doubt began to run into its natural ecological limits on Europe’s northern fringe. Without knowing anything further I suspect that the death of the southern Sami culture in the face of Norwegian and Swedish expansion in the early modern period was probably driven by the emergence of more systematic agricultural science, which could push the ecological limits beyond the long-standing equilibrium established in the Iron Age.

But I don’t think this is just a story of ecology. It is clearly a story of culture. We assume that culture is easily transferable from society to society. In some ways it is. The original phonetic script of Upper Mesopotamia and Syria seems to have quickly triggered imitation and appropriation from India to Italy within a few centuries of its widespread use by the Aramaeans. But farming is not like the idea of writing. The original farmers seem to have expanded rather slowly initially out of the Middle East. Not only did they perfect the biological character of their crops, they probably perfected the customs and traditions which would go along with farming. A complex suite of explicit rules and implicit norms. Perhaps it was not so easy to simply copy the farming lifestyle? Or, perhaps more interestingly, the hunter-gatherers by and large did not want to copy the farming lifestyle? (this is a tendency among some modern non-farming groups, who would rather work temporarily on farms themselves rather than become full-time obligate peasants) The large genetic distances between the LBK and the hunter-gatherers around them may indicate not only the relatively endogenous growth of the LBK in “virgin” land (e.g., compare to the Yankees of New England in the 17th and 18th century), but, also the emergence of an ideological aversion to mixing with the “savages.” We have plenty of textually attested de-humanization of the “savage” and “barbarian” by the “civilized.” It is likely that the gap between the LBK and the hunter-gatherers of Europe was only somewhat smaller than that between the Aborigines of Australia and Tasmania and the European settlers!

Finally, there’s one last issue I want to highlight: the authors find that many presumably hunter-gatherer lineages are found among the LBK, while very common haplogroups (mtDNA and Y) in Europe today are not found among the LBK or the ancient hunter-gatherers. The clear inference then would be that Europe went through several periods of demographic change and migration within the last 10,000 years. A simple two-way admixture scenario will not suffice. Yesterday I posted this bar plot which contrasted the pattern of ancestry of French vs. French Basque using ADMIXTURE at K = 10:

French

The green element is nearly 100% in Sardinia, and drops off to nearly nothing somewhere around Iran. The light blue component is modal around the Caucasus, though is widely distributed, from Spain to Bengal (yeah, that’s me!) to Sweden. A simple model would be that the light blue arrived with Neolithic agriculturalists, as the Basques are the descendants of the original Ice Age Europeans. But this may not be correct, and our impression of the Basques may be totally false. It is not out of the question now that the Basque culture may have arrived via the ancient leap-frogging of agriculture from fertile regions around the Mediterranean before the seafarers passed into the Atlantic and swept around the western fringe of Iberia. What we may be seeing is a palimpsest of agriculturalists, where the Basques simply lack the last layer.

In any case, one can speculate a lot right. Ancient DNA has allowed us to refute maximalist versions of pots-not-people, but has also overturned our ability to hold to simple robust models. In science you prefer parsimony, unless parsimony simply can’t explain the patterns at hand. I think we’re there at this point.

Citation: Wolfgang Haak, Oleg Balanovsky, Juan J. Sanchez, Sergey Koshel, Valery Zaporozhchenko, Christina J. Adler, Clio S. I. Der Sarkissian, Guido Brandt, Carolin Schwarz, Nicole Nicklisch, Veit Dresely, Barbara Fritsch, Elena Balanovska, Richard Villems, Harald Meller, Kurt W. Alt, Alan Cooper, & Genographic Consortium (2010). Ancient DNA from European Early Neolithic Farmers Reveals Their Near Eastern Affinities PLoS Biology : 10.1371/journal.pbio.1000536

August 2, 2010

Ancient DNA and Norden

Filed under: Ancient DNA,Genetics,Genomics,Scandinavia — Razib Khan @ 2:29 am

Genetics is now being brought to bear on whether there were non-trivial population movements in the prehistorical period. Or more precisely, a combination of genetics and archaeology, whereby the archaeologists retrieve and extract genetic material which the geneticists amplify and analyze. This has helped establish that European hunter-gatherers were not lactase persistent. This is totally unsurprising, but was a nice proof of principle. When it comes to ascertaining genetic relationships among populations, as opposed to specific traits whose genetic architecture is well established, it’s a bit trickier. Who knows how many population movements may have interposed themselves between the present and a particular period in the past from which you have samples?

A new paper in PLoS ONE reports findings which do little to clarify, though add weight to skepticism as to the definitiveness of earlier results, Genetic Diversity among Ancient Nordic Populations:

Using established criteria for work with fossil DNA we have analysed mitochondrial DNA from 92 individuals from 18 locations in Denmark ranging in time from the Mesolithic to the Medieval Age. Unequivocal assignment of mtDNA haplotypes was possible for 56 of the ancient individuals; however, the success rate varied substantially between sites; the highest rates were obtained with untouched, freshly excavated material, whereas heavy handling, archeological preservation and storage for many years influenced the ability to obtain authentic endogenic DNA. While the nucleotide diversity at two locations was similar to that among extant Danes, the diversity at four sites was considerably higher. This supports previous observations for ancient Britons. The overall occurrence of haplogroups did not deviate from extant Scandinavians, however, haplogroup I was significantly more frequent among the ancient Danes (average 13%) than among extant Danes and Scandinavians (~2.5%) as well as among other ancient population samples reported. Haplogroup I could therefore have been an ancient Southern Scandinavian type “diluted” by later immigration events. Interestingly, the two Neolithic samples (4,200 YBP, Bell Beaker culture) that were typed were haplogroup U4 and U5a, respectively, and the single Bronze Age sample (3,300–3,500 YBP) was haplogroup U4. These two haplogroups have been associated with the Mesolithic populations of Central and Northern Europe. Therefore, at least for Southern Scandinavia, our findings do not support a possible replacement of a haplogroup U dominated hunter-gatherer population by a more haplogroup diverse Neolithic Culture.

Here’s a review of an earlier paper on this topic. Here’s an important section from the discussion of the current paper:

…Given our small sample sizes from these crucial time periods further studies are certainly required. However, the frequency of Hg U4 and U5 declines significantly among our more recent Iron Age and Viking Age Danish population samples to the level observed among the extant Danish population. Our study therefore would point to the Early Iron Age and not the Neolithic Funnel Beaker Culture as suggested by Malmström et al. (2009)…as the time period when the mtDNA haplogroup frequency pattern, which is characteristic to the presently living population of Southern Scandinavia, emerged and remained by and large unaltered by the subsequent effects of genetic drift. In contrast to Hg U4, which is only found in the Neolithic and Early Bronze Age samples, Hg U5 was observed in ~9% (5/53) of the remaining ancient samples and identified at all sites except Kongemarken and Skovgaarde.

I wouldn’t put too much stock in these specific results. The sample sizes and representativeness issues are probably such that each new paper is going to change our assessment. But, I think the section which I emphasized points to a shift in the Zeitgeist. Until recently there’s been a very strong bias among historical geneticists to assume that the genetic variation is more strongly affected by deep time events, and that recent replacements and perturbations will have less impact. I think there were good reasons for this assumption, and still are, generalizing from broader patterns. But the over-extrapolation of the rule-of-thumb may have led to models which will soon be falsified in many specific instances.

On a slightly bittersweet note, ancient DNA will be able to answer questions about the origins of many circumpolar populations, but will have far less to tell us about societies and cultures further south, simply because of less favorable conditions for preservation. The main exception to this truism will presumably be desert societies. For example, Tutankhamun has been typed as of the R1b Y lineage.

April 20, 2010

Of pigs, people and porcine polygenism

800px-Wild_Pig_KSC02pd0873Jared Diamond famously argued in Guns, Germs and Steel that only a small set of organisms have the characteristics which make them viable domesticates. Diamond’s thesis is that the distribution of these organisms congenial to a mutualistic relationship with man shaped the arc of our species’ history and the variation in wealth that we see (though his a human-centric tale, we may enslave them, eat and use them as beasts of burden, but these are also species which have spread across the world with our expansion). This thesis has been challenged, but the bigger point of putting a focus on how humans relate to their domesticated animals, and the complex co-evolutionary path between the two, is something that we need to consider. In a plain biological and physical sense animals have utility; we eat them, and for thousands of years they were critical to our transportation networks. Some have argued that the rise of Islam, Arab monotheism, was contingent on the domestication of the camel (which opened up interior trade networks previously unaccessible). In The Horse, the Wheel, and Language: How Bronze-Age Riders from the Eurasian Steppes Shaped the Modern World the argument is made that the distribution of the Indo-European languages has to do with the facility of Central Eurasian plainsmen with their steeds. And of course there is the domestic dog, arguably the one creature which is able to read our emotions as if they were a con-specific.

I suspect that the evolution and ethology of domesticated animals will offer a window into our own evolution and ethology. Konrad Lorenz famously believed that humans were going through their own process of domestication all the while that they were selecting organisms suited to their own needs. More pliable, less intelligent, faster growing and maturing, and so forth. Know thy companions, and know thyself, so to speak.

What about an animal as intelligent as a dog, but famously tasty? (the combination of the two characters causing some ethical tension in the minds of many) I speak here of the pig. A few years ago research came out which showed that pig-culture was introduced to Europe from the Middle East. That is, Middle Eastern pigs came with Middle Eastern people in all likelihood. But modern European pigs do not derive from these lineages, rather, by comparing modern genetic variation with ancient DNA the authors showed that the Neolithic pigs had been replaced by local breeds. Just as pigs can go feral and fend for themselves rather easily, it seems that their basic morph can be derived from wild boar populations easily as well (by contrast, it will perhaps take some effort to derive a pekingese from wolf populations, offering a reason for why small dogs seem to have emerged once). A new paper explores the evolutionary history and phylogeography of the pigs of the swine-loving societies par excellence, those of East Asia. Patterns of East Asian pig domestication, migration, and turnover revealed by modern and ancient DNA:

The establishment of agricultural economies based upon domestic animals began independently in many parts of the world and led to both increases in human population size and the migration of people carrying domestic plants and animals. The precise circumstances of the earliest phases of these events remain mysterious given their antiquity and the fact that subsequent waves of migrants have often replaced the first. Through the use of more than 1,500 modern (including 151 previously uncharacterized specimens) and 18 ancient (representing six East Asian archeological sites) pig (Sus scrofa) DNA sequences sampled across East Asia, we provide evidence for the long-term genetic continuity between modern and ancient Chinese domestic pigs. Although the Chinese case for independent pig domestication is supported by both genetic and archaeological evidence, we discuss five additional (and possibly) independent domestications of indigenous wild boar populations: one in India, three in peninsular Southeast Asia, and one off the coast of Taiwan. Collectively, we refer to these instances as “cryptic domestication,” given the current lack of corroborating archaeological evidence. In addition, we demonstrate the existence of numerous populations of genetically distinct and widespread wild boar populations that have not contributed maternal genetic material to modern domestic stocks. The overall findings provide the most complete picture yet of pig evolution and domestication in East Asia, and generate testable hypotheses regarding the development and spread of early farmers in the Far East.

They used conventional phylogeographic techniques to catalog the variation in modern populations, as well as supplementing their data set with ancient samples. Here the genetic variance they’re looking at is the mtDNA, the maternal lineage. Easy to get at, and easy to analyze (lots of it, and non-recombinant). In general they seem to have found that there is a common genetic heritage of East Asian domestic pigs, who are embedded geographically among varieties of wild pig who exhibit localized genetic variants. Additionally, there are other varieties of domestic pig in Southeast and South Asia who seem to have arisen from their own boar populations (though there is a Pacific pig variant which seems to have been from mainland Southeast Asia, but that original source population has now been replaced by East Asian pigs). Finallythey find a strong continuity between ancient domestic East Asian pigs and the modern populations. This is a contrast with the findings in European which exhibited disjunction between past and present. Perhaps this has to do with the fact that East Asian pigs are more genuinely indigenous, derived from local wild lineages with regional adaptations, while the Middle Eastern pigs brought to Europe were short-term kludges easily superseded by domesticates derived from European boar populations.

pigfig2This figure shows the nature of haplotype sharing between wild and ancient & contemporary domestic pigs. The larger the pie, the more frequent the haplotype. The slices of the pie by color show wild (black), ancient (red) and modern domestic (white) shares of that haplotype. The line across the networks show the putative separation between the genetic variants relatively private to the wild populations, and those which lean toward a mix of wild & domesticates. The wild populations seem more diverse. 45 haplotypes out of 167 samples are found only in wild specimens, 92 haplotypes out of 339 samples are found only in domestic specimens, and 21 haplotypes are found in both 87 wild and 582 domestic pigs. One assumes that the domesticates are derived from a small subset of wild pigs, and that population underwent demographic expansion within the last 10,000 years. That’s not too different from our species, we’re descended from a small subset of H. sapiens, and we’ve undergone major demographic expansion. Our “wild” cousins among the great apes tend to have a lot more genetic variation even within their small populations because their demographic history has presumably been a bit more staid. As man was, so shall he turn his domesticates. And yet a major difference between the domestic pig and man seems to be that some variant of multiregionalism, the evolution of modern pigs from local lineages, and their subsequent hybridization to produce a genetically unified species, has been operative. One major caution with these studies is that they’re looking at mtDNA. The dog genomics work has been modified and overturned when they shifted from the mtDNA that most phylogeographers focus on to the total genome. One does not know the evolutionary history of an organism by one locus alone.

The pig is a peculiar beast, retaining its feral nature as evident by the periodic reemergence of morphs from released domestic populations which have no difficulty in going “wild.” There are 4 million feral hogs in the United States, and they can get quite large indeed. What would the pekingese do in a world without man? Probably be some other creature’s meal. But generalists like the pigs would no doubt flourish. The story of the pig is a story of piggybacking, so to speak, on the success of the upright ape and spreading across the world on the backs of the other white meat.

Let me finish from the author’s conclusion:

The evidence presented here suggests the following evolutionary history of pigs in East Asia. Having originally evolved in ISEA [Island Southeast Asia], wild Sus scrofa migrated (without human assistance) across the Kra Isthmus on the MalayPeninsula into Mainland Asia. From here, they spread across the landscape and, after traveling over land bridges, onto the islands of Japan, the Ryukyu chain, Taiwan, and Lanyu where they evolved unique mitochondrial signatures. After millennia of hunting and gathering, a major biocultural transition occurred early in the Holocene during which human populations in East Asia domesticated a variety of plants and animals, including pigs. This process took place at least once in the Yellow River drainage basin wheremilletmay have been first domesticated as early as 10,000 B.P…and may have also taken place independently in the downstream Yangtze River region where rice may have been domesticated…Two things are clear from the ancient DNA evidence presented here. First, unlike Europe, modern Chinese domestic pigs are the direct descendants of the first domestic pigs in this region. Second, despite the occurrence of a genetically distinct population of wild boar throughout modern China, this population has neither been incorporated into domestic stocks nor exterminated.

Citation: Larson, G., Liu, R., Zhao, X., Yuan, J., Fuller, D., Barton, L., Dobney, K., Fan, Q., Gu, Z., Liu, X., Luo, Y., Lv, P., Andersson, L., & Li, N. (2010). Patterns of East Asian pig domestication, migration, and turnover revealed by modern and ancient DNA Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0912264107

Image credit: NASA

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