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

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.

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