Sunday, May 21, 2017

Do Yamnaya Autosomal Genetics Derive From Caucasian Mail Order Brides?

Davidski at Eurogenes proposes a model for the formation of the Yamnaya genotype that makes a couple of key assumptions set forth below (emphasis in the original): 
- It's likely that low population densities in Eastern Europe during the Eneolithic ensured the rapid spread and rise of admixture from the Caucasus across much of the Pontic-Caspian steppe, which then plateaued at around 50% during the Yamnaya period, when population densities on the steppe may have become high enough so that continued gene flow from the Caucasus no longer had much of an impact. 
- The process that led to the Yamnaya genotype eventually led to its extinction by the Late Bronze Age, due to the large scale spread of Middle Neolithic European farmer ancestry across the entire Pontic-Caspian steppe, probably from its western half, resulting in the formation of the Steppe_MLBA genotype, exemplified by the Sintashta and Srubnaya people. 
- Ancient DNA suggests that Bronze Age steppe groups were highly patrilocal, and if so, it's likely that most of the mixture on the steppe at this time was facilitated via female exogamy (i.e. foreign brides), which would explain the lack of typically Caucasian Y-haplogroups, such as J2, in Bronze Age steppe and derived ancient groups sampled to date, such as the Corded Ware people and eastern Bell Beakers.
He also notes that Steppe people were highly mobile and that the circumstances encouraged admixture of Steppe peoples. There are some key unstated points are key as well (my statements, not a quotation):
* Steppe men are overwhelmingly Y-DNA R1, while this is rare among Caucasian men of any time period. Actually, Yamnaya culture men are predominantly Y-DNA R1b in the case of the Yamnaya, although the ancient DNA of Steppe men further to the North and in the Corded Ware Culture (which also had a more northern orientation) overwhelmingly reveals Y-DNA R1a.  (In both cases, of high derived version of each haplogroup, not the most basal versions.)
* Yamnaya autosomal genetics can be decently approximated by blending ancient DNA from Eastern Hunter-Gatherers (60%) and Caucasian Hunter-Gatherers (40%). This is what makes it necessary to hypothesize an influx of foreign Caucasian brides. See, e.g., here (migration from the Steppe paper) and here (earlier Eurogenes post analyzing ancient DNA samples). Genetic evidence of Caucasian admixture in ancient DNA is absent on the steppe until the Eneolithic (i.e. the Copper Age).
* Early Neolithic Caucasian farmers appear to have been largely derived from Caucasian Hunter-Gatherers, in situ, rather than largely replacing existing hunter-gatherers in places where they expanded as first wave Neolithic people in Europe derived from Anatolian Neolithic people did.  (See also here).
* There is strong physical evidence of patrilocality and large numbers of wives who migrate to their husband's homes from considerable distances from the remains of central European Bell Beaker men who have significant steppe ancestry. Other physical anthropology studies of Bronze Age European women have reached similar conclusions.
There is also anthropological corroboration for this hypothesis (again, my observations and not a quotation):
* Much of the metallurgy technology package that will subsequently be spread across Europe by people with significant steppe ancestry was probably first developed in the Caucasus mountains area. 
* The source of the steppe farming and herding package is less obvious, with David Anthony making a fairly solid case from archaeology in The Horse, The Wheel and Language for a basically Balkan origin of farming and herding technologies on the steppe. In general, there is considerable confusion among historical linguists and pre-historians over why Proto-Indo-European seems to have so many horticulture specific  and maritime shared root words for a society often conceptualized as made up of herders. Hunter-gatherers tend to transition much more easily to a pastoralist lifestyle (i.e. herding) than to horticulture that requires a much greater lifestyle and worldview change. But, the Early European Farmer autosomal DNA does not show up in Steppe populations until the Middle Bronze Age, so it is hard to seem Balkan farmers playing an important demic role in the formation of the steppe genotype. 
* The Caucasus mountains is also one possible source of a religious theme of a fire cult (also here) that is visible in the Zoroastrian religion which was established by Indo-Iranians (although also possibly in BMAC temples in the Harappan trade network and there is some linguistic evidence to support the idea that they shared a language with the Harappans from substrate words in Indo-Iranian and Indo-Aryan languages), although the center of gravity of this cult seems further east and later in time to be a good fit. A possibly residual fire cult can also be found in the cult of Hephaestus in the Aegean (with parallels in both Indo-European and non-Indo-European deities elsewhere in the region), although Hephaestus may have pre-Mycenean Greek roots as this blacksmith god is closely associated with the island of Lemnos which was one of the last non-Indo-European language hold outs in the region which was spoken until the 6th century BCE. The God's deformity is characteristic of Bronze Age rather than Iron Age smiths, who were often poisoned by arsenic associated with tin needed to make Bronze. 
The link between Caucasian brides and the people of the Pontic-Caspian steppe which is pretty much the only credible source of the main expansion (at least) of the Indo-European languages, however, is complicated. 

Did Caucasian brides and their children come to speak their husband's steppe language?  Or, could Indo-European be a Caucasian origin language passed from mother to child on the steppe?

The Caucasian mountains are currently home to several families of non-Indo-European languages that appear in a variety of respects (e.g. elaborate grammar and uncommon phonemes) to relict languages from a very long time ago (probably back at least to the early Neolithic in the region) that have not been heavily influenced by language contact or first language learners in highly isolated mountain valleys. The evidence from ancient mtDNA tends to confirm population genetic continuity in the Caucasus, although the ancient Y-DNA evidence seems to show more change over time.

So, the former hypothesis, that brides and their children came to speak their husband's language, seems more likely and has some corroboration in the pattern observed in similar situations that are historically attested.

One possibility is that Southern steppe people were not originally Indo-Europeans and had a strongly Caucasian influenced language (possibly related to Minoan and Basque), but that northern steppe people were Indo-Europeans. The fact that early Iberian Bell Beaker culture people seem to largely lack steppe ancestry, even though non-Iberian Bell Beaker people have it to a great degree, however, complicates the story.

New ancient DNA to come from ancient Minoans (non-Indo-European pre-Greeks of Crete), Mycenaeans (the first Greek speaking people of the Aegean), Maykop culture people (a technological source culture at the foothills of the Caucasus mountains near the Black Sea coast), and Harappans (pre-Indo-Europeans of the Indus River Valley civilization) will help shed some light on the different possible hypotheses in the near future.

How To Rule Out The Extra Dimensions Of String Theory

A blog post by Sabine Hossenfelder at Backreaction reviews a new paper that discusses how the existence of more than four dimensions, even is accessible only to gravitons, as required by string theory (which needs ten or eleven dimensions), could be ruled out with the LIGO gravitational wave detector and two comparable independent gravitational wave detectors.

Current experiments place bounds on the size of any extra dimensions as follows:
The mass of these excitations is inversely proportional to the radius (in natural units). This means if the radius is small, one needs a lot of energy to create an excitation, and this explains why he haven’t yet noticed the additional dimensions. . . .
From the current lack of observation, one can then derive bounds on the size of the extra-dimension. These bounds depend on the number of extra-dimensions and on their intrinsic curvature. For the simplest case – the flat extra-dimensions used in the paper – the bounds range from a few micrometers (for two extra-dimensions) to a few inverse MeV for six extra dimensions (natural units again).
Basically, gravitational waves, even the ordinary ones we can detect with LIGO (as opposed to the excited ones that involve exited gravitons that have excitations in extra dimensions) behave differently in a world with extra dimensions (even compactified dimensions that only gravitons can enter), than they do in general relativity.

If extra dimensions exist, there are excited states of all particles, gravitons have additional polarizations, and they exhibit a kind of oscillation known as a "breathing mode" that does not exist in general relativity. 

Absolutely ruling out the possibility is a technically difficult feat, but placing profoundly more strict limits on their number and size is considerably more straight forward. Only a narrow sub-type of extra dimensions is particular hard to detect with this methodology. So, it would be fairly easy to limit any possible string theory to the kind with that narrow subtype of extra dimensions.

Saturday, May 20, 2017

Sometimes Simple Things Are Complicated

In quantum physics, some interactions that seem simple, like two photons that interact in a manner that ultimately produces two photons, are actually wickedly complex. This is because you have to consider every conceivable way consistent with the laws of quantum mechanics that you can get from the starting state to the end state.

In this simple scenario, called light by light scattering, that means considering the possibility that the two photons have paths that include all sorts of mesons (two quark particles) that are created and destroyed in intermediate steps in the interactions, either as "real" particles or "virtual" ones (called "off shell").

A recent paper tackling some of the deep complexities of this seemingly simple interaction by analyzing fifteen of the most important intermediate meson paths is here.