Wednesday, October 18, 2017

Neutron Star Collision Tightens Neutrino Parameter Space

The latest big splash in physics, the observation of two neutron stars colliding into each other with both gravity waves and light, provides an independent observation of the Hubble constant that when combined with the Planck data tightens the parameter space of neutrino physics.

The sum of the three neutrino masses goes from less than 1.11 eV with Planck data alone, to less than 0.77 eV with this measurement and the Planck data combined. But, without this new data point, but combined with other data sets, the limitation was already even tighter with combinations as low as 0.11 eV, and the minimum value from neutrino oscillation data is 0.06 eV in a normal hierarchy. It isn't clear how much impact this new data point has on the earlier combination value.

The effective number of neutrino types (Neff) goes from 3.11 ± 0.25 with Planck data alone to 3.09 ± 0.25 with the addition of this measurement. But, as of 2015, the constraint with Planck data and other data sets was 3.04 ± 0.18. Neff equal to 3.046 in a case with the three Standard Model neutrinos and neutrinos with masses of 10 eV or more not counting in the calculation. It isn't entirely clear what the Neutron Star measurement of Hubble's constant adds, if anything, to the combined estimates, but it might, for example, slightly reduce the margin of error which would increase the significance by which the four neutrino case was ruled out. Neff and the Hubble constant are strongly correlated, but the combination value for Hubble's constant is very close to the new value from this observation.

So, the four neutrino case is ruled out at a more than 5.3 sigma level already, which is a threshold for a scientific discovery that there are indeed only three neutrinos with masses of 10 eV or less, ruling out the sterile neutrino hypothesis for a stable sterile neutrino of under 10 eV (when a best fit of potential anomalies from reactors predicts a sterile neutrino mass of about 1 eV also here). A 2015 pre-print on notes that:
The 95% allowed region in parameter space is Neff < 3.7, meff s < 0.52 eV from PlanckTT + lowP + lensing + BAO. This result has important consequences for the sterile neutrino interpretation of short-baseline anomalies. It has been shown that a sterile neutrino with the large mixing angles required to explain reactor anomalies would thermalize rapidly in the early Universe, yielding ∆Neff = 1. The preferred short-baseline solution then corresponds to ms of about 1 eV and ∆Neff = 1 and is strongly excluded (more than 99% confidence) by the above combination of Planck and BAO data.

Sunday, October 15, 2017

Ancient DNA Refines New World Settlement Paradigm


new pre-print based on analysis of ancient DNA from a number of far northern populations in North America and Siberia has refined what we know about the waves of pre-Columbian migration to North American after the founding population of American entered North America from the Beringian land bridge not later than about 14,000 BCE.[1] 

There is genetic variation that is found in Canada and the United States, but not in Latin America, which is not attributable to later waves of pre-Columbian migration. This was due either to founder effects in a population that rushed down the Pacific coast over a period of less than 2000 years and became ancestral to all indigenous Americans outside of Canada and the United States, or due to population structure in the original founding population of the Americas.

All pre-Columbian genetic ancestry extant in modern gene pools in the Americas (with the possible exception of part of the ancestry found in certain Amazonian tribes whose source is not well understood which is sometimes described as "Paleo-Asian" [2]) is attributable to this initial founding population (often called "First Americans" in the literature) or two main subsequent migrations from Siberia from genetically related (but not identical) Siberian populations. 

The first wave of subsequent pre-Columbian migration, ca. 3500 BCE (the current pre-print makes the case for an approximate 3000 BCE date [3]) gave rise to the Paleo-Eskimos who ceased to exist as a distinct population around the time that the "Neo-Eskimos" arrived, which is also the source, a few centuries later, of the distinctive genetic ancestry of the Na-Dene people (such as the Tlingit people of Southeast Alaska, and the Apache and Navajo people of the American Southeast), which is otherwise shared with the founding population of the Americas.

The second wave sometime between 1100 BCE and 200 BCE (probably closer to 200 BCE, given the archaeological evidence) gave rise to the Neo-Eskimos such as the Inuits, after substantial admixture with people descended from the founding population of the Americas. (The Na-Dene people were not a source of admixture with the Neo-Eskimos.)

Incidentally, this three wave model had already been predicted based upon archaeological and linguistic evidence several decades before genetic evidence confirmed this model.

There have been a few other well documented instances of much more incidental pre-Columbian contact after the founding population arrived with the Americas (for example, Vikings ca. 1000 CE in Eastern Canada), but none of the other cases left any genetic legacy in the Americas. Also notably, all of those credibly documented contacts post-date the Paleo-Eskimo migration, and all but one doubtful case of a single alleged New World apple seed found in India that was carbon dated to 2000 BCE, post-date both of these migration waves. For example, the Asian War Complex arrives ca. 700 CE, about the same time as the Neo-Eskimos reach Alaska proper, an a previous major bow and arrow technology innovation arrives ca. 400 BCE around the time of first contact between pre-Neo-Eskimos from Asia and and indigenous populations in the Bering Straight. Critically, all First American populations were still isolated from all other modern humans for more than 10,000 years (a time depth that, among other things, is so great that it impossible to reconstruct a proto-Amerind language from known indigenous American languages).

The Story Of The Na-Dene

The genetic distinctiveness of the Na-Dene (a.k.a. Athabaskan) indigenous populations of North America derives from admixture of Paleo-Eskimos in Alaska ca. 3000 BCE plus or minus a few centuries (genetic and archaeological evidence suggests that they arrived ca. 3500 BCE [4], giving rise to the Saqqaq culture and started to admix several centuries later) with indigenous First Americans (whose ancestors arrived over the Beringian land bridge by not later than about 14,000 BCE). The initial admixture percentage in ancient Northern Athabaskans was roughly 25%-40%. Over time, the Paleo-Eskimo admixture percentage in the Na-Dene has been diluted to about 10%, but all Na-Dene populations have this ancestry which is almost completely absent from non-Na-Dene language speakers.

Genetic evidence also supports the Dene-Yeniseian linguistic hypothesis that the Na-Dene languages have a linguistic family relationship with the Yeniseian language family of Siberia of which the sole surviving language is the Ket language. Specifically, ancient DNA from a population in the Ust'-Belaya culture of Chukotkan in Siberia, which is ancestral to the Paleo-Eskimos, shows evidence of genetic relatedness to both the Paleo-Eskimos and a western Siberian group related to Kets. The pre-print notes that: 
striking parallels in archaeological and genetic results suggest that admixture between proto-Paleo-Eskimos and Siberian lineages in Chukotka took place not long after they diverged [ca. 4300 BCE], indicating that cultural contact between these groups at this time almost certainly occurred as well. This result has implications for archaeology and historical linguistics[.]
At least two "push factors" drove Na-Dene tribes such as the Navajo to migrate to the American Southwest around 900 CE, and may have also been involved in the migration of the Tlingit Na-Dene tribes to southeastern Alaska and Pacific coastal Canada. One was the arrival of "Neo-Eskimos" (e.g. the founding population of the Inuits who were part of the Birnirk and Thule cultures) in western Alaska ca. 650 CE to 850 CE. The other was a massive volcanic ash fall ca. 900 CE. There was little or no Na-Dene genetic introgression into Neo-Eskimo populations. 

In the American Southwest, there was also a "pull factor" which was the control vacuum created when the society of the ancient Puebloan people (a.k.a. the Anasazi) collapsed, in part due to a major drought associated with the Medieval Climate Anomaly (which quite possibly was related, in part, to the volcanic eruptions that took place at about the same time that the MCA began) that may have partially encouraged Na-Dene people who had already migrated to the Canadian heartland to migrate South, or may have made them more successful as a potentially competing collapsed when, or not long after, they arrived.

It is also worth noting that there were major innovations in bow and arrow technology ca. 2500 BCE, which had previously been stagnant since 10,000 BCE, which might have coincided with the arrival of the Paleo-Eskimos and the increasing prevalence of the Arctic Small Tool tradition.

The Story of the Modern Eskimos

Some members of the population the Siberian population ancestral to the Paleo-Eskimos migrated to North America. Others remained in Siberia and gave rise to a sub-population ca. 1500 BCE (about 1500 years after the first wave admixed with the Na-Dene) that was ancestral to modern "Neo-Eskimos" such as the Inuits who arrived in western Alaska ca. 650 CE to 850 CE and are known to archaeologists as the Birnirk and Thule cultures. About 43% of the modern Neo-Eskimo population's ancestry is attributable Asian migrants from this population and this population is also the source of mtDNA D2a in Neo-Eskimos. 

The balance of Neo-Eskimo ancestry [5], arises from northern non-Na-Dene descendants of the first wave of migrants to North America, and in particular, from descendants of the indigenous southwestern Alaska and Kodiak Island Ocean Bay tradition (which flourished ca. 4800 BCE to 2500 BCE) in which a lot of the technological innovations involving year round maritime hunting were invented. In contrast, early "Paleo-Eskimo people used marine resources on a seasonal basis only, depended for the most part on hunting caribou and muskox, and lacked sophisticated hunting gear that allowed the later Inuit to become specialized in whaling." Thus, while the new influx of genes and their language had Asian origins, their maritime hunting technologies were made in North America.

The admixture that gave rise to the modern Neo-Eskimo population had already substantially run its course by 200 CE when ancient DNA reveals an already admixed Neo-Eskimo who was part of the Old Bering Sea culture which commenced ca. 200 BCE.

The admixture that gave rise to the ethnogenesis of the Neo-Eskimos predated the Ipiutak culture of western Alaska which emerged ca. 300 CE.

It is possible that some or all of the admixture and ethnogenesis that gave rise to the Neo-Eskimos took place in the Old Whaling culture of western Alaska (ca. 1100 BCE to 700 BCE), the Choris culture of western Alaska (ca. 700 BCE to 500 BCE) or the Norton culture of western Alaska (ca. 500 BCE to 200 BCE). There isn't enough ancient DNA data available to be more specific. Archaeological data tends to support a date at the more recent end of that range, such as the tail end of the Norton culture [6].

End notes and language from the body text of the paper supporting this summary, with editorial emphasis, additional hyperlinks to prior posts at this blog, and headings, appear below.