Tuesday, October 28, 2014

More Evidence Of Pre-Columbian Contact With Asia

In a recent post at this blog I recapped the evidence for pre-Columbian contact with the Americas after the Beringian land bridge was inundated. There were three main waves of subsequent migration:

* Saqqaq and Dorest Paleo-Eskimo populations are traceable to a wave of migration ca. 3500 BCE. These two archaeological cultures represent a single wave of demic migration and have genetic identity with each other (but admixed very little with other Native Americans before their demise).

* Early Na-Dene people arrive in Alaska ca. 1500 BCE.

* The 6th to 7th century CE Berginian Birnirk culture (in turn derived from Siberian populations) is the source of the proto-Inuit Thule people, who were the last substantial and sustained wave of pre-Columbian peoples to migrate to the Americas.

There were also several instances of less substantial and sustained migration, many of which have just become widely known:

* Around 1000 CE, Lief Erickson led a small population of Vikings to a short lived agricultural settlement called Vinland in maritime Canada. Recent discoveries announced in National Geographic in November of 2012 established that there were trade relations between the Vikings and indigeneous Arctic people at around the same time at the Northern tip of Canada's Baffin Island.

* From around 900 CE to 1100 CE, the "people who lived . . . in what today is the Lambayeque region, about 800 kilometers (500 miles) north of Lima, [Peru] had genetic links to the contemporaneous populations of Ecuador, Colombia, Siberia, Taiwan and to the Ainu people of northern Japan." These people were practitioners of the Middle Sican culture. It is not clear to what extent this contact was Austronesian in origin.

* I said then that:
Late in the period of Austronesian expansion (probably not earlier than 700 CE, with radiocarbon dated examples found in the Cook Islands by 1000 CE), perhaps from a final launching point at Easter Island, the kumara, a yam-like plant native to South America and possibly native to Peru, entered Austronesian territory and became a staple food. But, no genetic traces of the New World are found in Austronesian populations. It is possible that the kumara's arrival in Oceania and the Asian genetic influences found in Middle Sican graves involved the same instance of cultural Old World-New World contact.
The evidence now contradicts that contention insofar as it claims that there is no genetic evidence of Precolumbian contact in Polynesia. Razib Khan notes a new paper demonstrating that Easter Islanders have approximately 8% Native American admixture in their autosomal DNA arising from a Precolumbian admixture event. He notes that: "The rough dates for Amerindian ancestry admixture are in the range of 1300 to 1400 A.D., which match reasonably well with when Easter Island was settled."

* Razib also notes in the same post a new paper whose abstract states:
[W]e present 14C dates, and morphological, isotopic and genomic sequence data from two human skulls from the state of Minas Gerais, Brazil, part of one of the indigenous groups known as ‘Botocudos’. We find that their genomic ancestry is Polynesian, with no detectable Native American component. Radiocarbon analysis of the skulls shows that the individuals had died prior to the beginning of the 19th century. Our findings could either represent genomic evidence of Polynesians reaching South America during their Pacific expansion, or European-mediated transport.
As Razib sums up the finding of the paper in that post:
[H]eretofore the reasonable assumption about these Polynesian remains in interior Brazil were the product of escaped slaves, but there is an 80-90% probability that they died before any such enslavement of Polynesians could have occurred. In fact both remains may be pre-Columbian!
Both of the new papers appear in the most recent issue of the Journal Current Biology.

* Wikipedia summarizes many, but not all of these contacts, and adds others that I have not listed above.  Few are credible, but there is plausible evidence to support a few additional minor Old World-New World contacts.

* The Austronesians, the culture behind both of the latest two finds, are the same seafaring people who transplanted a community of their people from Indonesia, language intact, all of the way to Madagascar, with a small amount of South Asian admixture picked up along the way.

Of course, while these contacts did occur, none of them had the epic consequences that the era of European contact with the Americas begun by expedition of Christopher Columbus did.

New Insights From 45,000 Year Old Siberian Ancient DNA

A new study has the oldest modern human ancient DNA sequence every analyzed, from 45,000 in Siberia, assigned the name Ust’-Ishim.

Some key points:
The Y chromosome sequence of the Ust’-Ishim individual is . . . inferred to be ancestral to a group of related Y chromosomes (haplogroup K(xLT)) that occurs across Eurasia today. . . . The Ust’-Ishim mtDNA sequence falls at the root of a large group of related mtDNAs (the ‘R haplogroup’), which occurs today across Eurasia. . . . 
Based on genotyping data for 87 African and 108 non-African individuals, the Ust’-Ishim genome shares more alleles with non-Africans than with sub-Saharan Africans (|Z| = 41–89), consistent with the principal component analysis, mtDNA and Y chromosome results. . . . Among the non-Africans, the Ust’-Ishim genome shares more derived alleles with present-day people from East Asia than with present-day Europeans (|Z| = 2.1–6.4). . . . However, when an ~8,000-year-old genome from western Europe (La BraƱa) or a 24,000-year-old genome from Siberia (Mal’ta 1) were analysed, there is no evidence that the Ust’-Ishim genome shares more derived alleles with present-day East Asians than with these prehistoric individuals (|Z| < 2). This suggests that the population to which the Ust’-Ishim individual belonged diverged from the ancestors of present-day West Eurasian and East Eurasian populations before—or simultaneously with—their divergence from each other. The finding that the Ust’-Ishim individual is equally closely related to present-day Asians and to 8,000- to 24,000-year-old individuals from western Eurasia, but not to present-day Europeans, is compatible with the hypothesis that present-day Europeans derive some of their ancestry from a population that did not participate in the initial dispersals of modern humans into Europe and Asia.

Assuming that this corresponds to the number of mutations that have accumulated over around 45,000 years, we estimate a mutation rate of 0.43 × 10−9 per site per year (95% CI 0.38 × 10−9 to 0.49 × 10−9) that is consistent across all non-African genomes regardless of their coverage. This overall rate, as well as the relative rates inferred for different mutational classes (transversions, non-CpG transitions, and CpG transitions), is similar to the rate observed for de novo estimates from human pedigrees (~0.5 × 10−9 per site per year14, 15) and to the direct estimate of branch shortening. As discussed elsewhere, these rates are slower than those estimated using calibrations based on the fossil record and thus suggest older dates for the splits of modern human and archaic populations. [Ed. by Dienekes This is a very direct confirmation of the "slow" autosomal rate of ~1.2x10-8 mutations/generation/bp using a technology much different than those used before to estimate this. The slower mutation rate implies that major splits in human history (such as the Out-of-Africa event) took place much earlier than the Upper Paleolithic revolution and the spread of humans across Eurasia.] . . .

[W]e estimate that the admixture between the ancestors of the Ust’-Ishim individual and Neanderthals occurred approximately 50,000 to 60,000 years BP, which is close to the time of the major expansion of modern humans out of Africa and the Middle East.
In other words, this person lived just 5,000-15,000 years after Neanderthal admixture, a time which is on the recent end of other estimates of the timing of this event.

As a minor point, the discovery of 45,000 modern human remains in Siberia at all, provides a strong direct calibration point from which modern humans were known to be present at a specific out of Africa location.

Friday, October 17, 2014

Scalar and Tensor Dark Matter Doesn't Interact With SM Matter

All particles are either fermions, with total angular momentum J=0.5+N, or bosons, with total angular momentum J=0+N, in each case for N=0, 1, 2, 3 . . .  and the lower possible boson values for total angular momentum (aka "spin") have names.

spin-0 is called "scalar"
spin-1 is called "vector" and
spin-2 is called "tensor"

A recent measurement made by the ATLAS experiment at the Large Hadron Collider (LHC) set upper bounds on the cross-section of interaction between hypothetical scalar and tensor (i.e. spin-0 and spin-2) dark matter particles and the particles of the Standard Model.

These cross-sections of interaction can't be more than something on the order of 10-42 for scalar dark matter candidates with masses of between 1 GeV and 200 GeV+, and on the order of 10-40 for tensor dark matter candidates with masses of 1 GeV to 10 GeV, and as weak as 10-38 for dark matter candidates up to about 500 GeV. The LUX direct dark matter detection experiment imposes even stronger limitations on the cross-section of interaction of scalar dark matter candidates with masses of 10 GeV to 200 GeV+ of less than 10-44.

Furthermore, multiple lines of observational evidence strongly disfavor "cold dark matter" models.  Models with particles with more than 200 GeV masses are particularly strongly disfavored.

Thus, any scalar or tensor dark matter candidates must have interactions with ordinary matter than are many order of magnitude weaker than the slight interactions of neutrinos with other forms of ordinary matter.

Realistically, if scalar or tensor dark matter exists at all, it doesn't interact at all with particles outside the dark sector.  In all likelihood, they simply don't exist at all, because if they interacted with other dark matter particles that had higher cross-sections of interaction, you would at least see indirect evidence of their existence as they couple to other forms of dark matter with higher cross-sections of interaction with the Standard Model.

Now, in fairness, almost nobody in the astronomy community is seriously proposing massive scalar or tensor dark matter candidates, and in the most popular models that have a boson in the dark sector, self-interacting cold dark matter models, the typical candidate, sometimes called a "dark photon" is a vector boson (i.e. spin-1) with a mass of around 100 MeV (about ten times lighter than the low end of the scale proposed by ATLAS in this experiment).

So, while this ATLAS result ruled out one subtype of conceivable dark matter, it didn't do much to rule out the leading contenders in the race to explain dark matter phenomena (e.g., light axion dark matter, keV mass scale sterile neutrino-like fermionic dark matter, and GeV mass scale fermionic dark matter such as gravitinos and the fermion partners of Standard Model bosons in SUSY theories).

But, this is a blow to SUSY, because minimal SUSY theories create a myriad of new massive scalar particles, the lightest of which are good dark matter candidates, and some non-minimal SUSY theories often also create new massive tensor particles.

In particular, all R-parity conserving SUSY theories should have one or more massive scalar dark matter candidates, and R-parity violating SUSY theories are hard pressed to explain why SUSY dark matter still exists.  In R-parity violating SUSY models, one has to manipulate the SUSY model to prevent thermal relic, R-parity violating SUSY dark matter from all decaying to Standard Model particles over the life of the universe, something inconsistent with dark matter observations from astronomy.

META OBSERVATION:  Since the Dispatches at Turtle Island blog was created, there have been 101 posts including this one, that address the issue of dark matter to some extent (about 15% of all of the posts at this blog).  This is appropriate because dark matter is the most obvious and practically relevant of the unsolved problem in physics.