Wednesday, August 9, 2017

Modern Humans Reached Indonesia Shortly After The Toba Erruption

The Toba mega volcano eruption took place about 75,000 to 74,000 years ago.
Toba Lake in northern Sumatra is the world's largest active volcanic caldera. The volcanic eruption that resulted in Lake Toba (100 x 30 km) 74,000 years ago, is known to have been by far the biggest eruption of the last 2 million years. This mega-bang caused a prolonged world-wide nuclear winter and released ash in a huge plume that spread to the north-west and covered India, Pakistan, and the Gulf region in a blanket 1–5 metres (3–15 feet) deep. Toba ash is also found in the Greenland ice-record and submarine cores in the Indian Ocean, allowing a precise date marker. . . . the Toba eruption is the most accurately dated, dramatic, and unambiguous event before the last ice age.
Very shortly after that eruption, we find the oldest reliably dated modern human presence in what is now called Sumata, Indonesia, which is in Island Southeast Asia. We also know that modern humans were present in India prior to the Toba eruption based upon lithic tools of types (of the same type across the Toba ash barrier) associated only with modern humans that have been found both above and below Toba ash there.

It seems likely that the Toba eruption opened up a biogeographic barrier that had confined modern humans to India before then, possibly by thinning out dense forests and shrinking existing hominin populations in the region. 

I'd love to see ancient DNA from the teeth sampled here to see if they had Denisovan admixture, to look at the timing of their Neanderthal admixture, and to see how much of the genetic diversity present in these samples is still found in Southeast Asian populations. It would also clarify the timing of different waves of migration associated with particular uniparental genetic markers.
Genetic evidence for anatomically modern humans (AMH) out of Africa before 75 thousand years ago (ka) and in island southeast Asia (ISEA) before 60 ka (93–61 ka) predates accepted archaeological records of occupation in the region. Claims that AMH arrived in ISEA before 60 ka have been supported only by equivocal or non-skeletal evidence AMH evidence from this period is rare and lacks robust chronologies owing to a lack of direct dating applications, poor preservation and/or excavation strategies and questionable taxonomic identifications. 
Lida Ajer is a Sumatran Pleistocene cave with a rich rainforest fauna associated with fossil human teeth. The importance of the site is unclear owing to unsupported taxonomic identification of these fossils and uncertainties regarding the age of the deposit, therefore it is rarely considered in models of human dispersal. 
Here we reinvestigate Lida Ajer to identify the teeth confidently and establish a robust chronology using an integrated dating approach. Using enamel–dentine junction morphology, enamel thickness and comparative morphology, we show that the teeth are unequivocally AMH. Luminescence and uranium-series techniques applied to bone-bearing sediments and speleothems, and coupled uranium-series and electron spin resonance dating of mammalian teeth, place modern humans in Sumatra between 73 and 63 ka. This age is consistent with biostratigraphic estimations, palaeoclimate and sea-level reconstructions, and genetic evidence for a pre-60 ka arrival of AMH into ISEA. Lida Ajer represents, to our knowledge, the earliest evidence of rainforest occupation by AMH, and underscores the importance of reassessing the timing and environmental context of the dispersal of modern humans out of Africa.
K. E. Westaway, et al., "An early modern human presence in Sumatra 73,000–63,000 years ago" Nature(August 9, 2017) (Pay per view) doi:10.1038/nature23452

A newspaper account of the paper's findings with photographs can be found here.

Tuesday, August 8, 2017

Muons Are Weird. Is An Electron-Phobic Scalar Boson To Blame?

In the Standard Model, a muon is simply an electron with a bigger mass. 

But, measurements of the radius of muonic hydrogen and the muon magnetic dipole moment (muon g-2), show a fairly significant discrepancy between theory an experiment in that respect, at the five sigma and three sigma levels, respectively. There are also indications from B meson decays that the lepton universality is violated by the charged leptons (i.e. the muons and electrons do not behave identically apart from the differences predicted as a result of their respective masses).

A new PhD thesis by Yu-Sheng Liu at the University of Washington explores what kind of new physics could give rise to this discrepancy. The thesis concludes that a scalar boson whose couplings to the charged leptons differ by the ratio of the muon mass to the electron mass (mu/me)^n for some n>1 could resolve the subtle discrepancy between theory and experiment. In this theory, the new scalar boson couples more strongly to muons than to electrons. 

The  thesis then looks at the experimental bounds on the relevant coupling constants of this minimally flavor violating scalar boson. A vector boson or much of the rest of the parameter space (e.g. n<1) is ruled out.

The paper does not suggest how such an electron-phobic scalar boson would fit into any larger theoretical model, for example, at high energies.

While there are many possible explanations for the observed discrepancies, the most plausible of which involve experimental measurement issues, understated error bars and flawed theoretical calculations using Standard Model physics, this humble but thorough study presents one of the most plausible beyond the Standard Model theories to explain these phenomena that I have seen to date.

Ancient People In Forests and Jungles

While some of the dates are doubtful or probably represent archaic hominins, rather than modern humans, there is no doubt that people have lined in forests and jungles for a very long time.
In the last ten years, the archaeologically-acknowledged start date of human inhabitation of tropical forests has quadrupled in age. There is now clear evidence for the use of tropical forests by our species in Borneo and Melanesia by c. 45 ka; in South Asia by c. 36 ka; and in South America by c. 13 ka. There are suggestions of earlier rainforest occupation c. 125 ka in Java, c. 60 ka in the Philippines, c. 100 ka in China, and in Africa perhaps from the first appearance of Homo sapiens c. 200 ka, though further research is required to verify these cases.

Early modern humans adapted to diverse tropical forest formations, ranging from the sub-zero temperatures of montane forests to dense, humid, evergreen rainforests, undertaking sophisticated forest mammal hunting and plant processing. Moreover, people did not just adapt passively to these environments, but from the onset modified them in fundamental ways, with outcomes that have affected the natural histories of these forests to the present day.