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This Week in Science: Nov 9, 2018

In Science this week, a multi-institute team reports sequencing 15 ancient genomes from humans who lived in areas ranging from Alaska to Patagonia — some more than 10,000 years ago — and were most closely related to Native Americans. The study reveals complex and dynamic population histories from North to South America, with evidence of rapid dispersal and early diversification — including previously unknown groups — as people moved south. This activity resulted in multiple independent, geographically uneven migrations, including one that provides clues of a Late Pleistocene Australasian genetic signal, and a later Mesoamerican-related expansion, the researchers write.

Meanwhile, in Science Advances, an international research group presents a time series of ancient whole genomes from the Andes of Peru, dating back to as far as 7,000 years before present, and compares the sequences to ones from highland and lowland populations prehistoric and modern South American populations. The researchers identify a split between low- and high-elevation populations that occurred between 9,200 years and 8,200 years ago; a population collapse after European contact that is significantly more severe in South American lowlanders than in highland populations; and evidence for positive selection at genetic loci related to starch digestion and plausibly pathogen resistance after European contact. Notably, they did not find genes related to adaptation to hypoxia, "which may suggest more complex modes of genetic adaptation to high altitude."

GenomeWeb has more on these and a related study, here.

And in Science Translational Medicine, a European team of investigators presents a new method for detecting circulating tumor DNA that exploits the differences in ctDNA fragment length. The scientists  surveyed ctDNA fragment sizes in 344 plasma samples from 200 cancer patients using low-pass whole-genome sequencing, then performed tumor-guided personalized deep sequencing in 19 of the patients to establish the size distribution of mutant ctDNA. They discovered that focusing on fragments between 90 and 150 base pairs long improved ctDNA detection in patients with brain, renal, and pancreatic cancer. Analysis of the size-selected cfDNA identified clinically actionable mutations and copy number alterations that were otherwise not detected. "Our results indicate that exploiting the endogenous biological properties of cfDNA provides an alternative paradigm to deeper sequencing of ctDNA," the authors write.