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[QUOTE]Originally posted by DD'eDeN: [QB] The roots of the mammalian family-tree have long been shrouded in mystery: when did the placental mammals go their separate ways? Now, researchers say they've found where the family tree of placental mammals first branched apart, and when it happened. Placental mammals consist of 3 main groups that diverged rapidly, evolving in wildly different directions: - Afrotheria: elephants, tenrecs... - Xenarthra: armadillos, sloths... - Boreoeutheria: all others. The relationships between them have been a subject of fierce controversy, with multiple studies coming to incompatible conclusions over the last decade, leading some researchers to suggest that these relationships might be impossible to resolve: - which is the oldest sibling of the 3? - did the mammals go their separate ways, due to S.America & Africa breaking apart? - when did placentals split up? Tarver: "This has been one of the areas of greatest debate in evolutionary biology, many researchers consider it impossible to resolve. Now we've proven these problems can be solved: you just need to analyze genome-scale data-sets, using models that accurately reflect genomic evolution.² The researchers assembled the largest mammalian phylogenomic dataset ever collected, before testing it with a variety of models of molecular evolution, choosing the most robust model, and then analysing the data, using several supercomputer clusters: "We tested it to destruction. We threw the kitchen sink at it." Slavish Mirarab: "A complication in reconstructing evolutionary histories from genomic data is that different parts of genomes can & often do give conflicting accounts of the history. Individual genes within the same species can have different histories. This is one reason why the controversy has stood so long: many thought the relationships couldn¹t be resolved.² To address the complexities of analysing large numbers of genes shared among many spp, the researchers paired 2 fundamentally different approaches: - concatenated & - coalescent-based analyses. When the dust settled, the team had a specific family-tree: Atlantogenata (African Afrotheria + S.American Xenarthra) is the sister group to all other placentals. The team then tested 3 of the most influential rivals against each other with the same model. All of the previous studies suddenly fell into line, their data agreeing with Tarver cs. The researchers folded in another layer: a molecular clock analysis. Mario Dos-Reis: "The molecular clock analysis uses a combination of fossils & genomic data, to estimate when these lineages diverged from each other: Afrotheria & Xenarthra diverged from one another c 90 Ma." Previously, scientists thought that when Africa & S.America separated >100 Ma, they broke up the family of placental mammals. But the researchers found that placental mammals didn't split up until after Africa & S.America had already separated. Bob Asher: "We propose that South America's living endemic Xenarthra colonized the island-continent via overwater dispersal." At the time, the proto-Atlantic was only a few 100 miles wide. New World monkeys crossed the Atlantic later, when it was much bigger, probably on rafts formed from storm debris. [b]Note: SAm monkies & cavvies left Africa into Antarctica, then South America via Patagonia; there was NO rafting on storm debris.[/b] And, of course, mammals repeatedly colonised remote islands like Madagascar. Carver: "You don't always need to overturn the status-quo to make a big impact. All of the competing hypotheses had some evidence to support them: that¹s precisely why it was the source of such controversy. Proving the roots of the placental family tree with hard empirical evidence is a massive accomplishment." The Interrelationships of Placental Mammals and the Limits of Phylogenetic Inference James E Tarver, Mario dos Reis, Siavash Mirarab, Raymond J Moran, Sean Parker, Joseph E O¹Reilly, Benjamin L King, Mary J O¹Connell, Robert J Asher, Tandy Warnow, Kevin J Peterson, Philip CJ Donoghue & Davide Pisani 2016 Genome Biol. Evol.8:330 doi 10.1093/gbe/evv261 <http://dx.doi.org/10.1093/gbe/evv261> Placental mammals comprise 3 principal clades: - Afrotheria, e.g. elephants & tenrecs, - Xenarthra, e.g. armadillos & sloths), - Boreoeutheria (all other placental mammals)... Previous analyses have found support for all 3 hypotheses: some concluded that this phylogenetic problem might be impossible to resolve, due to the compounded effects of - incomplete lineage sorting (ILS) & - a rapid radiation. Here we use - a genome-scale nucleotide data-set, - microRNAs & - re-analysis of the 3 largest previously published amino-acid data-sets: the root of Placentalia lies between Atlantogenata & Boreoeutheria. Although we found evidence for ILS in early placental evolution, we are able to reject previous conclusions that the placental root cannot be resolved. Re-analyses of previous data-sets recover Atlantogenata + Boreoeutheria: contradictory results are a consequence of poorly fitting evolutionary models. When the evolutionary process is better-modeled, all data-sets converge on Atlantogenata. Our Bayesian molecular clock analysis estimates - marsupials diverged from placentals 157-170 Ma, - crown Placentalia diverged 86-100 Ma, - crown Atlantogenata diverged 84-97 Ma. Our results are compatible with placental diversification being driven by dispersal rather than vicariance mechanisms, postdating early phases in the protracted opening of the Atlantic Ocean. _______ [/QB][/QUOTE]
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