英语阅读：地球最早灵长类生活在树上.docx

Earths earliest primates2 have taken a step up in the world, now that researchers have gotten a good look at their ankles. A new study has found that Purgatorius, a small mammal that lived on a diet of fruit and insects, was a tree dweller3. Paleontologists made the discovery by analyzing4 65-million-year-old ankle bones collected from sites in northeastern Montana. Purgatorius, part of an extinct group of primates called plesiadapiforms, first appears in the fossil record shortly after the extinction5 of non-avian dinosaurs6. Some researchers have speculated over the years that primitive7 plesiadapiforms were terrestrial, and that primates moved into the tree canopy8 later. These ideas can still be found in some textbooks today. The textbook that I am currently using in my biological anthropology9 courses still has an illustration of Purgatorius walking on the ground. Hopefully this study will change what students are learning about earliest primate1 evolution and will place Purgatorius in the trees where it rightfully belongs, said Stephen Chester, the papers lead author. Chester, who conducted much of the research while at Yale University studying for his Ph.D., is an assistant professor at Brooklyn College, City University of New York. Chester is also a curatorial affiliate10 at the Yale Peabody Museum of Natural History. Until now, paleontologists had only the animals teeth and jaws11 to examine, which left much of its appearance and behavior a mystery. The identification of Purgatorius ankle bones, found in the same area as the teeth, gave researchers a better sense of how it lived. The ankle bones have diagnostic features for mobility12 that are only present in those of primates and their close relatives today, Chester said. These unique features would have allowed an animal such as Purgatorius to rotate and adjust its feet accordingly to grab branches while moving through trees. In contrast, ground-dwelling mammals lack these features and are better