雅思阅读实战训练模拟试题(五)_雅思英语阅读

雅思阅读实战训练模拟试题(五)

　　Wash in，wash out

　　7.Because many palaeontologists base their work on the shape of fossils alone，their methods of conservation are not designed to preserve DNA，Geigl explains.

　　8.The biggest problem is how they are cleaned. Fossils are often washed together on-site in a large bath，which can allow water and contaminants in the form of contemporary DNA to permeate into the porous bones. Not only is the authentic DNA getting washed out，but contamination is getting washed in, says Geigl.

　　9.Most ancient DNA specialists know this already，says Hendrik Poinar，an evolutionary geneticist at McMaster University in Ontario，Canada. But that doesnt mean that best practice has become widespread among those who actually find the fossils.

　　10.Getting hold of fossils that have been preserved with their DNA in mind relies on close relationships between lab-based geneticists and the excavators，says palaeogeneticist Svante Pbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig，Germany. And that only occurs in exceptional cases，he says.

　　11.Pbos team，which has been sequencing Neanderthal DNA，continually faces these problems. When you want to study ancient human and Neanderthal remains，theres a big issue of contamination with contemporary human DNA, he says.

　　12.This doesnt mean that all museum specimens are fatally flawed，notes Pbo. The Neanderthal fossils that were recently sequenced in his own lab，for example，had been part of a museum collection treated in the traditional way. But Pbo is keen to see samples of fossils from every major find preserved in line with Geigls recommendations just in case.