史密斯博士说这些虱子不会在他的系统树上出现，除非它们转变成寄生在胎盘类哺乳动物身上的物种，而大多数虱子不会经常变换宿主。

These lice would not show up in his tree, Dr. Smith said, unless they had been able to transfer to the placental mammalian species, and most lice do not regularly switch hosts.

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当哺乳动物捕获这个基因，利用它在胎盘产生一层融合细胞，通过这层细胞母亲能够将营养物质输送给她们的胚胎。

When mammals captured the gene, they used it in the placenta to create a layer of fused cells through which mothers can send nutrients to their embryos.

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他说：“化石记录一再证实现代胎盘类哺乳动物的起源是在白垩纪-早第三纪分界点的时候或那之后。

“The fossil record continues to show that the origin of modern placental mammals postdates or is at the Cretaceous-Paleogene boundary, ” he said.

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