土卫十六密度较低，科学家因此推断它可能由多孔冰物质构成。
Its low density has led scientists to speculate that it may consist of porous, icy material.
因此，一些科学家推断，地球早期的大气与现在的不一样，主要由氢、甲烷和氨构成。
Therefore some scientists postulate that the Earth's earliest atmosphere, unlike that of today, was dominated by hydrogen, methane, and ammonia.
他们推断，虽然一些进化上的变化涉及基因的丢失，因此是不可逆转的，但其他的变化可能是基因被关闭的结果。
They reasoned that while some evolutionary changes involve the loss of genes and are therefore irreversible, others may be the result of genes being switched off.
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