Chemical fossils ( e . g . alkanes and porphyrins ) are often the only evidence for life in rocks of precambrian age 化学化石（烷烃和卟啉）是前寒武纪地层中生命存在的唯一证据。
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Chemical fossils particularly resistant organic chemicals present in geological strata that are thought to indicate the existence of life in the period when the rocks were formed 化学化石：残存于地质地层中的有机化学物质，当这样的地层被发现时，可以预测出那个地层形成时期的生命活动现象。
