Briggs Derek E G
Department of Geology and Geophysics and Yale Peabody Museum of Natural History, Yale University, PO Box 208109, New Haven, CT 06511, USA.
Curr Biol. 2015 Oct 5;25(19):R864-8. doi: 10.1016/j.cub.2015.04.047.
The sudden appearance of fossils that marks the so-called 'Cambrian explosion' has intrigued and exercised biologists since Darwin's time. In On the Origin of Species, Darwin made it clear that he believed that ancestral forms 'lived long before' their first fossil representatives. While he considered such an invisible record necessary to explain the level of complexity already seen in the fossils of early trilobites, Darwin was at a loss to explain why there were no corresponding fossils of these earlier forms. In chapter 9 of the Origin, entitled 'On the imperfection of the geological record', he emphasized the 'poorness of our palaeontological collections' and stated categorically that 'no organism wholly soft can be preserved'. Fortunately much has been discovered in the last 150 years, not least multiple examples of Cambrian and Precambrian soft-bodied fossils. We now know that the sudden appearance of fossils in the Cambrian (541-485 million years ago) is real and not an artefact of an imperfect fossil record: rapid diversification of animals coincided with the evolution of biomineralized shells. And although fossils in earlier rocks are rare, they are not absent: their rarity reflects the low diversity of life at this time, as well as the low preservation potential of Precambrian organisms (see Primer by Butterfield, in this issue).
自达尔文时代以来，标志着所谓“寒武纪大爆发”的化石突然出现，一直吸引着生物学家并引发他们的思考。在《物种起源》中，达尔文明确表示，他相信祖先形态“早在”其第一批化石代表出现之前就已存在。虽然他认为这样一个无形的记录对于解释早期三叶虫化石中已经看到的复杂程度是必要的，但达尔文却无法解释为什么没有这些早期形态的相应化石。在《物种起源》第九章“论地质记录的不完整性”中，他强调了“我们古生物学收藏的贫乏”，并明确指出“没有任何完全柔软的生物体能够被保存下来”。幸运的是，在过去的150年里有了很多发现，尤其是寒武纪和前寒武纪软躯体化石的多个实例。我们现在知道，寒武纪（5.41亿至4.85亿年前）化石的突然出现是真实的，而不是不完美化石记录的人为产物：动物的快速多样化与生物矿化外壳的进化同时发生。而且，尽管早期岩石中的化石很少见，但并非不存在：它们的稀有反映了当时生命的低多样性，以及前寒武纪生物的低保存潜力（见本期巴特菲尔德撰写的入门文章）。
