远古世界的分子窗口

A molecular window to the primeval world

在19世纪中叶,达尔文被大量动物化石在寒武纪地层中突然出现而深深困扰[1],而寒武纪之前的岩石中似乎找不到任何化石的踪影,动物看起来似乎是"一夜之间"出现的.他在试图用化石记录的不完整性来解释这样的剧变的同时,也意识到这个问题也许会动摇他进化理论的根基.在其后的100多年中,古生物学家在前寒武纪地层中陆续发现了丰富的化石看似解决了达尔文的难题,但实际上也带来了更多的未解之谜.埃迪卡拉生物群代表了紧邻寒武纪的埃迪卡拉纪晚期的生物面貌,这一独特的化石生物群在包括我国三峡地区[2]在内的全世界30多个地点[3]都有发现.

Exemplified by the Ediacara biota, Precambrian strata yielded some most enigmatic fossils in the history of life. They exhibited unique body plans that perished on the eve of the Cambrian explosion. The phylogenetic affinity of the Ediacara biota has long been a matter of debate. Their affinities wandered across various phyla of life, including algae, ctenophores, cnidarians, annelids, molluscs, protists, terrestrial lichens and even an independent kingdom, Vendobionta. Finding the roots of modern phyla back in the Precambrian is frustratingly difficult, due to the relatively scarce fossil records back in time. Fortunately, biomarkers derived from biomolecules can be preserved as fossils too. These taxon-specific molecules served as windows into the primaeval world. They provide precious information about life perished in deep time from a unique perspective. In 2018, cholesteroids, a group of sterols characteristic of metazoans was detected from the iconic Ediacaran fossil Dickinsonia, provided another line of evidence for its animal affinity, along with existing morphological and ontogenic evidence, we can arguably confirm that Dickinsonia, as a member of metazoa, is more closely related to us than we thought. Hopanes extracted from disc-like fossils Beltanelliformis, illustrating that they may represent colonies of cyanobacteria. Two types of sponge biomarkers, 24-isopropylcholestane and 26-methylstigmastane, were found in 660–635 million years old rocks, potentially the sign of the earliest animals. However, learned from previous applications of biomarkers in the research of early life, two major hurdles must be overcome: contaminations and the possibility that the same biomarker produced by different taxonomic groups. Modern contaminations may come from various sources but can be minimised by adopting biomarker-clean protocols of sampling, e.g. the hydrocarbon-clean drilling method has been implemented on acquiring biomarkers from Archean rocks which are typically deficient of organic matter, hence prone to contamination. The latter hurdle is more serious and could significantly undermine the reliability of biomarkers. It is unlikely to fully investigate the triadic relationships between biomarkers, possible precursors and producers for all the groups of organisms. 24-isopropylcholestane and 26-methylstigmastane, once considered to be biomarkers exclusively for sponges. Their occurrences in the Neoproterozoic rocks were viewed as the first appearance of sponges. However, this evidence is probably no longer valid, because, for both biomarkers, their precursors were recently found common among Rhizaria, a supergroup of mostly unicellular protists. Even more cautions should be exercised while dealing with extinct groups, since we may never know what precursors they were capable to produce. As a result, biomarkers alone would not be enough, there have to be other lines of evidence to be convincing. When biomarkers contradict existing body fossil records, trust latter seems to be more promising at this moment.