科学家认为，地球上的所有生物———从人类到细菌，从蓝铃花到蓝鲸———都源自同一种实体，一种30亿年或40亿年前漂浮在“原始汤”周围的原胞。这种实体是什么样子呢？它又是如何生活，以及生活在哪里呢？在最新一期Nature上面，法国的科学家为我们揭开了谜团。
这种实体被称为“露卡(LUCA)”，也就是“第一个基本的共同祖先”之意，它没有留下任何已知的化石，也没有其他物理线索可揭示其身份，所以寻找“LUCA”的人们依靠间接证据来获取关于“LUCA”的生物信息及环境信息。
根据来自现存基因组的基因证据对先祖蛋白序列所做重建表明，“LUCA”所处环境很热，“LUCA”是嗜热生物。但核糖体RNA（rRNA）序列过去被认为与一个温度较低的环境相一致。一种新的“分子温度计”方法可能已经解决了这一明显的偏差。利用关于分子演化的最新数学模型对来自数百种现代物种的rRNA及蛋白序列所做分析表明，存在两个环境温度变化阶段。后来变成“LUCA”的生物先是嗜中温的，生活在大约60°C的水中，然后适应了较高的温度（高于70°C），产生了一个嗜热的共同祖先。随着之后海洋温度降低，细菌（Bacteria）和古细菌-真核生物（Archaea-Eukaryota）发生分化。（生物谷Bioon.com）
生物谷推荐原始出处：
Nature 456, 942-945 (18 December 2008) | doi:10.1038/nature07393
Parallel adaptations to high temperatures in the Archaean eon
Bastien Boussau1,3, Samuel Blanquart2,3, Anamaria Necsulea1, Nicolas Lartillot2,4 & Manolo Gouy1
1 Laboratoire de Biométrie et Biologie Evolutive, CNRS, Université de Lyon, Université Lyon I, 43 Boulevard du 11 Novembre, 69622 Villeurbanne, France
2 LIRMM, CNRS, 161 rue Ada, 34392 Montpellier, France
3 These authors contributed equally to this work.
4 Present address: Département de Biochimie, Université de Montréal, C.P. 6128, succursale Centre-Ville, Montréal QC H3C3J7, Canada.
Fossils of organisms dating from the origin and diversification of cellular life are scant and difficult to interpret1, for this reason alternative means to investigate the ecology of the last universal common ancestor (LUCA) and of the ancestors of the three domains of life are of great scientific value. It was recently recognized that the effects of temperature on ancestral organisms left 'genetic footprints' that could be uncovered in extant genomes2, 3, 4. Accordingly, analyses of resurrected proteins predicted that the bacterial ancestor was thermophilic and that Bacteria subsequently adapted to lower temperatures3, 4. As the archaeal ancestor is also thought to have been thermophilic5, the LUCA was parsimoniously inferred as thermophilic too. However, an analysis of ribosomal RNAs supported the hypothesis of a non-hyperthermophilic LUCA2. Here we show that both rRNA and protein sequences analysed with advanced, realistic models of molecular evolution6, 7 provide independent support for two environmental-temperature-related phases during the evolutionary history of the tree of life. In the first period, thermotolerance increased from a mesophilic LUCA to thermophilic ancestors of Bacteria and of Archaea–Eukaryota; in the second period, it decreased. Therefore, the two lineages descending from the LUCA and leading to the ancestors of Bacteria and Archaea–Eukaryota convergently adapted to high temperatures, possibly in response to a climate change of the early Earth1, 8, 9, and/or aided by the transition from an RNA genome in the LUCA to organisms with more thermostable DNA genomes10, 11. This analysis unifies apparently contradictory results2, 3, 4 into a coherent depiction of the evolution of an ecological trait over the entire tree of life.
