Data from: Horizontal gene transfer constrains the timing of methanogen evolution

Microbial methanogenesis may have been a major component of Earth’s carbon cycle during the Archaean eon, generating a methane greenhouse that increased global temperatures enough for a liquid hydrosphere, despite the Sun’s lower luminosity at the time. Evaluation of potential solutions to the ‘faint young Sun’ hypothesis by determining the age of microbial methanogenesis has been limited by ambiguous geochemical evidence and the absence of a diagnostic fossil record. To overcome these challenges, we use a temporal constraint: a horizontal gene transfer event from within archaeal methanogens to the ancestor of Cyanobacteria, one of the few microbial clades with recognized crown-group fossils. Results of molecular clock analyses calibrated by this horizontal-gene-transfer-propagated constraint show methanogens diverging within Euryarchaeota no later than 3.51 billion years ago, with methanogenesis itself probably evolving earlier. This timing provides independent support for scenarios wherein microbial methane production was important in maintaining temperatures on the early Earth.太古宙（Archaean eon）时期，微生物产甲烷作用（microbial methanogenesis）或许曾是地球碳循环的核心组成部分，其所产生的甲烷温室效应可提升全球温度，使液态水圈得以形成——尽管彼时太阳的光度相对偏低。此前，通过测定微生物产甲烷作用的起源时间以验证“昏暗年轻太阳”假说（faint young Sun hypothesis）的相关研究，始终受限于模糊不清的地球化学证据与缺乏鉴定性化石记录这两大瓶颈。为突破这些挑战，我们采用了一项时间约束条件：发生于古菌产甲烷菌（archaeal methanogens）内部向蓝细菌（Cyanobacteria）祖先的水平基因转移（horizontal gene transfer）事件，而蓝细菌是少数拥有公认冠群（crown-group）化石记录的微生物演化支之一。基于这一由水平基因转移事件确立的约束条件进行校准的分子钟分析（molecular clock analyses）结果显示，产甲烷菌在广古菌门（Euryarchaeota）内的分化时间不晚于35.1亿年前，而产甲烷作用本身的演化可能更早。这一时间节点为“微生物甲烷生成在维持早期地球温度中发挥了重要作用”的相关假说提供了独立支撑。