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：尽管彼时太阳光度较低，但其产生的甲烷温室效应可提升全球气温，足以维系地表液态水圈的存在。此前，通过确定微生物产甲烷作用的演化年代以验证“微弱年轻太阳假说（faint young Sun hypothesis）”的可行解释方案的相关研究，往往受限于模糊不清的地质化学证据，以及缺乏诊断性化石记录这两大瓶颈。为突破上述研究局限，本研究引入一项时间约束条件：发生于古菌产甲烷菌内部至蓝细菌（Cyanobacteria）祖先的水平基因转移（horizontal gene transfer）事件——蓝细菌是少数拥有公认冠群化石（crown-group fossils）记录的微生物分支之一。基于该水平基因转移约束校准的分子钟分析（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时间可能更早。该年代学结果为“微生物甲烷生成对维持早期地球气温发挥关键作用”的相关假说提供了独立佐证。