[3ch201-10] R22 [Regular Session]History of the Earth

[R22-O-7] [Invited]Life on the early Archean Earth – new insights from lenticular microfossils

*Kenichiro Sugitani¹ (1. Nagoya University)

Keywords:Archean, lenticular microfossils, photoautotrophs, Strelley Pool Formation, Farrel Quartzite , Pilbara Craton

世話人からのハイライト紹介：杉谷氏は，太古代のピルバラ（豪州）やカプバールクラトン（南アフリカ）の黒色チャート層から生物化石と思われる有機物を含むレンズ状物質を報告してきた．これは，35億年前の地球における生物活動や表層環境を考える上で非常に重要な証拠となる．このレンズ状微生物についての最新の成果および包括的な情報を提供し，初期地球や他の惑星での生態/進化の可能性を言及します．参考：ハイライトについて

Following several controversies, there now exist sufficient lines of evidence regarding life on the early Archean Earth (>3.0 Ga), which include isotopic and molecular signatures, biologically mediated sedimentary structures (stromatolite), and cellularly preserved microfossils. The early Archean cellularly-preserved microfossils are morphologically diverse, including septate and non-septate filaments, small to large (<1 to 60 µm in diameter) colonial spheres, and lenses (20 to 100 µm across). Among them, the lenses (lenticular microfossils) discovered from the 3.0 Ga Farrel Quartzite and the 3.4 Ga Strelley Pool Formation in the Pilbara Craton, Western Australia, and the 3.4 Ga Kromberg Formation in the Kaapvaal Craton, South Africa, are particularly important. They are composed of a central spheroid body surrounded by thin discoid flange. Although the biogenicity of lenticular structures has long been subjected to skepticism, it has currently been established and widely accepted through multidisciplinary studies performed by over 20 researchers from 6 countries. The next research target is to elucidate the ecology and biological affinity of lenticular microfossils. Our recent studies have shown that lenticular microfossils represent autotrophic and planktonic organisms that had acid-resistant robust envelopes and, if not all, an inner alveolar structure; they can be statistically classified into several groups based on minor morphological variations; and they reproduced by multiple fissions in addition to simple binary fissions. It is likely that lenticular microfossils were photoautotrophs, although it is still equivocal whether they produced oxygen or not. Their robust cell wall might be a consequence of adaptation to the harsh Archean environment, including e.g., violent volcanic activity and repeated asteroid impacts, although the high density of the cell wall had potentially reduced their planktonic habit. Lenticular microorganisms might have had enlarged cell volume and evolved flanges in order to counterbalance this disadvantage. Although comprehensive information about lenticular microfossils remains unavailable, they provide invaluable insights into the ecology and evolution of life on the early Archean Earth and potentially other planets such as Mars.