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A selection of microfossils (with interpretative drawings) shown in thin sections of the early Archean (3.5 Ga) Apex chert of Western Australia. Magnification is shown by scale. a: Archaeoscillatoriopsis maxima. b: Primaevifilum laticellulosum. (Courtesy of Bill Schopf.)

FIGURE 7-4.

Universal phylogenetic tree based on rRNA sequences. (After Pace 1997.) The scale bar corresponds to 0.1 changes per nucleotide.

FIGURE 7-4.

Universal phylogenetic tree based on rRNA sequences. (After Pace 1997.) The scale bar corresponds to 0.1 changes per nucleotide.

terrestrial environments in the Archean/early Proterozoic if thermophily was required at that time. As Knauth (1992) put it, the Earth's surface was an "open hydrothermal system in the Archean."

There is now some evidence that a thermophilic eucaryotic algae, Cyani-doschyzon, now living in CO2-charged water up to 57°C (Seckbach 1994), may be a living model of a primitive photosynthesizing eucaryote emerging some 2 to 2.5 billion years ago. Seckbach (1994) originally suggested that Cyanidoschyzon was a model for primitive mitochondrial eucaryotes, but recent phylogenetic evidence supports it being rooted on the tree after an-central protozoa (Seckbach 1997).

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