Conclusions Archean Life

Despite the fact that there are doubters for almost all of the evidence for life in the Archean, evidence for early life has expanded tremendously in the past 20 years, from body fossils to isotopic and geochemical studies. These data are supported in most cases by detailed stratigraphic and sedi-mentologic evidence of the environments of early life, and by knowledge of the depositional and metamorphic history of the rocks that contain ancient carbonaceous matter. Nisbet (2000) provided an excellent summary of the variety of environments for Archean life, including deep-sea hydrothermal vents, open ocean, lacustrine environments, hydrothermal sites around active volcanoes, and anywhere that micro-bial mats occur today, for example, in coastal sediments (FIG.2.18).

Perhaps more than in any other area of paleobiologi-cal research, the multidisciplinary approach has been used to address questions and provide answers relating to the earliest life on Earth. The advances made in phylogenetic and evolutionary microbiology have also contributed to an increased understanding of early life. For example, based on studies of modern taxa and on geochemical studies, it now appears likely that the first organisms were not photosyn-thetic cyanobacteria, but perhaps chemosynthetic organisms (e.g., chemolithotrophs) that lived around deep-sea vents, or anoxygenic photosynthesizers which could live closer to the surface (Nisbet and Fowler, 1999). Both Archaea and Eubacteria are known to live in these habitats today, and there is isotopic evidence that points to the presence of sul-fate-reducing bacteria and methanogenic Archaea around 3.5Ga. The knowledge that earliest life did not necessarily have to arise in a "warm little pond" has, in some ways,

Large complex cones

Cuspate swales

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Encrusting/domical laminites

Egg carton laminite

Cuspate swales

Small crested/conical laminite

Iron-rich laminite

Wavy laminite

FIGURE 2.15 Synoptic profiles of seven stromatolite facies from the SPC. (From Allwood et al., 2006.)

FIGURE 2.16 Example of bumpy stromatolite morphology (Pilbara Supergroup). Bar = 10 cm. (Courtesy J. W. Schopf.)

FIGURE 2.17 Dendritic carbonaceous material (Cleaverville Group). Bar = 100|im. (From Kiyokawa et al., 2006.)

Hydrothermal communities around andesite volcanoes

Lake communities

Coastal sediment S-microbial mats

Mid-ocean ridge chemotrophic community

Hydrothermal systems around komatiite shields

Hydrothermal communities around andesite volcanoes

Lake communities

Coastal sediment S-microbial mats

Mid-ocean ridge chemotrophic community

Hydrothermal systems around komatiite shields

Figure 2.18 Diagrammatic representation showing numerous sites where early life may have flourished. (From Nisbet, 2000.)

revolutionized the picture of early life on Earth. Nisbet and Fowler (1999) noted that anaerobic photosynthesizers, similar to modern green gliding bacteria, could have colonized shallow-water habitats. Bacterial sulfur metabolizers would have colonized anaerobic sites, perhaps at the bottom of microbial mats, where they would live off of decaying matter from the autotrophic organisms above them. Organisms similar to modern purple bacteria (Proteobacteria), which are anoxygenic photosynthesizers, would occur in more micro-aerophilic areas within the same microbial mat community, perhaps similar to some of those described by Allwood et al. (2006) from ~3.4Ga rocks. Cyanobacteria would be present on the top surface of such a mat. Similar microstrata occur in modern microbial mats. As oxygenic cyanobacteria diversified and their biomass and distribution increased, the world began to change.

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Responses

  • Luigi
    What fossil in archean rocks provide bacterial life?
    1 month ago

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