Around 1 billion years ago extremophilic bacteria arose. These bacteria were capable of living in extreme environmental conditions such as high heat, high salt concentration, high and low pH, etc). The extremophiles are now known to have been early members of the domain Archaebacteria. Suviving members of this group include K.O. Stetter & Reinhard Rachel, University of Regensburg. bacteria such as the acetogens, methanogens, thermophiles and halophiles. These organisms, like true bacteria, lack a nucleus, as well as organized internal membrane structures, vacuoles and internal organelles. The bacterium shown in Fig 4 (right), Aquifex, is genetically positioned as the oldest living fossil on the earth today.
The oldest fossils of life on earth are found in microfossils (3.8 Ga) and stromatolites (3.5 Ga). As shown in (Fig 5, 6 and 7) stromatolites appeared in the early shallow waters of the earth approximately 3.8 Ga. The artist's rendition in Fig. 5 (below, left) of the earth at that time accurately depicts the volcanic outgassing and undersea hydrothermal vent activity that would have been prevalent near the end of the Hadean eon. The earliest stromatolites were probably pre-Photosynthetic eubacteria ancestors of Chloroflex and Chlorobium, both photosynthetic archaean anaerobic thermophiles that preceded the Cyanobacteria.
Although early sedimentary rocks have undergone extensive metamorphosis (thereby destroying microfossil remains), scientists have introduced other techniques to determine an organism's position on the phylogenetic "tree of life". Biological macromolecules are studied for both function and sequence overlap (genetic homology, divergence, et al.) providing especially valuable insights. Indeed, this approach has become central to establishing early phylogentic relationships within the Eubacteria and Archaea. Extant biochemical pathways also lend evidence to early relationships, as well as prehistoric geochemical biomarkers, molecules and minerals left behind by species specific biological activity. Examples of this approach include the recent discovery of sterane, e.g., in 2.7 Ga sediments, demonstrating Eukaryotes were present as well as free oxygen for biosynthesis). Radioactive analysis may also prove useful for the determination of biochemical activity.
2. Bacterial Photosynthesis (3.8 - 2.7 Ga)
Introduction to the Photosynthetic bacteria: Thirty-six bacterial lineages (Eubacteria) have been identified, of which only five are capable of using chlorophyll-based energy conversion to create a Protonmotive Force (PMF) to drive ATP synthesis and reduce CO2 to sugars. Of the five bacterial groups capable of photoautotrophic photosynthetic growth, four, (the exception being the Cyanobacteria), perform photosynthesis under anaerobic conditions and do not oxidized water to molecular oxygen via the OEC (anoxygenic photosynthesis). Indeed, the Cyanobacteria are the only group of bacteria that have incorporated the OEC necessary for splitting water as a source of electrons. The majority of the Cyanobacteria are obligate photolithoautotrophs, having very limited capacity to photoassimilate organic compounds. The ability to photosynthesize and fix Nitrogen has resulted in many members of the Cyanobacteria becoming endosymbionts with plants, lower animals and fungi (lichens).