Major events in biological development on Earth

Major events in biological development on Earth





Directionality in the history of life: diffusion from the left wall or repeated scaling of the right?

Issues of directionality in the history of life can be framed in terms of six major evolutionary steps, or megatrajectories (cf. Maynard Smith and Szathmary 1995):

(1) evolution from the origin of life to the last common ancestor of extant organisms,
(2) the metabolic diversification of bacteria and archaea,
(3) evolution of eukaryotic cells,
(4) multicellularity,
(5) the invasion of the land and
(6)technological intelligence.

Within each megatrajectory, overall diversification conforms to a pattern of increasing variance bounded by a right wall as well as one on the left. However, the expanding envelope of forms and physiologies also reflects-at least in part-directional evolution within clades. Each megatrajectory has introduced fundamentally new evolutionary entities that garner resources in new ways, resulting in an unambiguously directional pattern of increasing ecological complexity marked by expanding ecospace utilization. The sequential addition of megatrajectories adheres to logical rules of ecosystem function, providing a blueprint for evolution that may have been followed to varying degrees wherever life has arisen.

1) http://isites.harvard.edu/fs/docs/icb.topic231281.files/Reading01_Lec24_OEB-113.pdf

http://www.as.utexas.edu/astronomy/education/fall08/scalo/secure/309l_nov04_life.pdf

http://www.chm.bris.ac.uk/motm/oec/motm.htm#Sec1

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).