Literature Review: Archean and Proterozoic

by Carl Strang

The Archean and Proterozoic Eons were immense periods of time in which the Earth cooled enough to form rocks and begin its Wilson Cycle of plate tectonic activity, the continents coming together and drifting apart in a Maypole Dance that goes on to this day. Life emerged from inorganic matter in the Archean, and achieved its first multicellular forms in the Proterozoic. We so often forget these eons in our rush to the Paleozoic, but research continues to fill the gaps in our knowledge and make more interesting these billions of years that constitute most of Earth’s (and life’s) history. Today I’ll share notes on some of the research that particularly struck me last year.

Chicken Islands, Georgian Bay. The Canadian Shield formed during the Proterozoic, but the plants and the glacier-rounded forms of the rocks tell how much has happened since.

S. Pizzarello, L. B. Williams, J. Lehman, G. P. Holland, J. L. Yarger. Abundant ammonia in primitive asteroids and the case for a possible exobiology. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1014961108     One problem in understanding the origin of life is the contradiction between the need for quantities of reduced nitrogen (e.g., ammonia) and evidence that the early atmosphere was neutral rather than reducing in its chemical activity. This group found that ammonia is carried in high concentrations to Earth within asteroidal meteorites, and this is a potential source of the material.

Anand Bala Subramaniam, Jiandi Wan, Arvind Gopinath, Howard A. Stone. Semi-permeable vesicles composed of natural clay. Soft Matter, 2011; DOI: 10.1039/C0SM01354D     They have mapped out some plausible steps for early cell formation. Montmorillonite clay, an abundant mineral, spontaneously can form spherical vesicles around air bubbles in water. The vesicles hold their structure as fluids and organics flow in through semipermeable pores in the vesicles and the air bubbles dissolve. Montmorillonite is known to act as a catalyst for the assembly of lipid membranes and RNA strands.

Shirey, Steven B., and Stephen H. Richardson. 2011. Start of the Wilson cycle at 3 Ga shown by diamonds from subcontinental mantle. Science 333:434-436. The mineral composition of inclusions within diamonds of different ages points to plate tectonics starting up 3 billion years ago. They argue that claims for earlier recycling of crust into the mantle need to be revised.

Lawrence A. David, Eric J. Alm. Rapid evolutionary innovation during an Archaean genetic expansion. Nature, 2010; DOI: 10.1038/nature09649     As described in a ScienceDaily article. They looked at the genomes of present-day organisms to trace innovative genetic events of the past. Their results indicate an event they call the Archean Explosion, a time 3.3-2.8 billion years ago when 27% of present-day gene families first appeared. The genes involved are mainly connected to electron transport in membranes, associated with oxygenic photosynthesis and respiration. Genes directly associated with the utilization of oxygen appear at the end of that period, connecting to geological evidence for the accumulation of oxygen in the atmosphere (the “Great Oxidation Event” of 2.5 billion years ago). This must have been a disastrous time for life, the oxygen leading to the death of then-dominant anaerobic forms. The researchers are hopeful that continued study can lead to the uncovering of still earlier genetic steps in the evolution of life.

T. Bosak, D.J.G. Lahr, S.B. Pruss, F.A. Macdonald, L. Dalton, E. Matys. Agglutinated tests in post-Sturtian cap carbonates of Namibia and Mongolia. Earth and Planetary Science Letters, 14 June 2011 DOI: 10.1016/j.epsl.2011.05.030     As described in ScienceDaily. The Snowball Earth glaciation was actually two separate periods when glaciers were pole to pole or nearly so, at 710 and 635 million years ago. This paper describes fossil shells, the first known, of microorganisms, possibly early shelled amebae, that lived soon after the first of these glacial episodes.

Biotechnology and Biological Sciences Research Council (2010, December 18). Rise in oxygen drove evolution of animal life 550 million years ago. ScienceDaily. Retrieved December 20, 2010, from http://www.sciencedaily.com­ /releases/2010/12/101217145647.htm     While describing research connecting oxygen detection mechanisms in humans with those in the simplest animal (Trichoplax adhaerens), the article points out that the rise of multicellular life 550 million years ago (late Proterozoic) coincides with an abrupt rise in atmospheric oxygen from 3% to 21%. Body size correlates with oxygen availability in multicellular animals.

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