Archaean Time
the Archaen - 3.8 to 2.5 billion years ago

 

Archaean Time:
Denoting Significant Events in:
Evolution, the Fossil Record, Paleontology and Paleobiology

 

 

 

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Archaean (3800 to 2500 mya)	Late Archaean (2800 to 2500 mya)	-
	Middle Archaean (3200 to 2800 mya)	-
	Early Archaean (3800 to 3200 mya)	Primitive Eukarya appear as a result of endosymbiosis Photosynthesis appears First life appears - Heterotrophic, anerobic, asexual Oldest fossils - Apex Chert of Australia (3.55 BYA) - prokaryotes dominate (Bacteria and Archaea); simple cell forms form stromatolite Oldest sedimentary rocks (3.8 BYA)

The genesis and evolution of life on Earth is a remarkable and complex story. Much of the story is based on theory that, while greatly advanced in the post-genomic era of molecular biology, will forever, we think, remain shrouded in some mystery. The Earth formed from a ball of gas that condensed to a spherical molten. After some 2 billion years, a crust formed on the Earth, a rudimentary atmosphere existed, and water vapor had begun to condense to form to beginnings of a marine environment essential for life. No matter whether the atoms of carbon, nitrogen, oxygen, and others were there at Earth's formation, or arrived later in alien bodies from space, they constituted the building blocks of life. These elements by intrinsic chemical nature formed organic compounds that were washed by rain into the seas.

Earth was a hostile place where only the raw ingredients of life existed. After some 2 billion years, a crust had formed on the earth, a rudimentary atmosphere existed, and water vapor had begun to condense to form to beginnings of a marine environment essential for life. The so-called primordial soup occurred from which life begun was in all likelihood the sea, but there were no eyewitnesses, and no laboratory can simulate the marine environmental conditions that we can not even define. Until recently, we could only reconstruct a history of life from the testimony of the earth's most ancient rocks. Science almost always marks its greatest progress at the conjunction of disciplines. The history of life, we conjecture, will not be an exception. Together, geology and paleontology, old sciences for sure, have found the eyewitnesses so long missing. Our advanced civilization has now entered the post-genomic era.

The technology now exists to unravel ancient lineages through the sequencing of the genomes of extant species. The immutable law of natural selection dictates that life will retain those features that foster survival. These features, many most ancient relics, are still retained genes of every living organism. Grayness will always shroud our complete understanding of the origins of life, and we are satisfied with that mystery, as well as the resiliency and eternity of life.

It could be said that organic chemistry (chemistry of carbon-based compounds) IS life. No matter whether the atoms of carbon, nitrogen, oxygen, and others were there at Earth's formation, or arrived later in bodies from space, they constituted the building blocks of life in a primordial soup. The very nature of carbon chemical bonding to itself and other atoms predetermines the formation of organic compounds, and the subsequent catalyzing of more complex organic compounds. And so it was on early-Earth, with all these organic compounds washed by rain into an increasingly rich marine environment. It is surprisingly easy for the chemical compounds believed present in the sea to have formed the classes of compounds found in all living cells, including amino acids, the basis for all proteins, sugars, and the chemical precursors of nucleotides that compose RNA and DNA.

All organisms, and all the cells that constitute them, are believed to have descended from a common ancestor cell through evolution by natural selection. Evolution is the central organizing principal of biology, helping us make sense of the bewildering diversity of life. Living cells probably arose on Earth some 3 ½ billion years ago through the spontaneous interaction of organic molecules. It is likely that autocatalytic mechanisms inherent to these organic systems began with the evolution of RNA molecules that could catalize not only their own replication, but provided a template for the production of additional protein catalysts. As life and cells became increasingly complex over the eons, a time was reached when the more chemically stable double helix of DNA replaced the relatively unstable RNA to transmit a greater amount of genetic information. RNA was relegated to a role of performing two primeval functions, templates for protein production, and catalysts for other RNA's. In this way, the appearance of living organisms on Earth was preceded by a period of chemical evolution, whereby the relative simple organic molecules gradually begat more complex macro-molecules that could replicate themselves.