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Poorna Pal, MS MBA Ph.D. Professor of Geology Glendale Community College Phone: (818)240-1000 x 5517 ppal@glendale.edu Office: CS-262 Office Hours (Spring 2004): Mon: 5-6 PM on days of TV class Tues & Ths: 2-4 PM, CS-262 Wed: 5-6.30 PM, CS-262

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Geological Time and Evolution

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Click on the image on the right to browse the online edition of USGS publication "Geological Time"

Lead-ratio (or the Pb²⁰⁷-Pb²⁰⁶ isochron) method provides the most direct means to estimate the Earth's age. As Pb²⁰⁷ comes from radioactive decay of U²³⁵ and Pb²⁰⁶ from that of U²³⁸, the plot of these two lead isotopes (relative to the non-radiogenic Pb²⁰⁴ or Pb²⁰⁸) should be linear if the solar system formed from a common pool of matter that was uniformly distributed in terms of the Pb-isotope ratios.  The older the sample, the higher the uranium-to-lead ratio and Pb²⁰⁶/Pb²⁰⁴ and Pb²⁰⁷/Pb²⁰⁴ values will be.

Source:

http://www.talkorigins.org/faqs/faq-age-of-earth.html#dal01

The slope of the straight-line fit to 5 meteorite and one terrestrial data thus yields an estimate of ~4.55 Ga. This is also the result from dating of meteorite samples by Rb-Sr and other radiometric methods.

• Life and the Geological Scale of Time

It is not clear whether life intrinsically evolved on the earth or, having originated elsewhere, proliferated on the earth after the first oceans appeared ~4 Ga ago. Based on the earliest evidence of life, the 3.7-4 Ga old stromatolites, the first 500-1000 Ma of earth’s history appears to have been altogether barren.

Based on the fossil evidence, we divide the geological time into the following:

• Phanerozoic: the most recent 570 Ma of earth's history with a well-preserved record of life, comprising 

  • Paleozoic: the 245-570 Ma era of primitive life-forms (vertebrate life evolved in the early part of this era, with Devonian as the age of fish);

  • Mesozoic: the 65-245 Ma era that began with the evolution of dinosaurs as also mammals but was dominated by the dinosaurs; and

  • Cenozoic: the most recent 65 Ma of earth's history that began with the extinction of dinosaurs and has been dominated by the mammals.

• Precambrian: the period since the earth's evolution, ~4.5 Ga ago, until the dawn of the Phanerozoic (circa 570 Ma), and therefore named Precambrian, it is divided into:

  • Proterozoic: an era of primitive life-forms that is usually divided into the

    • late (570-1250 Ma before the present)

    • middle (1.25-1.9 Ga before the present); and

    • early (1.9-2.5 Ga before the present); and

  • Azoic or Archean: the earliest 2-2.5 Ga of earth's history.

• Evolution of life over the geological times has followed three strands:

  • evolution of new species,
    e.g., the end-Permian appearance of dinosaurs and mammals,

  • extinction of some existing species,
    e.g., the end-Cretaceous extinction of dinosaurs, and

  • proliferation of some existing species,
    e.g., the Cenozoic domination of mammals.

• Darwinian evolutionary model sought gradual morphological changes, leading to the evolution of new species, as would result from adaptation to the environmental change. But, compared to this ‘gradualism’, the observed fossil record displays sudden appearance of new species following periods of prolonged mor­phological statis. The Eldredge-Gould model of ‘punctuated equilibrium’ (i.e., new species appear suddenly when, under envi­ronmental stress, portions of the gene pool of some existing species undergo rapid speciation) overcomes this problem.

See, for instance, “Punctuated Equilibrium at Twenty: A Paleontological Perspective” by Donald Prothero (Skeptic  vol. 1, no. 3, Fall 1992, pp. 38-47): http://www.skeptic.com/01.3.prothero-punc-eq.html and “Score One for Punk Eek: The fitful evolution of bacteria supports a controversial theory” by John Horgan (Scientific American, July 21, 1996): http://www.sciam.com/article.cfm?chanID=sa004&articleID=000DFABC-A1BF-1C76-9B81809EC588EF21

• Mass Extinction Events:

  Instances of sudden mass extinction events too exist. For instance, the end-Cretaceous dinosaur extinction, ~65 Ma ago, was also when 75% of the species disappeared and, at the end of the Paleozoic, ~245 Ma ago, an estimated 90% of all the species became extinct. Indeed, as the graph alongside shows, such events have recurred with a 25-30 Ma cyclicity that matches those in the records of bolide impacts as also volcanism. Hence the controversy about extraterrestrial catastrophism versus terrestrial cataclysms as the source of the environmental trauma that triggered these extinction events.