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Unformatted text preview: The History of Life on Earth WHEN did "it" happen? We're going to talk, in many instances, about "dates" - i.e., the amount of time before the present when organisms were present We're not going to look in detail at the methods, but it is essential that we understand something about how the dates are estimated "Stratigraphy" and various types of "radiometric dating" Stratigraphy The idea is that layers of rock closer to the surface are younger, while those further below the surface are older Radiometric Dating Fig 26.7 and handout These dates are measurements, based upon well-established properties of natural radioactive materials - they are not "guesses" By using various radioactive isotopes, times from 100 years to 4.6 billion years before the present can be measured (continued) Radiometric dating is typically carried out in igneous (e.g., granite or basalt) or metamorphic (e.g.,quartzite or gneiss) rocks Radiometric dating is actually measuring the time since the rock cooled sufficiently to prevent loss of the "daughter isotope" (e.g., K-40 ---> Ar-40; Ar-40 is a gas) When employing radiocarbon (continued) dating (only usable to ~ 100,000 years before the present) the fossil (or artifact) ITSELF is dated - not the surrounding rocks The margin of error in radiometric dating is typically less than 10% - so a rock dated to 10 million years is between 9 and 11 million years old Radiometric dates are "facts" These dates tell us nothing about HOW life began, or about WHAT mechanism has led to the great diversity of life, BUT they do set a time frame for the history of life of Earth The BIG Concept
Basalt dated to ~65 MYA Fossilbearing sedimentary rocks Quartzite dated to ~100 MYA How old are the fossils? ~ 4,600 million years ago (MYA) formation of the Earth ~ 4,000-3,500 MYA - origin of life ~ 3,500 MYA - oldest prokaryotic fossils Oldest prokaroytic fossils found in formations called stromatolites History of Life on Earth (Table 26.1) Stromatolites Fig. 26.11 History of Earth's Atmospheric Oxygen ~ 2,700 MYA - atmospheric oxygen concentration begins to increase How can we know ANYTHING about the atmosphere 2.7 billion years ago!! The types of rocks formed at various times (dates from radiometric dating) provide clues regarding the conditions under which the rocks were produced Banded Iron Formations ~2 billionyearold rocks with bands of iron oxide ("rust") Fig. 26.12 (cont.) ~ 2,200 MYA - oldest eukaryote fossils ~ 600 MYA - oldest animal fossils ~ 500 MYA - oldest plant fossils ~ 65 MYA - extinction of the dinosaurs ~ 0.2 MYA - origin of modern humans ~ 0.00002 MYA - your birth ...
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This note was uploaded on 03/30/2008 for the course BIOL 212 taught by Professor Shen during the Fall '08 term at Iowa State.
- Fall '08