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Chapter 8

ANT203H5 Chapter Notes - Chapter 8: Electron Paramagnetic Resonance, Fluorine, Cosmic Ray

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Esteban Parra

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-“Lucy” is one of the most famous fossils in human evolution
oSpecimen of hominin species different from our own, one that walked on to legs
but had a small ape-size brain
-Paleoanthropology – The study of primate and human evolution
Dating the Past
-Relative Dating – A comparative method of dating the older of two or more fossils or
sites, rather than providing a specific date
-Chronometric Dating – The method of estimating the specific date of fossils or sites
-Different nomenclature has been used to refer to geological and historical dates
oB.P. – Before Present (1950), the internationally accepted form of designating
past dates
Relative Dating Methods
-Can tell us the basic time sequence of fossil and archaeological sites and provide us with
a range of possible dates based on comparison of the fossils or geology with other sites of
known age
oStratigraphy – A relative dating method based on the fact that older remains are
founder deeper in the earth because of cumulative build-up of the earth’s surface
over time
oYou still do not know the age of the fossils or the exact amount of time between
the two, but you have established which is older in geologic time
oWhere earth’s crust has folded and broken through the surface of the ground, the
usual stratigraphic order is disturbed
-Other Relative Dating Methods
oFluorine Dating – A relative dating method, based on the accumulation of
fluorine in a bone, that tells if two bones from a site are of the same age
The rate of this process varies from sit to site, so we cannot tell exactly
how old a bone is by using this method
It does allow us to determine if two bones found at the same site are the
same age by comparing their fluorine levels
oBiostratigraphy – A relative dating method in which sites can be assigned an
approximate age based on the similarity of animal remains to those from other
dated sites
oPaleomagnetic Reversal - A method of dating sites based on the fact that the
earth’s magnetic field has shifted back and forth from the north to the south in the
past at irregular intervals
Sedimentary rocks preserve a record of these past changes in polarity
Chronometric Dating Methods
-Chronometric dating relies on physical and chemical processes in the universe that
remain constant
-Carbon-14 Dating
oDue to cosmic radiation, some of the carbon in the atmosphere is a radioactive
isotope known as carbon-14 (14C)
oWhen an organism dies, no additional 14C is ingested, and the accumulated 14C
begins to decay
o14C is said to have a half-life of 5,730 years
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Notes From Reading
oHalf-Life – The average length of time it takes for half of a radioactive substance
to decay into another form
oCarbon-14 Dating – A chronometric dating method based on the half-life of
carbon-14 that can be applied to organic remains, such as charcoal, dating back
over the past 50,000 years
oCarbon-14 dating is useful only for sites dating back over the past 50,000 years at
oIt is not useful for dating most of earth’s geological history
-Argon Dating
oPotassium-Argon Dating – A chronometric dating method based on the half-life
of radioactive potassium that can be used to date volcanic rock older than
100,000 years
Requires rocks that did not possess any argon gas to begin with
Though we cannot date a fossil directly with this method, we can assign
a date based on the relationship of a fossil find to different levels of
volcanic ash
oArgon-Argon Dating – A variation of potassium-argon dating that can be
applied to very small samples of volcanic rock
Can be applied to small samples – even a single crystal
oIs more accurate because the final estimates can be calculated from a single
extraction of argon gas, whereas the potassium-argon dating method requires two
separate samples for assessing potassium and argon
-Other Chronometric Dating Methods
oDendrochronology – A chronometric dating methods based on the fact that trees
in dry climates tend to accumulate one growth ring per year
oFission Track Dating - A chronometric dating method based on the number of
tracks made across volcanic rock as uranium decays into lead
oThermoluminescence – A chronometric dating method that uses the fact that
certain heated objects accumulate trapped electrons over time, which allows the
date when the object was initially heated to be determined
oElectron Spin Resonance (ESR) – A chronometric dating method that estimates
dates from observation of radioactive atoms trapped in the calcite crystals present
in a number of materials, such as bones and shells
Reconstructing the Past
Interpreting Fossils
-Identifying Species
oThe biological species concept provides a test to determine if individuals from
two populations belong to the same species – capable of reproducing naturally
and giving rise to fertile offspring
oWe cannot find evidence of interbreeding based on fossils by using the biological
species concept, instead we make our species assignments based on inferences
from physical appearance, or morphology, of the fossils
oPaleospecies – Species identified from fossil remains based on their physical
similarities and differences relative to other species
-Variation within Species
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