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Chapter 8


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Esteban Parra

Notes From Reading C HAPTER  8: HE FOSSIL RECORD  PGS. 193­213) Introduction - “Lucy” is one of the most famous fossils in human evolution o Specimen 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 – Acomparative 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 o B.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 - Stratigraphy o Stratigraphy – Arelative 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 o You 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 o Where earth’s crust has folded and broken through the surface of the ground, the usual stratigraphic order is disturbed - Other Relative Dating Methods o Fluorine Dating – Arelative 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 o Biostratigraphy – Arelative dating method in which sites can be assigned an approximate age based on the similarity of animal remains to those from other dated sites o Paleomagnetic Reversal - Amethod 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 o Due to cosmic radiation, some of the carbon in the atmosphere is a radioactive isotope known as carbon-14 (14C) o When an organism dies, no additional 14C is ingested, and the accumulated 14C begins to decay o 14C is said to have a half-life of 5,730 years Notes From Reading C HAPTER 8: THE FOSSIL RECORD  PGS. 193­213) o Half-Life – The average length of time it takes for half of a radioactive substance to decay into another form o Carbon-14 Dating – Achronometric 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 o Carbon-14 dating is useful only for sites dating back over the past 50,000 years at most o It is not useful for dating most of earth’s geological history - Argon Dating o Potassium-Argon Dating – Achronometric 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 o Argon-Argon Dating – Avariation 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 o Is 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 o Dendrochronology – Achronometric dating methods based on the fact that trees in dry climates tend to accumulate one growth ring per year o Fission Track Dating - Achronometric dating method based on the number of tracks made across volcanic rock as uranium decays into lead o Thermoluminescence – Achronometric 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 o Electron Spin Resonance (ESR) – Achronometric 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 o The 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 o We 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 o Paleospecies – Species identified from fossil remains based on their physical similarities and differences relative to other species - Variation within Species Notes From Reading CHAPTER  8: HE FOSSIL RECORD  PGS. 193­213) o We must be aware of the amount of variation that exists within a species and make sure that we do not confuse differences arising within a species with differences between species o There is a difference between male and females: we have to consider levels of sexual dimorphism before creating conclusions about fossils and species status o Sometimes we can estimate sex from skulls; overall size and shape of certain features on the skull can be used to separate males and female
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