D3: Human Evolution

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D3 Human Evolution
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D3 Human Evolution

• D.3.1 outline the method for dating rocks and
  fossils using radioisotopes, with reference to
  14C and 40K
• D.3.2 Define half-life
• D.3.3 Deduce the approximate age of materials
  based on a simple decay curve for a radioisotope
  
• Pages 484-492 Campbell

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The Fossil Record

• Fossils are any form of preserved remains from a
  living organism
• Like bones in rock, insects in amber, frozen
  mammoths
• The fossil record is the ordered array in which
  fossils appear within layers of sedimentary rocks
  
• Paleontologists collect and determine fossils
• How do we determine their age?
• Contributions and limitations of the fossil
  record

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How are fossils formed?

• If a dead organism gets buried in sedimentary
  silt, it will decay slowly and leave a place in
  the surrounding silt
• They become solid, and fill the exact gap the
  organism left behind (like a cast)
• The silt around this solidifies, becoming
  sedimentary rock
• In the rock you find a different looking stone
• Looks like the dead organism ? fossil
• Very few circumstances that fossils are formed
• This is why the fossil record is incomplete
• Most individuals do not leave a fossil after
  death
• http//www.bbc.co.uk/sn/prehistoric_life/dinosaurs
  /making_fossils/

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Age of fossils

• We can use isotopes to determine the age of
  fossils
• Isotopes are
• Atoms which have a mass different from most other
  atoms
• Different number of neutrons
• Unstable will spontaneously change into one or
  more atoms of other elements, often emitting
  radiation

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Half life

• This change takes time
• Radioactive decay
• During a fixed interval (depending on the
  isotope), half of the amount present will decay
• As a result, at the end of the period, the
  radioactivity will be half of what is was before
• This is called half-life of the isotope
• Half-lives vary from fractions of seconds to
  thousand of years
• Half-life of14C is 5730 years

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So to repeat

• Half life the number of years it takes for 50
  of the original sample to decay
• Unaffected by temperature, pressure and other
  environmental factors
• http//www.colorado.edu/physics/2000/isotopes/radi
  oactive_decay3.html

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Carbon dating

• Method used to determine the age of organic
  material
• Involves radioactive 14C
• A normal atom of carbon is referred to as 12C
• 6 protons, 6 electrons, 6 neutrons
• 14C has 6 protons, 6 elections, 8 neutrons
• Unstable spontaneously change to 14N
• Emits radiation

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Carbon dating

• 14C is present in small amounts on Earth
• All living organisms 14C in the same proportion
  as is found in the atmosphere
• 14C is emitted by the sun ? photosynthesis
  (enters food chain)
• After an organism dies, the process of
  incorporating new carbon into the body stops
• After 5730 years, the amount of 14C present in
  the remains is half of what is was at the time of
  death
• After 11,460 years, it would be ¼

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Carbon dating

• Accurate and useful for young fossils
  (20,000-50,000 years old)
• Older material, another isotopes should be chosen
  (with a longer half-life)
• 40K
• Will decay to form 40Ar (argon)
• Half life 1300 million years
• 238Uranium (volcanoes)

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How can we use a graph to figure this out?

• At time 0, the fraction of 14C is 1 (the original
  amount present the organism just died)
• When only ½ the original amount of 14C is present
  a time equal to the half-life of the isotope has
  passed
• 5730 years
• When only 0.125 of the original amount of 14C is
  present, 3 half lives have passed
• The specimen died 17190 years ago

12
Review

• A paleontologist estimates that when a particular
  rock formed, it contained 12 mg of the
  radioisotope potassium-40. The rock now contains
  3 mg of potassium-40. The half-life of
  potassium-40 is 1.3 billion years. About how old
  is the rock?

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Answer

• 2.6 billion years
• Passed through 2 half lives

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Review

• Your measurements indicated that a fossilized
  skull you unearthed has a carbon-14 to carbon-12
  ratio of about 1/16 that of the atmosphere. What
  is the approximate age of the skull?

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Answer

• 22,920 years
• 4 half-lives
• 5730 years

16
Absolutely hilarious

• The Simpsons Take on Evolution and Creationism
• http//religiousfreaks.com/2006/07/10/evolution-vs
  -creationism-simpsons-style/
• Just click on Ned Flanders