D3: Human Evolution

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

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

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/

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

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

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/radioactive_decay3.html

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

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

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)

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

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?

Answer 2.6 billion years Passed through 2 half lives

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?

Answer: ~22,920 years 4 half-lives 5730 years

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