Dating Fossils http://www.uwo.ca/biology/images/undergraduate/trilobite.jpg

How old are fossils? Oldest fossils – 3.5 byo prokaryotes 1st life forms are probably older What is evolution? The scientific theory that explains how species change over time Today, species have changed both structurally & physiologically from what they were.

What are fossils? Remains/traces of once living organisms that no longer exist. Usually found in sedimentary rock – the formation process is quick, preventing bacteria from decaying the organism. Usually consists of ‘hard’ parts of the organism. Minerals may replace remains, preserving microscopic structures.

Other fossil formation methods Freezing – i.e. Mammoths Imprints – film of carbon left after decay Casts – sediments fill mold of organism Kinds of fossils =- Whole organisms, bones, plants; feathers; pollen grains http://images.google.com/imgres?imgurl=http://notexactlyrocketscience.files.wordpress.com/2007/01/800px-microraptor_fossil1.jpg&imgrefurl=http

How do we know how old fossils are? Relative age – position in layers of sedimentary rocks. Absolute age – radioisotopes w/unstable nucleii & a constant (known) rate of decay Half-life – length of time it takes ½ of radioactive material to decay http://razd.evcforum.net/Pictures/CvE/carbon14decay.jpg

What is an isotope? Atom with a different # of neutrons 12C and 13C= 6P, 6N (7N) and are stable 14C = 6P & 8N, unstable, ½ life = 5730 yrs How does dating work? 1) ratio of 14C production/decay in atmosphere is constant 2) organisms take in 12C & 14C at a constant rate

How does dating work? (con’t) 3) organism dies – 12C & 14C in atmosphere remain constant 14C in organism decays 12C : 14C ratio in organism changes 4) compare 12C : 14C ratio in organism to atmospheric ratio 5) scientists can date organisms up to 50K years using carbon dating.

How is all of this information applied to fossils we find in the dirt? What about old fossils? Use different isotopes – i.e. 40K (potassium 40), ½ life = 1.28 billion years So……. How is all of this information applied to fossils we find in the dirt? http://images.usatoday.com/tech/_photos/2006/11/07/fossil472.jpg