1. Evolutionary History – part 2 Chapter 20

2. History of Life  Fossils:  Any record of past life  Hard body parts: shells, bones, teeth  Traces or impressions: trails, footprints  Mostly found in sedimentary rock  Layers of sediments that have been compressed and hardened  The Law of Superposition: older strata below, younger strata above  However, strata may be disturbed by geological processes

3. Dating fossils  Relative dating – fossils found together are from the same time period  Index fossils – used to identify deposits made at the same time in different places on Earth; fossils found with the index fossils are from the same time period

4. Dating fossils…  Absolute dating – to tell the actual age of a fossil  Uses the decay of radioactive isotopes  These breakdown to a stable element at a known rate  Radiocarbon dating:  C-14 is a rare isotope of C-12 and is present in all living things (and most fossils)  It breaks down to N-14  5,730 year half-life (one half of the C-14 in an organism will be gone after that amount of time)  We compare the ratio of C-14 to C-12 in the fossil and this ratio to a living organism to calculate the age of the fossil  This is only good for fossils < 100,000 years old  Older fossils can use different radioactive elements

5. Other dating methods…  Molecular clock  Mutations in certain parts of the genome  They occur at a fixed rate  They are not tied to natural selection (which would cause the allele frequencies to change)  The number of DNA base-pair differences tells how long two species have been evolving separately

6. The geologic time scale  Precambian Time  Prokaryotes ~ 3.5 BYA  Eukaryotes ~ 2.1 BYA (endosymbiotic theory)  Multicellular protists ~ 1.4 BYA  Paleozoic Era  Cambrian explosion – lots of variation in ocean life  ~ 500 MYA life on land  Mesozoic Era  Flowering plants, dinosaurs  Cenozoic Era  Mammals

7. Mass extinctions  Large-scale extinctions over a relatively short period of time  The fossil record shows 5  Caused by either cataclysmic events (asteroid hitting Earth) or more gradual environmental change  We are currently in another one but this one is human-caused. 