Reading Earth's History: An Introduction to Fossils and the Geologic Time Scale

Geologic Time Scale: • The Earth is over 4.7 billion years old. • Life started as single-celled organisms and now has a great variety of multi-celled organisms. • Scientists study life’s history from fossil records arranged in a Geologic Time Scale. Organizing Earth’s History • The Geologic Time Scale is a timeline that organizes the events in Earth’s history, like a calendar. • Single-celled organisms dominated most of Earth’s history. • Plants and fish have only evolved within the past 500 million years. • Humans evolved about 110,000 years ago. • If the entire history of Earth were squeezed into a single day, humans would not evolve until the last few seconds. • The scale is divided into eons, era, periods, and epochs. • An Eon, billions of years ago, is divided by the Cambrian boundary when the variety of life forms exploded • An Era includes mass extinctions that mark the boundaries between eras. • A Period includes blocks of time when a unique rock series was laid down. Fossils • Fossils provide evidence for the geologic time scale. • They are imprints or remains of organisms that were once alive. Fossil Formation: • Fossils form when layers of sediment build up over a dead organism. • Over time, pressure can cause these layers to turn to rock and the remains are preserved. • Fossils form in different ways, depending on the material the fossil is forming in and the climate. Fossil Molds and Casts Process Notes Acids eat away the If the mold fills skeleton or shell and with minerals, it leaves an impression becomes a cast (mold) in the rock Tracks and Trails Imprints are left in Dinosaur tracks mud, which later are fairly hardens common Minerals in plant Petrifaction Common with cells crystallize; plants Soft tissue preservation Traps minerals enter openings or cavities in shells or bones. Mummification or frozen in ice Whole organisms locked in tar pits, asphalt, amber, etc. Fossils do not last long Fossils and Geologic Time Scale • Most fossils form in sedimentary rock. • Positions of fossils with in the rock help construct the geologic time scale because the lowest layer of rocks are the oldest, meaning the fossils would be the oldest too. • Therefore, fossils on the top are from organisms that lived in more recent times. Review Questions: 1. Which division in the geologic time scale is billions of years long? - Eon 2. During which period in the geologic time scale was there a great increase in the variety of life forms on Earth? - Cambrian Boundary 3. During which epoch in the geologic time scale did humans evolve? - Holocene 4. Which type of organism evolved first? Millions of Years Ago Event 360-286 First reptiles 410-360 First amphibians 440-410 First insects 505-440 First fish - first fish 5. How can the Grand Canyon be used to construct the geologic time scale? - Fossil algae at the bottom lived more than 2.5 billion years ago, seed plants in the middle layers lived more than 65 million years ago.