Probably very hot Frequent volcanic eruptions Atmosphere had no oxygen

◦ Called a “reducing atmosphere” Did have the following

◦ Water vapor◦ Carbon dioxide◦ Nitrogen

Earth is more than 4.6 Billion Years Old

3.4 – 3.9 Billon years ago◦ Earth cooled enough for water to condense and

become part of our oceans

Oldest rocks found on Earth are only 3.9 billion years old

Rocks are missing around 0.7 billion years of information, however, they are a very important source of information about the diversity of life on early Earth.

A. Fossils – Clues to the Past About 95% of the species that have lived on

Earth are now extinct Fossil: evidence of an organism that lived

long ago There are many different types of fossils

◦ Trace fossils◦ Casts◦ Molds◦ Amber-preserved or frozen fossils

A. Fossils – Clues to the Past◦ 1. Trace Fossils

Indirect evidence such as a footprint, trail or burrow◦ 2. Casts

When minerals fill in a space ◦ 3. Molds

forms when an organism is buried in sediment, then decays leaving an empty space

◦ 4. Amber-preserved fossil Sometimes an entire organism can get trapped in ice

or tree sap that hardens into amber.

B. Palentologists – “Dectectives of the Past”◦ Palentologists: scientists that study ancient life

and use fossils to understand events that happened long ago. also study fossils to also learn about ancient climates

and geography

C. Fossil Formation: Organisms usually have to be buried in

sediment (mud, clay, sand) They are covered, and natural pressure

causes the organism to compress and harden into a type of rock◦ Sedimentary rock

Most fossils are located in lakes, streams, or oceans

D. Relative Dating:◦ Relative dating works like this

The Deeper the layer where the fossil is found, the older the fossil is

The higher up in the soil the fossil is found, the younger the fossil is.

E. Radiometric Dating: Used to determine “actual” age of a fossil Utilizes radioactive isotopes in rocks

◦ Measures rate of isotope decay and compares to known standards

Unfortunately, Carbon-14 can only be used to date fossils that are less than 50,000 years old

Common radiometric dating isotope is Carbon-14 Potassium-40 measures actual date up to 1.3 billion

years old

“Geologic Time Scale”◦ Based on Earth’s layers of rocks and fossils

A. The Geologic Time Scale◦ Divided into 4 large sections or “Eras” *(PPMC)*

Pre-Cambrian Paleozoic Mesozoic Cenozoic

◦ Each “Era” represents a LONG period of time◦ Distinguished by the types of organisms that lived

during that period of time.

B. Precambrian Era◦ Oldest fossils found are 3.4 b.y.o.◦ Mostly fossils of photosynthetic cyanobacteria◦ Also find stromatolites

Another photosynthetic structure◦ Accounts for 87% of Earth’s history◦ In the beginning, there were only unicellular

prokaryotes (single-celled prokaryotic cells) Cells that have no membrane-bound organelles and

no true nucleus◦ Eukaryotic developed around 2.1 b.y.a.◦ End of precambrian came around 543 m.y.a.

C. Paleozoic◦ From 543 m.y.a. to 248 m.y.a.◦ Enormous increase in biodiversity

Variety of plants and animals◦ Ocean life

Worms, sea stars, unusual arthropods◦ Fish

Oldest animals with backbones◦ Ferns and early seed plants◦ Four-legged amphibians◦ Towards the end, reptiles appeared◦ 1ST MASS EXTINCTION OCCURRED AT THE END

D. Mesozoic◦ 248 m.y.a. to 65 m.y.a. ◦ Early mammals

Small, mouse-like◦ Dinosaurs were present◦ Modern birds evolved from dinosaurs◦ Flowering plants appeared◦ Plate tectonics occurred during this time◦ 2ND MASS EXTINCTION OCCURRED

This is the extinction that killed dinosaurs Meteorite collision

E. Cenozoic◦ 65 m.y.a. to present◦ Mammals begin to flourish◦ Primates appeared right at 65 m.y marker◦ Modern humans appeared 200,000 y.a.