Precambrian 4.6 billion – 540 million years ago

Earth formed about 4.56 billion years ago. During Precambrian time, the atmosphere and oceans formed and plate tectonics began to build up continental land masses.

Earth Forms Gravity pulls together ice, dust, and rock from space As earth grows, its gravitational pull increases and more ice, rock, and dust was added. High velocity impacts from rocks in space caused earth to melt – This melting allowed nickel and iron to sink into the center while less-dense silicates floated to the outside and cooled into rock The earth took several million years to cool

Hadean Earth

The Nebular Hypothesis

Atmosphere Forms As Earth cooled, gases trapped in molten rocks were released forming the early atmosphere The early atmosphere had water vapor, carbon dioxide, and nitrogen but NO OXYGEN Prokaryotic organisms evolved and converted the carbon dioxide to oxygen. At first, most of this oxygen combined with iron in rocks to form rust About 2.5 billion years ago, the available iron finished reacting and oxygen was able to build up in the atmosphere

Oceans Form As earth cooled, water vapor in the air condensed to form clouds and it began to rain. Most of the rain evaporated very quickly but this evaporation allowed earth to cool more quickly. As rains continued, low areas began to fill with water and the oceans formed Formation of the oceans reduced the amount of water vapor and carbon dioxide in the air. The atmosphere was mostly nitrogen at this point

Archean Earth

Continents Develop Small continents began to form ~500 million years after earth formed Plate tectonics and the rock cycle were already in action (uniformitarianism) Most Precambrian rocks were destroyed and those that remain are usually buried under more recent rock layers (law of superposition) Continents still contain large areas of Precambrian rocks called shields. – Most shields are deformed metamorphic rock, but they contain mineral ores that can give clues to the Precambrian environment Several supercontinents were formed and moved during Precambrian time

Precambrian Rock

Paleozoic Continents

The earliest life probably evolved in the oceans. Later, one celled organisms evolved that use light energy to produce food through photosynthesis. These organisms consisted of simple cells called prokaryotes. Slowly, more complex cells, called eukaryotes evolved. Late in the Precambrian, multicelled organisms with soft bodies evolved

Earliest Life Earliest life Single-celled, prokaryotes Similar to bacteria found in hydrothermal vents at the bottom of the ocean

Photosynthesis Later bacteria, similar to modern cyanobacteria, use carbon dioxide and sunlight to make food and produce oxygen – this will eventually form the ozone layer Stromatolites are fossils of ancient bacterial colonies. Called microfossils because they are so small

Cyanobacteria

The Endosymbiotic Theory eukaryotes evolved when prokaryotic cells joined together to make more complex cells

Eukaryotes 1.8 billion years ago, more complex organisms called eukaryotes evolved. These organisms are similar to red and green algae found on Earth today.

Multicellular Life The fossils earliest multicellular organisms are found in the Ediacara Hills in Australia Ediacaran fauna – resemble worms, jellyfish, and corals found in today’s oceans.

Ediacaran Fauna

Then it got cold….