Precambrian History

Hadean Eon (4.6–4.0 Ga)

• Solar nebula hypothesis: A cloud of gas and dust collapsed under gravity, rotated, and flattened into a disk. The Sun formed at the center, while planetesimals collided and accreted to form Earth.

• Differentiation: Early Earth was extensively molten. Dense iron and nickel sank to form the core, while lighter silicate materials formed the mantle and early crust.

• Giant impact: A large impact with a Mars-sized body likely ejected material that eventually formed the Moon.

• Cooling: Earth's surface gradually cooled, primitive crust developed, and water vapor condensed to form liquid water and early oceans.

• Evidence: Ancient zircon crystals, approximately 4.4 Ga old, suggest that early crust and liquid water existed surprisingly early. There is no confirmed fossil record from this eon.

Archean Eon (4.0–2.5 Ga)

• Stable portions of continental crust developed, along with early sedimentary and volcanic rocks.

• Granite-greenstone terrains preserve some of Earth's oldest continental crust. Greenstone belts contain altered volcanic and sedimentary rocks.

• Early oceans existed, but the atmosphere contained very little free oxygen.

• The earliest widely accepted evidence of life dates to approximately 3.5–3.4 Ga, including microbial structures and stromatolites.

• Cyanobacteria and other photosynthetic organisms released oxygen through photosynthesis.

• Oxygen reacted with dissolved iron in seawater, contributing to the formation of banded iron formations (BIFs).

Paleoproterozoic (2.5–1.6 Ga)

• The Great Oxidation Event, beginning around 2.4 Ga, caused atmospheric oxygen to rise substantially.

• Banded iron formations accumulated as oxygen reacted with iron in the oceans.

• Earth's first widely recognized major glaciation, the Huronian glaciation, occurred approximately 2.4–2.1 Ga; the Gowganda Formation preserves glacial evidence.

• The Trans-Hudson and Wopmay orogenies, among other events, contributed to continental growth and assembly.

• The Wilson cycle describes how rifting opens ocean basins and later plate convergence closes them, producing mountain belts.

Mesoproterozoic (1.6–1.0 Ga)

• Midcontinent Rift developed in North America, producing extensive basaltic volcanic rocks and rift-related sedimentary deposits.

• The Grenville Orogeny helped assemble the supercontinent Rodinia.

• Eukaryotes, cells with nuclei, evolved by this general part of Earth's history and became increasingly established.

Neoproterozoic (1.0 Ga–539 Ma)

• Rodinia began breaking apart around 750 Ma. Rifting and continental separation continued over an extended period.

• The Cryogenian Period experienced severe global glaciations, including the Sturtian and Marinoan events, associated with the Snowball Earth hypothesis.

• Glaciation altered Earth's climate, while weathering, volcanic activity, and carbon cycling contributed to later warming.

• Carbonate rocks deposited after glaciations, sometimes called cap carbonates, record major changes in ocean chemistry and climate.

• The Ediacaran Period saw diverse large multicellular organisms, including early animals, before the Cambrian explosion.

Precambrian history of life

1. Earliest evidence of life: more than 3.5 Ga, although older evidence has been proposed.

2. Photosynthetic microorganisms: released oxygen and helped form stromatolites and banded iron formations.

3. Great Oxidation Event: atmospheric oxygen increased around 2.4 Ga.

4. Eukaryotes: cells with nuclei developed by approximately 1.8–2.1 Ga, with uncertainty about the earliest evidence.

5. Ediacaran biota: diverse large organisms appeared during the last part of the Proterozoic.

6. Cambrian explosion: many animal groups diversified rapidly near the beginning of the Paleozoic.