Precambrian
Earth coalesced from orbiting material about 4,543 million years ago. A planet named Theia struck the young world shortly after its birth. This collision split off debris that eventually formed the Moon. Scientists call this event the Giant-impact hypothesis. Zircon crystals found in Western Australia date to 4,404 ± 8 million years old. These tiny minerals prove a stable crust existed by 4,433 million years ago. Heavy bombardment continued for hundreds of millions of years. The Hadean Eon ended when the Late Heavy Bombardment period concluded around 4,031 million years ago.
Carbon found in rocks from western Greenland dates back 3.8 billion years. Some scientists believe this carbon has organic origins. Microscopic fossils of bacteria older than 3.46 billion years exist in Western Australia. Evidence suggests life could have evolved over 4.280 billion years ago. The RNA world hypothesis claims RNA evolved before DNA genomes. Porous rock systems with heated air-water interfaces may have supported ribozyme-catalyzed RNA replication. Complex multicellular organisms appeared as early as 2100 million years ago. Cyanobacteria and other microbes formed prokaryotic mats covering terrestrial areas.
Molecular oxygen was not a significant fraction of Earth's atmosphere until photosynthetic life forms evolved. Photosynthesis produced oxygen as a metabolic byproduct. This shift caused an ecological crisis known as the oxygen catastrophe. Oxygen quickly combined with iron in Earth's crust initially. Massive banded iron formations laid down as iron oxides provide evidence for this process. After oxidizable surfaces ran out, oxygen began to accumulate in the atmosphere. Modern high-oxygen atmospheres developed only after these ancient chemical reactions ceased.
Glacial periods identified go back to the Huronian epoch around 2400, 2100 million years ago. The Sturtian-Varangian glaciation occurred between 850 and 635 million years ago. These ice ages may have brought glacial conditions all the way to the equator. Scientists describe this state as Snowball Earth. Evidence suggests the planet froze from pole to equator during these extreme events. Such global freezing would have drastically altered surface temperatures and ocean chemistry. These conditions persisted for millions of years before the ice finally melted.
The earliest known supercontinent was Vaalbara formed 3.636 billion years ago. Vaalbara broke up approximately 2.845, 2.803 billion years ago. Kenorland formed around 2.72 billion years ago and split into cratons like Laurentia and Baltica. Columbia or Nuna assembled between 2.1 and 1.8 billion years ago. Rodinia is thought to have formed about 1300 to 900 million years ago. This massive landmass included most continents before breaking apart around 750 to 600 million years ago. Plate tectonics drove the assembly and breakup of these ancient landmasses over billions of years.
A very diverse collection of soft-bodied forms appeared between 635 and 542 million years ago. These organisms are referred to as Ediacaran biota. Tracks from an animal with leg-like appendages were found in mud dated 551 million years ago. Hard-shelled creatures appeared toward the end of this time span. The increase in diversity during the early Cambrian period is called the Cambrian explosion. Fossils widely accepted as complex multicellular organisms date from the Ediacaran Period. A possible 1047 million year old Bangiomorpha red alga exists in the Canadian Arctic.
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Common questions
When did Earth form according to the Giant-impact hypothesis?
Earth coalesced from orbiting material about 4,543 million years ago. A planet named Theia struck the young world shortly after its birth and this collision split off debris that eventually formed the Moon.
What is the earliest evidence of life in Precambrian rocks?
Carbon found in rocks from western Greenland dates back 3.8 billion years. Microscopic fossils of bacteria older than 3.46 billion years exist in Western Australia and some scientists believe this carbon has organic origins.
How did oxygen levels change during the Great Oxidation Event?
Molecular oxygen was not a significant fraction of Earth's atmosphere until photosynthetic life forms evolved. Oxygen quickly combined with iron in Earth's crust initially before massive banded iron formations laid down as iron oxides provided evidence for this process.
When did Snowball Earth glaciation occur in the Precambrian era?
Glacial periods identified go back to the Huronian epoch around 2400, 2100 million years ago. The Sturtian-Varangian glaciation occurred between 850 and 635 million years ago when scientists describe this state as Snowball Earth.
Which supercontinent existed first in the Precambrian history?
The earliest known supercontinent was Vaalbara formed 3.636 billion years ago. Vaalbara broke up approximately 2.845, 2.803 billion years ago while Kenorland formed around 2.72 billion years ago and split into cratons like Laurentia and Baltica.