Precambrian Eon

The Precambrian eon, or Supereon, refers to the geological time comprising the eons that came before the Phanerozoic eon. This time spans from the formation of Earth around 4.5 billion years ago to the evolution of abundant macroscopic hard-shelled animals, which marked the beginning of the Cambrian era, the first period of the first era of the Phanerozoic eon. The Precambrian eon encompasses 86 percent of the Earth’s history, however, very little is known about this time period. In fact, the few fossil discoveries from this period were recently made in the late 20th century. Precambrian time can be further divided into three large eons, the Hadean, Archean, and Proterozoic eons.

The Precambrian’s oldest eon, the Hadean (4.5 to 3.9 billion years ago), predates most of the geologic record. During the Hadean, the solar system formed out of gas and dust, the sun began to emit light and heat, and Earth took shape. Meteors and other galactic debris showered the planet over the first half-billion years, making it entirely uninhabitable. Planet Earth was very hot during its initial formation. As the Earth began to cool and its mass increased, its gravitational field strengthened. This attracted meteorites and other debris, which continued to bombard the planet for at least another 500 million years, producing enough energy and heat to vaporize any water or melt any rock that may have been present.

Iron continued to sink to form the Earth’s core, while silicon, magnesium, and aluminum gradually rose toward the surface. Gases released from magma inside the Earth escaped through cracks in the surface and began to collect in the early atmosphere. The likely presence of methane and ammonia among the gases made for conditions that would be highly toxic to life as we know it. Because there was little to no free oxygen, no protective ozone layers existed and damaging ultraviolet rays showered the Earth at full strength.

As the meteorite bombardment finally slowed, Earth was able to cool, and its surface hardened as a crust, rocks, and continental plates began to form. Water began to condense in the atmosphere, resulting in torrential rainfall. After several hundred million years of falling rain, great oceans were formed. By about 3.9 billion years ago, Earth’s environment had been transformed from a highly unstable state into a more hospitable place. This marked the beginning of the Archean eon (3.9 to 2.5 billion years ago). It was early in the Archaean eon that life first appeared on Earth.