Planet Earth was formed roughly 4.54 billion years ago, not long after the Solar System began forming, with the Sun gaining mass in the center, and gaining enough heat for hydrogen fusion to begin. A spinning disc of gas and debris slowly gathered into bands and then into planets–the inner four rocky (Mercury, Venus, Earth & Mars), the outer four gaseous (Jupiter, Saturn, Uranus & Neptune).
The first 500 million years of earth’s existence–from 4.5 to 4 billion years ago (bya)–are known as the Hadean Eon. There was probably no ocean, and the planet surface was highly volcanic, because the planet’s interior was far hotter with radioactive decay. There are no fossilized traces of life. It is not well known what the atmosphere was. The Moon likely formed during this eon, possibly due to a small protoplanet colliding with earth and ejecting a large mass which began to orbit and became a satellite.
All remaining eons are known collectively as the “Phaneorozoic Eon”, from Greek meaning “life has appeared”. The Phanerozoic is broken down into five smaller eons, the first being the Archean (4-2.5 bya). Early single-celled organisms known as archea appeared (called “prokaryotes”, because the cells do not have separated, organized nuclei), some of which performed photosynthesis. By this means oxygen began to appear in the atmosphere, joining large concentrations of carbon dioxide and methane. The oxygen-rich atmosphere–its current state, known as earth’s third, came into existence.
Next was the Protoerozoic Era, 2.5 bya – 254 million years ago (mya).
Continents grew in size and with the oxygen-rich atmosphere the archea disappeared–poisoned out of existence by their own byproducts—and eukaryotic (cells with nuclei) organisms appeared. It is the emergence of eukaryotic cells which led to the evolution of multicelled organisms. It is theorized–though not consensus–that the emergence of oxygen as a major component in the atmosphere helped bring about more than one snowball, i.e. fully frozen, earth. Several major glaciations did occur, whether they reached the Equator or not.
Tomorrow: a brief history of the earth 2: the Paleozoic, Mesozoic and Cenozoic eras.
This post was previously published on Dailykos.com and is republished on Medium.
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