The Earth formed 4.6 billion years ago, with the Moon forming from a collision with a smaller planet. The Hadean Eon saw the formation of the Earth’s crust and the emergence of primitive life, while the Archean Eon saw the evolution of autotrophs and the “oxygen catastrophe.” The Proterozoic, Paleozoic, Mesozoic, and Cenozoic eras saw the evolution of complex multicellular life.
Planet Earth formed about 4.6 billion years (4,567,000,000 million years) ago, in a completely molten state, from the protoplanetary debris disk orbiting the newly formed Sun. Another smaller Mars-sized planet called Theia is thought to have formed in the same orbit as Earth around this time. After 28 – 34 million years (4.527 – 4.533 billion years ago), orbital wobbles caused Theia to hit the Earth, ejecting a large amount of crust and forming the Moon. This is the best current theory of the formation of the Moon, called the giant impact hypothesis.
The earliest era in Earth’s history is known as the Hadean Eon (named after the underworld of Greek mythology, Hades), which spans from the formation of the Earth to 3.8 billion years ago. The oceans began forming during the early Hadean, perhaps as late as 4.2 million years ago. The Earth cooled from a molten state, forming a solid crust in just 100-150 million years. Heavier elements like iron and nickel have sunk into the Earth’s core, leaving smaller amounts on the surface. The entire planet differentiated into layers. During the latter part of the Hadean, about four billion years ago, primitive life emerged, perhaps as small self-replicating strands of DNA or RNA that used energy-rich chemicals for fuel (heterotrophs).
The next era of Earth’s history, known as the Archean Eon, lasted from 3.8 billion to 2.5 billion years ago. Molecular dating studies have indicated that the last universal common ancestor (LUCA) of contemporary life lived during the early Archean, 3.5 billion years ago or earlier. This is around the same time that the first autotrophs evolved. An autotroph is an organism capable of using photosynthesis to extract energy from sunlight. Oxygen is a waste product of photosynthesis, and was released in large quantities during this time. Because oxygen was poisonous to most living organisms then, there was a mass extinction and a reshaping of air and surface chemistry, called the “oxygen catastrophe.”
The Aeons that follow the Hadean and the Archean are the Proterozoic (“primitive era of life”), which lasts from 2.5 billion to only 542 million years ago, the Paleozoic (“old era of life”) which lasts from 542 to 251 million years ago, the Mesozoic (“middle era of life”) which lasted from 251 to 65 million years ago, and the Cenozoic (“recent era of life”) which lasted from 65 million years ago to the present. In the Proterozoic, except for the last tens of millions of years, the only known life forms were primitive microbes. In the last 542 million years alone, about 12% of the Earth’s history, complex multicellular life evolved.
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