The Archean Eon began 3.8 billion years ago and ended 2.5 billion years ago. It is defined by date boundaries rather than fossils. Life originated in the early Archean, confirmed by stromalite fossils. Earth had no free oxygen, a carbon dioxide atmosphere, and unstable continents. The oceans were more acidic due to volcanic activity and carbon dioxide.
The Archean Eon is the second geological eon in Earth’s history, beginning at the end of the Hadean Eon 3800 million years ago (mya) and extending to the beginning of the Proterozoic Eon 2500 million years ago. Unlike more recent eons when life was abundant, the Archean is defined by arbitrary date boundaries rather than the presence or absence of certain fossils. It would be safe to say that the Archean Aeon was long ago, as reflected in its name, which means old.
The origin of life occurred in the early Archean, about 3.5 billion years ago. This is confirmed by fossil evidence in the form of stromalites, the remains of simple microorganisms that created thick layers of biotic muck as they died. The Archean begins shortly after a long series of events called the Late Heavy Bombardment, during which pieces of solar debris that hadn’t yet formed into planets were pulled into the gravity well that is Earth. Some biologists believe this bombardment was enhanced by the presence of the massive planet Jupiter, which would have sucked up some of the largest asteroids before they could hit our fragile planet.
In the Archean, Earth’s heat flux was about three times what it is today, and there was no free oxygen, the atmosphere was composed mostly of carbon dioxide. In fact, oxygen was poisonous to most of the microorganisms existing at the time. Instead of rust, the surface had deposits of free iron. There were no stable continents – indeed, they were constantly being shuffled due to geological activity and comet impacts.
Some of the oldest rocks in the world date back to the Archean, or even earlier, in the Hadean. According to geologists, only 5-40% of the present continental crust was formed during the Archean. Volcanic activity would have been much greater than it is today, and many of the earliest proto-continents were probably formed by huge volcanic eruptions. Because of the large amount of carbon dioxide in the atmosphere, the oceans would have been more acidic than today and hostile to all but the most extremophile organisms today.
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