Level Head (level_head) wrote in how_life_began,
Level Head

The Story of the Universe So Far

These things are well understood, and supported by many independent lines of evidence:

-- The universe's origin roughly 13.7 billion years ago (bya),
-- the solar system's condensation from a cloud of stellar dust and gases about 4.6 bya,
-- the Earth's cohesion into a planet about 30,000 years after the Sun lit up 4.55 bya,
-- the then-slightly-smaller Earth getting whacked by another planet, creating the present day Earth and Moon,
-- the formation of very primitive life less than a billion years later,
-- the "poisoning" of Earth's atmosphere with oxygen more than 2 bya
-- the formation of early multicellular life about a billion years after that,
-- that the world was covered in ice around 700 million years ago (mya),
-- the beginnings of "modern" groups of organisms once ice released its hold around 600-550 mya,
-- the colonization of the land first by plants, then arthropods, then finally vertebrates about 350 million years ago.

The material that makes up the present day continents has been accumulating for a while -- it is the "froth" on top of the tectonic plates, light enough not to get drawn back down inside the Earth. The continents have swirled and collided and broken up and recollided in new shapes about two dozen times that we know of. Currently, the Atlantic is getting wider as new lava is inserted at a "seam" that runs north-south down its middle. This seam passes through Iceland, for example, which gets about 2cm (nearly an inch) wider every year.

Since the Earth's magnetic field affects rocks that harden at the time, we can tell that the poles have "swapped" many times (about 29 cycles). And we can see the "stripes" of rock spreading on the sea floor with the different magnetic orientation, and know that the Atlantic was a continental split that has been widening for about 54 million years. Earlier, the Iapetus ocean was absorbed when the continents drew together -- that old boundary runs through Greenland -- and England, as the new split crosses the old join.

I'd be pleased to take a shot at any "how do we know?" questions on any of this -- I'm working from memory, and it will be good exercise to look up good source material.

===|==============/ Level Head
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