Where We Came From--and When

A little more than four and a half billion years ago, when the universe was already nine or ten billion years old, a floating mass of gas and dust some 15 billion miles across began to come together where we are now in the Milky Way galaxy.  More than 99 % of it formed itself into a star, our Sun.  The remainder of the dust and gas, little more than interstellar debris, began collecting in what we call our solar system, and gradually the largest collections of the debris formed into the planets that orbit the Sun.  Including Earth of course.

That all happened in a short space of time, some 200 million years.  Then, still in its planetary infancy, Earth was struck by a huge asteroid or planet which split off a big chunk of its surface and sent it hurtling into space, stopping some 240,000 miles away where it began orbiting the Earth it used to be a part of.  This of course is the moon.

An atmosphere of carbon dioxide, nitrogen, methane, and sulfur gradually formed above the Earth.  Carbon dioxide, the powerful greenhouse gas we hear so much about today, has a warming effect that threatens to melt Arctic ice caps and raise coastal water levels.  That same carbon dioxide  was also at work in the early years of Earth's formation when warming saved the planet by preventing it from freezing over.  The principal at work here may be that if you wait long enough good turns to bad and bad to good, at least in the case of carbon dioxide--and maybe everything else if you have time to wait and see.

It took another 500 million years before life formed and another four billion years after that before we showed up.  Those thoughts may get their own posting some day, but for now, however, it is remarkable to think even this briefly about where we came from--and when.


With thanks to the brilliant and always entertaining Bill Bryson, this time in his Short History of Nearly Everything (2003).