Origin of Earth.
Origin of Life...

Timeline 2 - 5 billion years ago to present

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The Earth apparently originated out of a cloud of cosmic gass some 4.6 billion years ago.  4.6 billion years is 4,600,000,000 years - but that doesn't help much.  What does 4,600 million mean in concrete terms?  There would seem to be nothing in our experience that could help us to grasp such a number.  We know that billions of computations flow through microprocessors; we know there are billions of stars out there.  We know because we've been told so, but our knowledge is beyond our understanding.

Still, trying to to conceptualize the age of the Earth may prove to be a useful exercise.  In part, because we are inhabitants of the Earth, and as we learn to understand it, we may also learn to inhabit it in better ways.  In part, because we are surrounded by vast numbers - billions of dollars, billions of people, billions of bits of information, billions of microbes in an infected reservoir, millions of degrees at Ground Zero in Hiroshima.  As long as these numbers remain hazy abstractions, they will continue to have little impact on our minds, though they may have considerable effects on our lives.

Let us think in terms of units of space and distance, which are more concrete and tangible than the invisible flow we call time: A millimeter is a unit of distance that it easily grasped.  It is about the thickness of a wooden match.  A kilometer is another tangible unit.  It's a short walk, somewhat less than a mile.  We can easily imagine ourselves moving that distance.  Nevertheless, there are 1 million millimeters to a kilometer.

Now let us imagine a strange clock that ticks only once a year.  Every year, at each tick, it moves a small marker one millimeter forward along a thin rail that reaches right across the Atlantic.  Every year, the marker moves one millimeter.  Every 10 years, it moves one centimeter.  10 centimeters is a very long lifetime for a human being.  One meter is 100 centimeters, that's 1000 years.  If 10 centimeters brings us back in history to 1900 AD, a meter brings us back to the early middle ages.  And a kilometer brings us a million years back.  That's well before Homo Sapiens had evolved fully as a species, though his and her predecessor, Homo Erectus, may already have developed a social life fairly similar to ours.  Or to put it another way: a kilometer is about 15 ice-ages ago.

Now 10 kilometers is a good day's walk, it brings us back 10 million years.  The 30 million year long uplifting of the Himalayas, Alps, Sierras and Rockies had just been completed 10 million years ago, and only distant, chimpanzee-like ancestors of Homo Sapiens existed.

100 kilometers is nearly the distance, as the crow flies, from San Francisco to Sacramento, California.  Moving by slow, one-millimeter-per-year ticks, that brings us 100 million years back.  That's back to the time of the dinosaurs, which had by then already been flourishing for 100 million years, and still had some 25-30 million years left on Earth, before they disappeared, along with countless other species, in the last of the Earth's great extinctions.  What causes the great extinctions remains a mystery, though a theory has been gaining ground recently, that the last great extinction was caused by the impact of a giant meteorite that hit the Earth at the site of the present-day Gulf of Mexico.

1000 kilometers is approximately the distance (as the crow flies) between Manchester (England) and Berlin (Germany).  The clock has by now been ticking for 1,000 million years, and that takes us back beyond the Varangian glaciations (850 - 600 million years ago), when the Earth was for a time almost completely covered by ice, back before the origin of true animals - although bacteria and other simple life-forms had existed in the seas for at least 2400 million years already.  Scientists are constantly pushing the limit for the origin of life backward.  But it is already clear that life is nearly as old as the Earth itself.

The age of the Earth is 4600 million years.  The marker on our clock has moved 4600 kilometers since then.  Starting in Boston, Massachusetts, it has travelled almost across the Atlantic to Lisbon, Portugal, moving one millimeter a year, 7-8 centimeters in the course of a normal human life-span.  It has passed through a semi-molten state, through a long process of crust formation, which has left us with the fairly stable landscape forms that characterize the Earth as we know it.  At the end of this process, about 1000 million years ago, at the end of what geologists call the Precambrian era, the Earth seems to have cooled down so drastically that it nearly ended as an ice-ball planet.  Then, for unknown reasons, the climate warmed again, and throughout the last 550 million years it has continued to stay habitably warm.  It is only during this rather short period that the Earth as we know it, has existed.

Homo Sapiens Sapiens originated 130 meters ago - but the age of the Earth is nearly the width of the Atlantic Ocean.  We occupy a very small place in the general order of things.

Sources

Ice-ball glaciation, ca. 2400 mill. years BP: Caltech Media Relations, News Releases, March 19th 1997.
Precambrian mass extinctions: http://hannover.park.org/Canada/Museum/extinction/cammass.html 
Abundance of oxygen in atmosphere, first ice-age, ca. 2400 mill. BP: Scientific American, Explore! (online), Nov. 1st 1999.
First land-based life, 2,200 mill. BP: Scientific American, Explore! (online), Nov. 1st 1999.
Cambrian explosion, Rapid Polar Wander during Cambrian: Scientific American, Explore! (online), Aug 25th 1997.
Ice-ages, 750 and 600 mill. BP: Scientific American, Explore! (online), Nov. 1st 1999.
Varangian Glaciation, supercontinent Rodinia, crust formation, earliest life: Precambrian Tectonics, Climate, and Life - http://www.lehigh.edu/~inpal/