Ask GeoMan...
How long has the earth been here?
...a very long time, at least according to the way humans measure time. Based on relative and absolute age dating techniques, geologists now think that the earth was formed about 4,600,000,000 years ago. One of the challenges of any study of the earth is being able to comprehend just how long this is, and for what an extremely small part of it humans have been here.
Let's make a simple scale model to illustrate: Spread your arms straight out from your shoulders. Pretend that all of earth history is represented by the distance between your fingertips. If you start at your left hand, time passed to your left wrist before we know ANYTHING at all - all rocks and other evidence are lost before that time. Sometime between your wrist and elbow, it seems certain that simple, primitive, one-celled life forms appeared. Also by this time, the earth's differentiation process had proceeded far enough to start building some mighty impressive continental land masses (probably nothing living on them, but they're getting ready). One-celled organisms stick around for quite awhile (actually they're still here), but by your shoulder the atmosphere begins to become enriched in free oxygen. The doom of one-cell supremacy is approaching. But it doesn't happen right away. Well past your head, and beyond the far reaches of your right shoulder, the earth finally sees two-celled critters. The expansion of life really picks up from here on, and we have evidence of some pretty complex beings by mid-forearm. It isn't until your right wrist that organisms developed hard parts (shells, bones, teeth) which could be preserved as "fossils" for us to find. Dinosaurs existed between the joints of your fingers. How about humankind? Take a nail file and gently scrape it along the fingernail of your longest finger. Way to go - you just wiped out all of human history (and more).
How many years has the earth existed? The same as how many birds are in a flock - more than you can count!
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