In reply to Bojo:
> I read something the other day saying that if the history of the Earth was compressed into 24 hours then humans arrived 78 seconds ago!
This arm comparison is one I've used before; as it relates to a part of the body, pretty much everyone can relate to it. The description below, pinched off someone's internet page, is about as exhaustive as you'll ever need.
T.
Hold your arm out with your fingers stretched, which ever you like, your choice. Place the tip of the index finger of your other hand in the middle of your chin, this the formation of the Earth, ~4560 million years ago. The next check point is where your neck and should meet. Here the Earth’s surface first solidifies and is then destroyed when an object the size of Mars collides with the Earth. The debris from this collision becomes our familiar moon. Next, we come to your armpit, it is here, around 4000 million years ago, that the first life appears.
About 3500 million years ago, round about your upper bicep, oxygen is suddenly more abundant in the Earths atmosphere and the chemistry of our planet is changed forever. Currently the blame for this ‘Great Oxidation Event’ is placed on the evolution of photosynthetic cyanobacteria. After this, nothing much happens until we get to your elbow, around 1600 million years ago. A bacteria swallows another bacteria, and for some reason the larger bacteria does not eat its victim. This is endosymbiosis, the evolution of the complex eukaryotic cell, the smaller bacteria eventually becomes the mitochondria of our own cells. You would think an event like this would revolutionise biology, and it does eventually. But like the bacteria before them, the first eukaryotes do nothing much for a really long time.
This period of ‘nothing much for a really long time’ lasts from your elbow to your wrist, around a billion years.
From around 800 to 600 million years ago, the base to the top of your wrist, a lot happens. Some eukaryotes do a second endosymbiosis with cyanobacteria and become photosynthetic algae, some also start working together in a single body and become the first sea weeds, who also invent sexual reproduction. Some amoeba start eating other amoeba, who in response either make the first shells, or learn to swim. This triggers a predator-prey arms race that rapidly escalates into a savage war for survival that rages to this day. At 550 million years, the base of your palm, some of the combatants grow large and start leaving visible fossils in the rocks. Some argue these creatures are plants or fungus, but most agree they are simple animals, or at least animal style organisms. At 542 million years, the middle of your thumb pad, life enters its nuclear age. Large shelled organisms appear, things with eyes and mouths and teeth and perhaps the greatest biological innovation of them all; an anus that is separate from the mouth.
By the top of your palm, large life forms are everywhere. They fill the oceans, they cover the land, they have infiltrated every single niche available, and they have made as many new niches. In the middle segment of your index finger, big lizards appear. A bit later they vanish when a rock the size of small mountain, and travelling at 250 thousand km per hour, tries to visit Mexico. Very quickly the lizards are replaced by furry warm-blooded, live birthing organisms called mammals. Around the middle of the last segment of your index finger the Earths vast tropical forests are replaced by grass lands. As grass is shorter and harder to climb than trees, some of the arboreal mammals called apes decide to live on the ground. To avoid being eaten they start walking upright to see over the grass. By the last quarter of the last segment of your index finger, some of those apes have started using simple stone tools to help them catch and prepare food. Shortly after, they also figure out how to make fire and how to attach a sharp stone to a long stick so they can catch food from a distance. Suddenly the Earth gets very cold and the upright apes decide to leave their warm homeland and travel to some of the coldest parts of the Earth.
This brings us to the very end of your index finger, the final fraction of a millimetre of your fingernail. This thinnest sliver of your body, and of Earth History, represents all of recorded human history and civilisation, it represents every ‘modern’ human who has ever lived or ever will live. Around the last 300 thousand years or so. The entirety of human history, both written, archaeological and biological, is just the thinnest wisp of breath in the lifetime of our planet