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Originally Posted by zen_tom
Which way? I took what I thought was a conservative, rounded figure of 500 million from what I read here |
That URL has life's age between 4.5 Gyr and 3.5 Gyr.
The Cambrian era starts with the "Cambrian explosion", when life exploded into lots of different forms that formed pretty fossils. I believe that hard shells became relatively common during this period (or, at least, in this period's fossil record!). Life before the "Cambrian explosion" is harder to find in the fossil record. The pre-cambrian Vendian period goes back to 650 million years, and contains relatively common, soft-body, earlier fossil life.
More data:
http://www.ucmp.berkeley.edu/cambrian/camb.html
The oldest rocks we've found on earth are 4.03 billion years old (near Great Slave Lake in Canada). Asteroids seem to top out at 4.6 billion years old. The oldest evidence for live on earth places it back 3.5 to 4.0 billion years.