On Fri, 23 Aug 96 hanson@dosh.hum.caltech.edu (Robin Hanson) Wrote:
>>On earth life seems to have started as soon as it
>>possibly could, as soon as the planet cooled enough
>>to allow liquid water.
>I'm not sure we know that. There was a billion years
>(a long time really) between the time the Earth formed and
>the oldest known life fossils.
Earth formed about 4.6 billion ago, but life didn't have a chance for it's
first billion years or so, it was far too hot and the Earth suffered impacts
so large it would make the one that killed the Dinosaurs look like a joke.
One popular theory about the formation of the moon is that about this time
The Earth was hit by a Mars sized object, the ejected debris formed a ring
around the earth that later coalesced into The Moon. Even rocks couldn't
survive that, the oldest rocks ever found are the "Isua" rocks of south west
Greenland and they are only 3.8 billion years old. Interestingly some reported
signs of biological activity in the Isua rocks, but this interpretation
remains highly controversial. The oldest undisputed fossils are a little over
3.5 billion years old and were found in western Australia and South Africa.
>>Having got to this stage it was not easy to leave
>>it, life stayed stuck here for over 2 billion years.
>Only 1.7 billion years, actually.
Best estimate I've seen is that Eukaryotic cells evolved between 1.2 an 1.4
billion years ago, well over 2 billion years after the first life. Well OK, I
remember reading a report somewhere about 1.6 billion, but I don't think
that's generally accepted.
>For Great Filter issues, "hard" would be expected times of a
>trillion years or more. An expected times 700my is far from
>hard enough to matter.
Obviously we have zero chance of finding anything a trillion years old, but I
don't think 700 million is a trivial percentage of life's 3.5 billion year
history.
>The really hard stages, I think, do not cluster near us in
>time.
If so then we must have successfully passed through the hard stages long ago,
it's hard for me to believe that we won't get to the really difficult stuff
for another billion years.
John K Clark johnkc@well.com
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