Re: How Long has Earth been habitable?

On Jan 25, 11:26=A0am, "Mike Dworetsky"
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> "Mike Dworetsky" <platinum...@[EMAIL PROTECTED]
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> > "Paul Ciszek" <nos...@[EMAIL PROTECTED]
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> >> When did Earth first have an atmosphere with enough O2 and a low
enough=

> >> CO2 level that an unprotected human could breathe? =A0Have there been
a=
ny
> >> times since then that these conditons were not met?
>
> >> --
> >> Please reply to: =A0 =A0 =A0 =A0 =A0 =A0 =A0 | "Evolution is a theory
t=
hat accounts
> >> pciszek at panix dot com =A0 =A0 =A0 | =A0for variety, not
superiority.=
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> >> Autoreply has been disabled =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0
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=A0 -- Joan Pontius
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> > I'm told (via some googling at a few sites) that the best estimates
are
> > that the atmosphere reached O2 levels similar to today around 400 MY
ago=
..
> > It seems doubtful that humans could have survived during the late
> > Cambrian, around 530 MY ago. =A0There was some oxygen but probably not
> > enough (a few %). I don't think the high levels ever disappeared or
> > reduced to below land-animal survivability once they appeared.
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> > --
> > Mike Dworetsky
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> > (Remove pants sp*mbl*ck to reply)
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> Found a recent article in Nature on this very question. =A0Essentially
> confirms my previous post. =A0For a long time the level was around 1-10%
o=
f
> present level, but it rose around 400-500 MYA.
>
> http://www.nature.com/nature/journal/v451/n7176/full/nature06587.html
>
> --
> Mike Dworetsky
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> (Remove pants sp*mbl*ck to reply)- Hide quoted text -
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> - Show quoted text -

  Here's another showing we could have breathed  the atmosphere going
back to the Cambrina explosion.

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=3D34224

  Incidentally, there was  supposedly a big in oxygen levels to close
to 35% during the carboniferous, when we got those giant dragon flys.
Anything over 35%, and you easily get run away fires, bringing the
oxygen level back down again- A. McIntire