Re: How Long has Earth been habitable?

Paul Ciszek <nospam@[EMAIL PROTECTED]
> wrote:

> When did Earth first have an atmosphere with enough O2 and a low enough
> CO2 level that an unprotected human could breathe?  Have there been any
> times since then that these conditons were not met?

They don't really know.

On the other hand, agriculture seems to have been infeasible during
glacial periods--the climate was too chaotic--so you might find that
there were other factors that would rule out settlement.

-- 
Harry Erwin <http://www.theworld.com/~herwin>
My neuroscience wikiwiki is at 
<http://scat-he-g4.sunderland.ac.uk/~harryerw/phpwiki/index.php>