Hi All

I was just browsing a book store and found a book that covers the Earth's geological history by Ian Plimer, a geologist who bankrupted himself trying to get some creationists legally called frauds in court. Not such a successful move, but his book on this occasion had some interesting details on what the Earth was like in the last billion or so years.

http://www.abcshop.com.au/browse/product.asp?productid=157459 ... A Short History of Planet Earth

Apparently CO2 has maintained quite high levels even into the last 500 million years. Our current Ice Age is one of the few eras in which it has dipped below the percent mark - even in the age of animals (after 600 mya) it has hit as high as 18%. Since 5% is toxic to most of us after a short time the fact that it got so high suggests the Earth has sweltered and been rather toxic. Even just 120 mya it hit 6%. Nasty.

http://www.ipa.org.au/pubs/special/climate/plimerpaper.html

Oxygen has gone up and down dramatically too. Even in the Cambrian oxygen was only 5%, eventually rising to just 13% into Devonian times, peaking at 35% at one point, dropping, rising to 27% then dropping again to 16% then rising to current levels. For a good portion of history the Earth wasn't habitable by humans, even after animal life evolved.

As for the deeper past according to the majority of temperature indicators the Earth was HOT ~ 90 - 60 degrees C. CO2 and CH4 helped keep it hot even when the Sun was dimmer than at present.

http://au.arxiv.org/abs/astro-ph/0305214 ... here Charles Lineweaver covers the thermal history of the biosphere and its implications.

So I wonder how many biocompatible planets will seem uninviting to us cool-loving O2 breathers and yet be home to life?

Adam