In article ,
Everett Hickey wrote:
Earth has a predominantly nitrogen atmosphere, with oxygen making up only a
5th of it and everything else negligible. Much of this is because the far
heavier methane atmosphere we used to have, along with the not
inconsiderable co2 content, have been chemically leeched out...

Current belief is that Earth's atmosphere never had significant methane
(or ammonia). It was always an oxidizing atmosphere, dominated by CO2.
Earth's early atmosphere almost certainly looked a whole lot like Venus's
current atmosphere, except cooler and having more water vapor. In fact,
that's what Venus's atmosphere looked like then too.

(The key difference is just distance from the Sun: Earth's surface, even
with a thick CO2 atmosphere, never got warm enough to boil the oceans. On
Venus, once the oceans started to boil, there was no going back, because
water vapor is a powerful greenhouse gas: the oceans boiled dry, surface
temperature soared, solar UV broke down the water into hydrogen and
oxygen, and the hydrogen was lost. Earth never got that hot, and the
oceans slowly converted the CO2 into carbonate rock.)

...Had there been no life, we'd have either an almost
entirely nitrogen atmosphere w/ higher co2 levels...

There probably wouldn't be a lot of CO2 left. The CO2 was taken out by
chemical processes in the ocean; life was not involved in that until the
very end.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |