Hi LJK

All the new findings on the Earth's early days make it seem nastier than ever -

first we get a reducing atmosphere,

then hydrogen/carbon-dioxide,

then N2, CO2 & CH4, with seas full of sulphide... no serious oxygen until the late Proterozoic.

Then a killer Ice Epoch, with the nicest conditions like the Arctic, interspersed with 120 degree F winters when CO2 rises to ~ 3%...

Things stabilise and oxygen takes 400 megayears to rise from 1% to ~ 35%, before crashing below 10% at the end of the Permian and taking ~ 90% of species with it.

Then for whole of dino-time it sits at ~ 10%... like being in the high Andes.

Finally a slow crawl back up to ~ 23% and a slight dip back to 21%. And where next? In about 500 million, maybe, the CO2 will crash to zero and all land-life will grind to a halt. The sea biosphere will persist thanks to dissolved carbonate, until Earth's average temp. rises enough to saturate the stratosphere and... whoosh! Oceans gone with hydrogen into space thanks to UV by 1 billion AD.

Man... when we look at "Earth-like" planets for much of their history they'll be uninhabitable to all except anoxic microbes.