Hi All

3,000,000,000 BC - Twin planets existing close to a G2 star - both are covered in clouds, dense N2/CO2 atmospheres, and near scalding (about 343 K) oceans. That's the picture of Venus and Earth prior to their divergence...

Ancient Earth...
http://www.stanford.edu/dept/news/pr/2004/lowegeo-62.html

Ancient Venus...
http://www.newscientist.com/news/news.jsp?id=ns99994136

...one planet stayed hot, and got hotter. The other got cold.

The hot one boiled away its oceans, loading its atmosphere with water vapour and solar UV obligingly busted it up and the solar wind blew away hydrogen from the planet's exosphere. The planet dried up throughout its crust. Without lubricant the crust seized up and interior heat was bottled up until it erupted and re-paved the surface in lava. The planet cooled from the clouds, then grew hotter from the extra greenhouse gases, perhaps a few times before becoming present day hellish lava-plains.

The cold one became cold because it buried most of its CO2 in mineral form, then chilled out when the CO2 dropped enough for the stratosphere to turn red from methane derived aerosols. The cold planet never got as hot as it was ever again, but it got cold time and time again. Damn cold, perhaps 223 K every so often. But it also got warm from time to time too.

But once the two worlds were very similar.

Adam