If earth would be enveloped by clouds, like Venus is today, there would be a totally different environment on the planet. Some research suggest that was the case circa 35000 years ago and before that. This renders for example carbon-14 dating inaccurate because the molecule breaks down differently in that atmosphere, also, most of the waters of todays oceans would be spread in the atmosphere so heck of a lot more land mass available. (check "underwater cities, pyramids etc..) Also, the environment would have a composition in the atmosphere which would promote biological systems to grow very large. Maybe also very old, I don't know.
This is an interesting theory.