Scientists examining the planet Venus have long wondered about why "Earth's Twin" has such a radically different climate than our shiny blue marble. On theory now being bandied about egghead circles is that the planet evolved very rapidly in geologic terms, but then more or less halted after losing its lsupply iquid water, which it quite likely had at an early age. Water provides the ley lubricant for Earth's plate tectonic system, which helps keep the planetary climate stable by locking carbon into the planetary crust.
"In the early stages of the Solar System, Venus seems to have evolved very rapidly compared to the Earth. Data from Venus Express supports the theory that the Earth’s twin once had significant volume of water covering the surface but it appears that these oceans were lost in a very short geological timescale.
As a result of the loss of water, the geological evolution of the surface of Venus slowed right down because it was unable to develop plate tectonics like the Earth. Biological evolution was prevented altogether. Thus, in terms of Venus being another Earth in climate and habitability terms, it evolved too quickly at first, then too slowly."
Unfortunately for Venus, plate tectonics never got started, so a great deal of carbon dioxide remains in the planetary atmosphere, leading to a runaway greenhouse effect that sends temperatures on the surface to a blistering 450 degrees Celsius. Both the European Venus Explorer and a Japanese probe called the Venus Climate Orbiter are scheduled to study the planet in even more detail through the 2009-11 timeframe.