There's long been a theory amongst some scientists that Venus may have once harbored life. New computer models increase the likelihood that the planet had, for an extended period of time, shallow oceans and temperatures suited to our kind of life.
Now, Venus is a hellscape. Why?
Earth is kept habitable by plate tectonics. Earthquakes and the like cause surface changes that sequester carbon dioxide. Venus does not have a strong enough magnetic field and these processes stopped a long time ago. Carbon dioxide and water vapor then built up into a runaway greenhouse effect. Essentially it hit a point of negative feedback. Venus' water boiled away into space (although a few scientists still think there may be surviving Venusian life in the upper atmosphere of the planet).
(As bad as climate change is, don't worry, we would have to burn ten times as much coal and oil as exists in the Earth's crust to trigger a full blown runaway. It is likely that this is the ultimate end of life on Earth, but only in a billion years or so).
So, what does Venus tell us?
First of all, it reminds us that for long term habitability a planet needs a magnetic field.
Second of all, we may be able to colonize Venus due to the fact that there's a point in the high atmosphere with gravity and pressure and temperature the same as Earth. In Venus' thicker atmosphere, our concept of breathable air is a lifting gas. Which is why cloud cities on Venus are slowly becoming a science fiction staple.
Third of all, Venus is likely a better candidate for terraforming than Mars is.
Fourth of all, Venus may have been habitable for three billion years. Remember those dinosaurs on Venus?
In fact, there may have once been Venusians...
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