Mark,

Because the Sun is a main sequence star, we know that the Sun as a young star had much less luminosity. Over the eons, the Sun has slowly been growing in luminosity. It is believed that Venus once had an ocean and viable atmosphere. There is evidence that biology started on Earth almost immediately after formation of the ocean. It is quite possible that life started on Venus when that world still had its ocean. Unfortunately for Venus, as the Sun's luminosity increased, the Venusian surface temperature passed a critical limit and the atmosphere went into thermal runaway. Even more unfortunate, this same fate of thermal runaway awaits the Earth. Sometime in the next 2 billion years due to increasing solar luminosity, the Earth will become another Venus.

Now when we apply this same logic to Mars it becomes clear that Mars has probably never been warmer than it is today. Many people believe that all water on Mars is frozen (it is obviously unknown whether or not there is liquid water deep underground on Mars). Given that Mars has probably always been ice cold and probably never had an ocean makes the argument against biology on Mars a "slam dunk". IMHO, its only funding politics that keeps exobiology based Mars exploration going. We need to get Mars exploration shifted over to human settlement preparation as soon as possible. Also we should spend more resources exploring Venus which is actually a very interesting planet.

Gary