|Posted: 2010-October-02 at 10:50pm | IP Logged
Most ocean floor chemosynthetic life depends on the geological activity of the world, not the atmosphere. Being larger than Earth, Gl 581g should have a larger core, and be active longer. Thus, the ocean floor would be an ideal place of life. Admittedly, it would also be almost impossible to detect that life, which is why we didn't know about Earth's deep-sea chemosynthesizers until relatively recently.
You're probably right about shallow water or land life, unless the world is very active (like Io?), which it probably isn't. So, if we landed there, and there were no photosynthesizers, we probably wouldn't see much, if any, evidence of life.
Photosynthesis may have started here on the seafloor, as there are photosynthetic organisms around the same deep-sea vents. Even with no visible light, they manage to collect infrared from the hot rocks and water. If the same types appeared at Gl 581g, they would be ideally suited to colonozing the shallower water, and ultimately the land, as the sunlight contains a much higher proportion of infrared that Sol's. Even that doesn't mean an oxygen atmosphere, as there are photosynthesizers that don't release oxygen, but it should produce some type of detectable change in the atmosphere.
"Advanced life" is another issue entirely. Multicellular life is a relatively recent development on Earth, and may or may not be as common as "life". But that's another subject entirely... The best estimate of Gl 581's age seems to be about 4.3 billion years, so there has been time for advanced life to appear, if life has evolved at the same rate as on Earth, if there's any life at all.