So some researchers have found signs of methane emissions on Mars, which are pretty hard to explain except in terms of life.
That's sort of cool, but also sort of depressing. If there's life on Mars, it's almost certainly tiny -- bacteria, basically -- and subterranean. More interestingly, it's life that's almost totally irrelevant to anything but itself. Life on Earth has totally transformed everything about Earth: the atmosphere, the oceans, plate tectonics, the composition of the crust. Life on Mars, on the other hand, seems to have done nothing to or for Mars; the planet would probably be much the same whether it existed or not. It's just... marginal.
Apropos of which, here's a rather overwritten but still interesting article about the new "Medea Hypothesis":
According to the paleontologist Peter Ward, however... life isn't naturally nourishing - it's poisonous. Rather than a supple system of checks and balances, he argues, the natural world is a doomsday device careening from one cataclysm to another. Long before humans came onto the scene, primitive life forms were busily trashing the planet, and on multiple occasions, Ward argues, they came close to rendering it lifeless. Around 3.7 billion years ago, they created a planet-girdling methane smog that threatened to extinguish every living thing; a little over a billion years later they pumped the atmosphere full of poison gas. (That gas, ironically, was oxygen, which later life forms adapted to use as fuel.)
The story of life on earth, in Ward's reckoning, is a long series of suicide attempts. Four of the five major mass extinctions since the rise of animals, Ward says, were caused not by meteor impacts or volcanic eruptions, but by bacteria, and twice, he argues, the planet was transformed into a nearly total ball of ice thanks to the voracious appetites of plants. In other words, it's not just human beings, with our chemical spills, nuclear arsenals, and tailpipe emissions, who are a menace. The main threat to life is life itself.
"Life is toxic," Ward says. "It's life that's causing all the damn problems."
Ward, a paleontologist at the University of Washington and a scholar of the earth's great extinctions, calls his model the Medea Hypothesis, after the mythological Greek sorceress who killed her own children. The name makes clear Ward's ambition: To challenge and eventually replace the Gaia Hypothesis, the well-known 1970s scientific model that posits that every living thing on earth is part of a gargantuan, self-regulating super-organism.
Ward himself admits that the name "Medea Hypothesis" is a bit of a piss-take, but what the hell: I'm good with it. I lived on an island for several years; island ecosystems are stable until something happens and then, well, they're not. So the whole Gaia thing always seemed a bit dubious to me.
But back to Mars: assuming the methane blooms are biogenic, there are still some obvious questions. Are the bugs native to Mars or imported from Earth by impact events? (Or is Earthlife descended from Mars?) Has life always been marginal, or was there a lot more of it in the past? If the latter, was its decline endogenous, or caused by some external event?
Meanwhile, here's a speculation: what if the Mars model is the norm? If the universe is full of life, yep, sure enough -- and virtually all of it consists of small, marginal groups of microbes barely clinging to existence? And Earth, with its teeming biosphere, wildly disequilibriated atmosphere, and great lumbering multicellular organisms everywhere, is the far far end of the distribution curve?
I'm sure there's a catchy name for this scenario, but I can't think of one this late at night. Hum: what's something that is ubiquitous, but irrelevant?