It means that any Earth-like planets we find may be populated by… yeast. But that counts. It’s life. And life does something special: it ingests chemicals and excretes other chemicals.
One such chemical is oxygen. On Earth, we breathe it in, but plants breathe it out. There’s a lot of it in our air; our atmosphere is more than 20% oxygen. If we found a planetary atmosphere with lots of oxygen gas, that would almost certainly be an indicator of life.
As it turns out, we’re on the verge of being able to do just that. Planets are dim and huddle close to their stars, but there are techniques to separate the light from the two objects. Oxygen has a signature, like a fingerprint, that can be detected in that light. It will take an extremely sensitive telescope and very clever techniques to see it, but we have the technology now to build such machines. One such is the James Webb Space Telescope, due to launch in 2018. It should be able to detect oxygen in an alien planet’s air. And many other instruments are being planned and built that can carry out similar observations.
Our technology is getting so good so quickly that finding alien biological atmospheric signatures is probably our best bet. To me, the numbers add up better than for the other strategies: there must be lots of this type of planet out there, life seems to arise easily, and biology messes with a planet’s chemistry in a detectable way. We don’t know if Mars or those watery moons have life at all, and even if they do it could take a long time to find it. And who knows if smart aliens are out there, and want to talk to us? But it may only be a few more years until we point a telescope at a fleck of light, absorbing those photons one by one, sifting through them, and finding in them – literally – the breath of life.
So when will we find life in space? If it's out there, then my hope is: very soon.