Suggested at New Scientist: There may be life out there that was like life on Earth when Earth was a very different environment:
In the 4.5 billion years our planet has existed, it has experienced dramatic transformations: ice ages and warming periods, times when the atmosphere was impossible to breathe, when large areas were desert, or when lush tropical forests hugged the poles. Throughout the vast majority of this turbulent history, life has somehow clung on.
If, armed with a spotters’ guide to the world we inhabit today, we found exoplanets resembling those early Earths, would we even recognise them for what they were? Maybe not. We know how to seek comparatively advanced signs of intelligent life, such as cacophonous radio transmissions and the bright lights of megacities. If a planet has less sophisticated inhabitants, however, we must rely on identifying other signatures …Kelly Oakes, “The clues to finding alien life could lie in Earth’s deep past” at New Scientist (paywall)
It’s kind of like a religious quest. It’s probably good for them to keep looking so one doesn’t tell them to stop even if one fears that the Grail isn’t out there.
See also: At Forbes: About extraterrestrial life, “fancy probabilistic analysis” just isn’t science
How can lots of water worlds be “bad news for life”?
At Scientific American: Maybe aliens live too fast or too slow for us to recognize
SETI reacts to the new study that says not to wait up for extraterrestrials
Researchers: We have dissolved the Fermi Paradox! (They’re NOT Out There)