Scientists have found many planets that likely either were or are habitable, yet evidence of life on any of them has proven elusive. A team of astrobiologists in Australia has a theory about why that's the case.

Scientists have found many planets that are likely - or may have once been - habitable, yet evidence of life on any of them has proven elusive. 
A team of astrobiologists from the ANU Research School of Earth Sciences in Australia has a theory about why that's the case.
They believe alien life is already extinct. 
In their paper, published in Astrobiology, the scientists point out the evolutionary process resulting in advanced, durable beings takes an immense amount of