All you have to do is look up at the night sky to see how many stars are out there, many with Earth-like worlds orbiting the star. Scientists searching for alien life have often wondered why with so many potentially habitable planets orbiting the plethora of stars in the universe, why haven't we heard any signs of other intelligent life.
Astrobiologists from the ANU research School of Earth Sciences say the reason we haven't heard any signals from extraterrestrials is that early life is fragile and becomes extinct quickly. Dr. Aditya Chopra says that early life is fragile and rarely evolves quickly enough to survive.
Chopra goes on to say that the reason we haven't found signs of alien life is less because of the likelihood of the origin of life or intelligence. It has more to do with the rarity of the rapid emergence of biological regulation of feedback cycles on the surface of planets in the universe.
In a nutshell, the scientist says that the problem for early life on planets is that the planet experiences runaway cooling or heating leading to the death of any early life on the planet before the organism can regulate greenhouse gasses like water and carbon dioxide to stabilize surface temperatures. This has been called the Gaian Bottleneck. One prediction of the bottleneck is that the majority of fossils on planets that did have early life will be from extinct microbial life, not from multicellular species.