Contrary to popular belief, speculation that we are not alone in the universe is as old as philosophy itself. The essential steps in the reasoning were based on the atomic theory of the ancient Greek philosopher Democritus. First, the laws of nature are universal. Second, there is nothing special or privileged about Earth. Finally, if something is possible, nature tends to make it happen.
Philosophy is one thing, filling in the physical details is another.
Although astronomers increasingly suspect that bio-friendly planets may be abundant in the universe, the chemical steps leading to life remain largely mysterious. It follows that the likelihood of its happening again elsewhere in the cosmos is infinitesimal. Life bucks this trend only because it is a statistical fluke. Similar reasoning applies to evolution. According to the orthodox view, Darwinian selection is utterly blind.