The blue marble, NASA

Are we alone in the universe? Is there alien life out there somewhere? These are questions that have long intrigued humankind. Answers have ranged from the optimistic postulate by Frank Drake forty years ago of more than 1,000 civilizations in our galaxy alone to the much more pessimistic opinion of the astrobiologists, Ward and Brownlee, in 2000 that complex life may be extremely rare in the universe at large. Whereas the latter authors contend that intelligent life may be scarce, they express the belief that simple (microbial) life is abundant in the universe.

The likelihood of simple life being widespread depends, of course, on the probability of life’s origin. On this question there is again a wide range of opinion among scientists. Ward and Brownlee believe that life’s origin on Earth was highly probable and microbial life is common in the universe because it occurred so…