In a paper just published in the Proceedings of the National Academy of Sciences, David Kipping from Columbia University used a Bayesian approach to assess how likely life—including intelligent life—is to exist beyond Earth. Bayesian inference is a good statistical method to test a hypothesis when data is very limited and where some facts are more likely than others. Another advantage is that biases can be built into the analysis, as in Kipping’s work.
We know that life arose quickly on our own planet. If evidence from the first terrestrial microfossils is used in the Bayesian analysis, Kipping’s results show that the likelihood of life appearing rapidly on other worlds is about three times greater than the likelihood of its being rare. If life appeared even earlier on Earth, as suggested by Elizabeth Bell and co-authors based on isotopic evidence in minerals, the chances are nine times greater.
Regarding the evolution of intelligence, Kipping’s results were a bit less clear. They suggest that the odds are 3 to 2 that intelligence may be rare. These results are not surprising, since they’re consistent with what many of us would think intuitively. Life arose early on Earth, ergo life is likely to be common. Intelligent life—such as ours—arose very late, so it must be quite a bit more rare.
But there may be a caveat. Perhaps an early origin of life is necessary for intelligence to develop. After all, Earth will be habitable for humans only for another billion years or so. If the evolution toward humans had taken 20 percent longer, we would have run out of time. Of course, this calculation may not apply to other types of stars different from our own, where the habitable period of orbiting planets can extend much longer.
Kipping’s analyses seem to indicate that the Universe likely teems with life. I don’t think it provides much insight, however, on the Fermi Paradox—the question of why we haven’t already made contact with technologically advanced extraterrestrials. Then again, his two-step model only includes the origin of life and the evolution of intelligence as transitions. William Bains and I identified in our Cosmic Zoo hypothesis other critical transitions in the evolution toward technologically advanced life, such as the invention of photosynthesis, the rise of eukaryotes, and the dawn of multicellularity—each of which could possibly act as a filter toward reaching the next phase of complexity. I’m wondering what the results would be if these transitions were included in Kipping’s analysis?
The advantage of using Bayesian statistics is that results are independent of our inherent biases. But the conclusions are still based on N=1: life on Earth remains the only example of life in the Universe. So any result, however clever the method employed, has to be taken with a grain of salt.