On Earth, life emerged rapidly — practically as soon as the planet reached sustainable homeostasis. By contrast, the first multicellular organism from which we descend took nearly 4 billion years to evolve. But despite our knowing when life emerged on our world, scientists know little about how it happened — a mystery with crucial import to the question of life on alien worlds.
However, a statistical technique called Bayesian inference might shed new light on how complex alien life evolves beyond the Earth, according to a new paper published in the Proceeding of the National Academy of Sciences.
Bayesian inference to alien life on new worlds
In the new paper, Assistant Professor at Columbia's Department of Astronomy David Kipping shows how using a statistical technique known as the Bayesian inference could advance astronomers' understanding of how complex extraterrestrial life could evolve on alien worlds.
"The rapid emergence of life and the late evolution of humanity, in the context of the timeline of evolution, are certainly suggestive," said Kipping. "But in this study it's impossible to actually quantify what the facts tell us."
To carry out his analysis, Kipping adapted our chronology of the earliest evidence of human life. He then asked how often life — and intelligence — should happen if Earth's history were repeated. From the beginning till now, he ran the clock over and over again.
Bayesian statistical inference of alien life
Kipping framed the problem in four different ways: Life could be common and intelligence usually follows, life might be rare but usually develops intelligence, life is common but rarely with intelligence, and lastly, life is rare, and rarer still intelligent.
This mode of using Bayesian statistical inference — which updates the probability for a hypothesis as more evidence or information becomes available — produces prior beliefs regarding the modeled system, and then combines them with new data to cast probabilities of disparate outcomes.
In other words, it's a feedback loop, self-referencing through time as new data becomes available.
"The technique is akin to betting odds," said Kipping. "It encourages the repeated testing of new evidence against your position, in essence a positive feedback loop of refining your estimates of likelihood of an event."
Good odds for alien life, not so for alien intelligence
From the four possible cases above, Kipping applied Bayesian mathematical formulas to weigh the models against one another. "In Bayesian inference, prior probability distributions always need to be selected," said Kipping. "But a key result here is that when one compares the rare-life versus common-life scenarios, the common-life scenario is always at least nine times more likely than the rare one."
The analysis relied on evidence suggesting life on Earth began 300 million years following the formation of Earth's oceans as specified in carbon-13-depleted zircon deposits — very fast in the context of Earth's total lifetime. Kipping reiterated that the ratio of life emerging was nine-to-one or higher, depending on the real-world value of the probability that life develops.
Kipping's conclusion found that if planets with similar conditions and evolutionary development timelines to Earth are common, then the analysis shows it shouldn't have issues spontaneously emerging on alien worlds.
As to the odds for complex, distinct, and intelligent alien life? This is where Kipping says the odds aren't as strong, reporting a mere three-to-two odds in favor of intelligent life on alien worlds. "If we played Earth's history again, the emergence of intelligence is actually somewhat unlikely," he said.
While the search for alien intelligence carries on, Kipping's findings lend statistical credibility to the main prerequisite: life itself. While his work doesn't do the same for intelligence on alien worlds, he reassures the global community: "The search for intelligent life in worlds beyond Earth should be by no means discouraged."