New study estimates the odds of life and intelligence emerging beyond our planet
From: Columbia University
May 18, 2020 -- Despite knowing when
life first appeared on Earth, scientists still do not understand how life
occurred, which has important implications for the likelihood of finding life
elsewhere in the universe. A new paper shows how an analysis using a
statistical technique called Bayesian inference could shed light on how complex
extraterrestrial life might evolve in alien worlds.
We know from the geological record that
life started relatively quickly, as soon as our planet's environment was stable
enough to support it. We also know that the first multicellular organism, which
eventually produced today's technological civilization, took far longer to
evolve, approximately 4 billion years.
But despite knowing when life first
appeared on Earth, scientists still do not understand how life occurred, which
has important implications for the likelihood of finding life elsewhere in the
universe.
In a new paper published in the Proceeding
of the National Academy of Sciences today, David Kipping, an assistant
professor in Columbia's Department of Astronomy, shows how an analysis using a
statistical technique called Bayesian inference could shed light on how complex
extraterrestrial life might evolve in alien worlds.
"The rapid emergence of life and
the late evolution of humanity, in the context of the timeline of evolution,
are certainly suggestive," Kipping said. "But in this study it's
possible to actually quantify what the facts tell us."
To conduct his analysis, Kipping used
the chronology of the earliest evidence for life and the evolution of humanity.
He asked how often we would expect life and intelligence to re-emerge if
Earth's history were to repeat, re-running the clock over and over again.
He framed the problem in terms of four
possible answers: Life is common and often develops intelligence, life is rare
but often develops intelligence, life is common and rarely develops
intelligence and, finally, life is rare and rarely develops intelligence.
This method of Bayesian statistical
inference -- used to update the probability for a hypothesis as evidence or
information becomes available -- states prior beliefs about the system being
modeled, which are then combined with data to cast probabilities of outcomes.
"The technique is akin to betting
odds," Kipping said. "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."
From these four hypotheses, Kipping used
Bayesian mathematical formulas to weigh the models against one another.
"In Bayesian inference, prior probability distributions always need to be
selected," Kipping said. "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 is based on evidence that
life emerged within 300 million years of the formation of the Earth's oceans as
found in carbon-13-depleted zircon deposits, a very fast start in the context
of Earth's lifetime. Kipping emphasizes that the ratio is at least 9:1 or
higher, depending on the true value of how often intelligence develops.
Kipping's conclusion is that if planets
with similar conditions and evolutionary time lines to Earth are common, then
the analysis suggests that life should have little problem spontaneously
emerging on other planets. And what are the odds that these extraterrestrial
lives could be complex, differentiated and intelligent? Here, Kipping's inquiry
is less assured, finding just 3:2 odds in favor of intelligent life.
This result stems from humanity's
relatively late appearance in Earth's habitable window, suggesting that its
development was neither an easy nor ensured process. "If we played Earth's
history again, the emergence of intelligence is actually somewhat
unlikely," he said.
Kipping points out that the odds in the
study aren't overwhelming, being quite close to 50:50, and the findings should
be treated as no more than a gentle nudge toward a hypothesis.
"The analysis can't provide
certainties or guarantees, only statistical probabilities based on what
happened here on Earth," Kipping said. "Yet encouragingly, the case
for a universe teeming with life emerges as the favored bet. The search for
intelligent life in worlds beyond Earth should be by no means
discouraged."
Story Source:
Materials provided by Columbia University.
Original written by Carla Cantor. Note: Content may be edited for style
and length.
Journal Reference:
- David
Kipping. An objective Bayesian analysis of life’s early start and
our late arrival. PNAS, 2020 DOI: 10.1073/pnas.1921655117
https://www.sciencedaily.com/releases/2020/05/200518162639.htm
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