What Story Does Earth's Life Tell About ET?
This artist's conception from NASA shows a young, hypothetical planet around a cool star. A soupy mix of potentially life-forming chemicals can be seen pooling around the base of the jagged rocks.
JPL-Caltech/NASA hide caption
This artist's conception from NASA shows a young, hypothetical planet around a cool star. A soupy mix of potentially life-forming chemicals can be seen pooling around the base of the jagged rocks.
JPL-Caltech/NASAHow easy is it to create life? It happened here at least once. Does that give scientists studying the creation of life from non-life (abiogenesis) much to work with? Mark Twain once said of science: "One gets such wholesale returns of conjecture out of such a trifling investment of fact." Twain's comment is often aptly on display when scientists use one simple fact about Earth life — it started here fast — to make inferences about life everywhere else.
The Earth was formed 4.5 billion years ago. So how long afterwards did life first appear? Based on Carbon isotopic data, it appears that life emerged on Earth at least 3.85 billion years ago. It's likely that frequent impacts from large asteroids kept the planet sterile before this time, though it is possible that life did form more than once, only to be wiped out.
So the Earth forms and then, less than a billion years later, life forms. From this one fact it would appear (with a nod to Twain) that we could tell a great story. If the conditions are correct, meaning a planet can potententialy support, then after just a short time it will support life. The rapid formation of life on Earth seems to give the optimists out there (me included) a warm fuzzy feeling that Universe is teeming with life.
Attempts to explore this conclusion in detail were cautiously bolstered in 2002 in a paper titled "Does the Rapid Appearance of Life on Earth Suggest that Life is Common in the Universe?" In that study, Charlie Lineweaver and Tamara Davis of the University of New South Wales asked what can be inferred statistically from Earth's early creation of life. As they say at the beginning of the paper, the idea they want to explore is simple.
"Over a given time period, more probable events happen more often (and thus more rapidly) than less probable events. Thus, the probability of winning a lottery can be inferred from how quickly a lottery winner has won."
Here life and its creation are the lottery. After building their statistical model, Lineweaver and Davis concluded that the probability of biogenesis on habitable planets was > 13% at the 95% confidence level. "Greater than 13%" could mean the true probability of forming life on a habitable planet could be 25%, 50% or even 100%. Still, Lineweaver and Davis were cautious and claimed their result in no way meant that "life is common in the universe." If you were an optimist, however, then the two scientists had certainly given you reason to be ... optimistic.
Now along come two Princeton researchers who have used a different statistical approach and their claim far gloomier than earlier results. David Spiegel and Edwin Turner used what is called Bayesian statistics to see what can be inferred from Earth's early formation of life. The Bayesian view of what statistics means is quite different than what you learned in high school (or college or never at all). For a Bayesian, statistics is about beginning with some expectation of what might happen in the world (like winning the lottery) and then using real data (like buying tickets and losing) to change those expectations. Folks can get pretty excited about Bayes and his approach because it really offers a radical way of approaching inference in science.
When the Princeton duo looked at Earth's abiogenesis through Bayesian goggles the conclusions were not optimistic. Part of the issue was that the early appearance of life on Earth had to be "folded" into another piece of data — the long wait (around 3.5 billion years) until sentient beings like ourselves were around to notice that life had started early. Adding that fact and the simplest set of assumption they could start with led Spiegel and Turner to conclude that the early emergence of life on Earth is consistent with life being very rare.
Bummer.
Its important to note that Spiegel and Turner aren't claiming life is rare. They are simply saying you can't conclude that it isn't based on its rapid appearance here, as they write at the end of the paper.
"A Bayesian approach to estimating the probability of abiogenesis clarifies the relative influence of data and of our prior beliefs. Although a "best guess" of the probability of abiogenesis suggests that life should be common in the galaxy if early-Earth-like conditions are, still, the data are consistent (under plausible priors) with life being extremely rare. Thus, a Bayesian enthusiast of extraterrestrial life should be significantly encouraged by the rapid appearance of life on the early Earth but cannot be highly confident on that basis."
Sorry Mark Twain, but the truth is that sometimes science forces you tell much more constricted stories.
You can keep up with more of what Adam Frank is thinking on Facebook and Twitter. His new book is About Time: Cosmology and Culture at the Twilight of the Big Bang.