We love to think that life is not just an insulated incident in the universe. Earth is swarming with 9 million species, including those clever enough to understand that intelligent life might occur elsewhere.
A string of discoveries has fuelled the idea that alien life is plentiful. A growing catalogue of exoplanets, many of which orbit in the habitable zone of their home stars, suggests that there is plenty of real estate for life.
Subsurface oceans on icy moons in the outer solar system are evidence of life-friendly conditions. The discovery of phosphine in Venus “toxic atmosphere suggests that life can thrive even in the most hostile places.
Given all this, it is easy to imagine intelligent life developing on a planet or one of the 100 billion stars in our galaxy. So simple that, given the vastness of the visible universe, we assume that there must be other technological civilizations out there.
But we haven’t heard from them yet. In the absence of evidence from space, when astronomers turn their focus back to Earth as the sole example of intelligent life, we will reconsider this question. And if we cannot find anything that is consistent with what biologists have been whispering about for a while, then the expectation of hearing about alien civilizations will fizzle out.
For decades, SETI researchers have swept the sky with radio telescopes hoping to discover messages from technological civilizations. Despite its fruitlessness, the effort has never been lacking in optimism.
When estimating the number of intelligent civilizations capable of transmitting or receiving radio signals from outside the Milky Way, we use a formula in 1961 by astronomer Frank Drake. The formula multiplies seven variables: the initial rate of star formation in the galaxy, the proportion of stars with orbiting planets, and what proportion of these planets are habitable. Thanks to the Kepler space telescope, which has discovered thousands of exoplanets, we now know that some stars harbor planets of which many could evolve life.
This means we can use solid numbers for several of Drake’s equations. But the calculations also include other biological variables which, aside from the example of Earth, we have very little to guide us about.
The conventional approach to reducing probabilities involves the production of statistics. As things stand, in relation to the Drake equation, the numbers that it spits out at the end that show that we are alone in the galaxy or that our civilization is one in a million. It all depends on what you put in there.
But there is another way to approach these probabilities, and it could change the way we think about the chances of finding an alien civilization with which to communicate. An enormous selection of Earth-like planets could be observed over billions of years to see how life formed and how it became intelligent. The problem is that we only have a sample size of one Earth and two data points that affect it. We know that life first appeared on Earth when it formed 4.5 billion years ago, and that intelligence was a recent development in the first 300 to 900 million years.
We name the Bayesian statistics after the 18th century mathematician Thomas Bayes. Bayes developed a way to calculate the probability of future events based on what has already happened and then update the odds when new information is available. Bayesian statistics offer a clever way to calculate probabilities with limited data. Simply put, probabilities depend not only on the data you have but also on your previous assumptions.
These past convictions or priorities are crucial here. Here, they refer to our belief that life first appeared on Earth during its birth, and that intelligence followed shortly after. By selecting a value for a predecessor like this, we can draw conclusions about the relative probability that these processes occur on Earth when we turn back the clock on another similar planet.
In 2012 David S. Spiegel of the Edwin Turner Institute for Advanced Studies in Princeton, New Jersey was the first to apply a Bayesian approach to the early appearance of life on Earth. We based it on what we called a single prior: if one divides Earth’s history into uniform chunks that stretch over, say, 100 million years, one can say that life began in one of these chunks. But Spiegel called his findings inconclusive. The early appearance of life on Earth suggests that its formation was common, but it could not draw strong conclusions.
David Kipping, an astronomer at Columbia University in New York, did these calculations with a set of priorities that promise robust results. This boils down to betting on the likelihood that life will appear on a habitable planet and on the likelihood that it will develop into an intelligent planet: 0 means that life will never occur, 1 means that it will occur, and so on, not some arbitrary value. It’s strange that 50 percent of Earth-like planets with the same conditions as Earth will end up with life, but if they do, “50 percent won’t,” Kipping says. This gives rise to four general scenarios that Kipping considers most likely than others: life is common and often develops intelligence, life is rare but often develops intelligence, life is common and rarely develops intelligence and life is rare and rarely develops intelligence.
Intelligence emerged a few million years ago as a tool of hominids and the advance of modern science just 400 years ago. Drake himself experienced a pivotal moment in the development of radio technology just over a century ago. In fact, Kipping argues, the date of shifting things from a few million years to a time frame of several billion years makes little difference to the result.
After scraping the numbers together, it’s almost 50-50, but it’s not worth getting hung up on, ‘It’s a soft preference, but it’s there. It tells us there is something out there.’
Matthew Cobb, a biologist at the University of Manchester in the United Kingdom, argues that people are too keen to assume that life has some tendency to increase complexity, not to mention intelligence. This is a minority position among astronomers and biologists, who have been pointing out for some time that we overestimate the probability that life will take root on habitable planets, and the chances that life will be there long enough for it to emerge and produce intelligence. Given the limited amount of data and sophisticated mathematics, our expectations of finding intelligence on Earth have moved toward a pessimistic view, Kipping says. His bet is that life is mundane, but intelligent life is rare.
In a chapter he contributed to the book Alien Worlds: Leading Scientists in the Search for Extraterrestrial Life in 2017, Cobb points out a myriad of barriers that make it difficult for simple life forms to access intelligence - ones that have never been clear in Earth history. The leap from simple organisms to multicellular eukaryotic organisms, which consist, for example, of complex membrane-binding cells with a central nucleus, is a complete fluke. It requires two simple cells to collide in a certain way, so that one absorbs the other, an event of “insane improbability,” Cobb says.
This suggests that we should exercise caution, if not pessimism, in assessing the opportunities for the development of intelligence and technological civilization. He notes it is easy to imagine an alternative Earth where, for example, the scientific revolution never took place. Evolution, when its preferred direction converges, will have certain useful qualities - such as intelligence. Unlikely, in his opinion, is the development of a culture without intelligence.
Evolution may not always do what it seems to do. It could find several ways to reach an animal’s eyes or wings. But it doesn’t increase the chances at all.
Human intelligence is a selective advantage for us, he says. Animals and life increase intelligence. Imagine if intelligence was at least as beneficial as seeing or flying… then we would expect intelligence to emerge when life takes hold.
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