Planet Earth has existed for the last 4.5 billion years: about the last third of the universe’s history. Through a combination of geology and paleontology, we can trace the existence of life back more than four billion years, teaching us that life originated on Earth very early. If it did not coincide immediately with the formation of our planet – and it may have been – it certainly occurred within the first few hundred million years of Earth’s history.
But intelligent life is a whole other story. Life survived and flourished billions of years before human beings appeared: apparently the first intelligent and technologically advanced species on our planet. How likely is life in the universe? How likely is intelligent life? Although we have not yet discovered any beyond Earth, a new study says it has determined whether life is “rare” or “common” on Earth-like worlds, concluding that life may be common, but intelligence is rare. That’s why.
It is very difficult to know what the true probability of life on Earth is, or what the chances are that intelligent, technologically advanced life would occur. To know such a thing, what we would ideally like to do is to create the same environment that was present on Earth at the time of its formation, watch as each of these environments evolves over ~ 4.5 billion years, and see what comes out.
This is how science works perfectly on a fundamental level: experimental. You want to know something about the behavior of a system, so you create that system and observe its behavior over and over again. When you have a large enough sample size, you see what the results are and draw your conclusions based on that. This is the simplest approach to any scientific question.
The question of how probable life or intelligent life is in an Earth-like world is, in fact, an impossible approach. To begin with, we have only one planet (Earth) for which we know where life exists at all, and it is not like having Suns and Earth-like planets just waiting for us to observe them over 4.5 billion annual time limits. The idea that we can take a large sample of them and conduct a controlled experiment is simply not possible in our realistic universe.
And that’s too bad, because the truest way to think about probability is to do just that. You take a large set of prepared samples, all of which are prepared identically, you let them develop under a controlled set of conditions and you see what comes out. The number of successes – whether you determine success by life, intelligent life or some other criteria – divided by the total number of attempts, will give you the probability of success.
Mathematically, we call this frequency probability. In fact, you may only have one planet around one star, but if you could know the results of a very large number of systems that are made from identical planets around the same stars, you will know the probability of getting your particular planet a particular result. Just as you know the probability of getting two six-sided dice to add up to 7 is one-sixth, you could know the probability of life (or intelligent life) occurring on Earth.
But in practice we cannot use this approach for the planet Earth. With one system alone, we cannot perform the experiment many times and determine the frequency of desired (and unwanted) results. However, this does not mean that we are completely crossed. There is a different approach we can take: one based on the Bayesian probability.
In the Bayesian probability, reasoning goes backwards instead of forwards. What you are evaluating is not the general probability of results, but rather the probability that a particular hypothesis is valid compared to all possible hypotheses. This is the best tool you can use when you have only one system with one result.
This is harder to understand, so let’s give an example: planet Earth. We know that life originated on Earth relatively early. The earliest fossils go back 3.8 billion years and there are zircon deposits that are thought to be of biological origin, dating from 4.1 to 4.4 billion years, and our planet is only about 4.5 billion years. On the other hand, complex life did not appear until before the Cambrian explosion (only 600 million years ago), and intelligent, technologically advanced life arose only with the arrival of human beings.
This is our only system, along with its result.
So what are the possible hypotheses that could lead to this? Actually, there are only four.
- Life usually originates on planets like Earth and often becomes intelligent.
- Life usually occurs on planets like Earth, but it rarely becomes intelligent.
- Life rarely occurs on planets like Earth, but when it does, it often becomes intelligent.
- Life rarely occurs on planets like Earth, and when it does, it rarely becomes intelligent.
If you were to take the frequenter’s dream, you could start with billions of planets around billions of stars, which definitely looked a lot like Earth, and watch what unfolds. Unfortunately, we absolutely cannot do that; we don’t even know what exactly makes a world like Earth in any meaningful way.
Can the holy TRAPPIST-1 be similar to the Earth? How about one of the planets around TOI-700? Or one of the planets around Alpha Centauri A or B? There are too many things we don’t know about to talk about Earth-like planets with any certainty.
But with Bayesian probabilities, you can do more than just throw up your hands in a show. As a starting point, you can assume that each of the relevant questions has a 50:50 probability, where there is a 50% chance that life will occur frequently and a 50% probability that it will occur only infrequently. Similarly, if we assume that life arises, then there is a 50% chance that it will become intelligent often and a 50% chance that it will become intelligent only rarely.
What you do then is a model based on the probabilities you have guessed, how often you get a result that is consistent with the observed results. (In this case, extracting life and / or intelligent life from them in a way that equates to what happened on Earth.) In a new document just published on May 18, 2020, David Kipping did just that, providing the first sound probability analysis of these four scenarios.
Bayesian probabilities cannot tell you what the actual odds of these results are, but they can tell you which hypothesis is more likely – and by how much – which is useful information about “betting odds”. If there is no meaningful information, the initial probabilities you have accepted (50:50 just in case) will be unchanged. However, if the analysis favors one hypothesis over another, you will see your betting odds shift in that direction.
The fact that life originated early in Earth’s history is a huge factor in shifting the odds. If you continue with the evidence for microfossils, this gives you better 3-in-1 betting odds that life occurs often, not infrequently; if you use (contested but still convincing) evidence of zircon deposits, the odds increase to a better 9 to 1 that life is common, not rare. If we retold the history of the Earth many times, we would expect life to occur often, not difficult at all.
But the same procedure actually gives very little information about intelligent living. If you start with the same odds (50:50) for intelligent life – assuming that life originated, intelligent life is common or rare – Bayesian analysis, which Kipping performed, slightly favors the scenario of rare intelligence. But the effect is slight, transforming the 1 to 1 odds into something more like a 3 to 2 scenario in favor of rarity.
However, this does not mean that intelligent life is rare. This means that the information we have does not do a very good job of limiting whether the emergence of intelligent life is rare or frequent. This is in stark contrast to the question of the origin of life in general: the data are good enough to conclude that the common life scenario is more likely than the rare life scenario. If we started with a branch of early Earth, life would probably appear, but we cannot come to a good conclusion about the origin of intelligent life.
All this you can expect without special statistical analysis. Our planet from the earliest times was like any other: a lottery ticket in the space lotteries for life and intelligent life. Venus and Mars, among the other worlds in our solar system, also had their own tickets, and it is extremely possible that life appeared on the three worlds in the beginning; it must have happened on Earth. Only in our world, however, has life been maintained and prospered, and ultimately spawned an intelligent appearance and a technologically advanced civilization.
We cannot turn the clock back and see how things turn out, nor do we have data on a technosignature or a secure Earth-like exoplanet to help us understand these cosmic probabilities. However, smart analysis can reveal that when it comes time to place your bets, bet on the emergence of life as a common phenomenon as opposed to the rare one. For intelligence, however, the only information is a slight push in the rare direction. To learn more, we will need data that we do not yet have.