On social media it seems believe in aliens is often put at the same level as believe in ghosts or believe in bigfoot. A large group of people seem to believe sentient lifeforms don't exist outside of our planet. In this post I want to discuss why the very idea that we are alone in the universe is really a naive ides, and that there should be no doubt "if" alien sentient lifeforms exist.
Life on earth
The earth is roughly 4,500 million years old, only about 20 to a 100 million years younger than our star, the sun. What is believed now is that the earth and moon formed by a collision with another proto planet. Remember this, this will turn out to be important when we get to the rare earth hypothesis.
The first evidence for life on earth is about 3,700 million years old. That means, if life started on earth, and if life only started on earth once and survived all this time, then there have been life forms on this planet for over 80% of the earths existence.
Life may have existed earlier, but there is no evidence to be found of it, so we assume the planet was lifeless for upto the first 800 million years of its existence.
If we assume water was needed for the formation of life, it is important to realize that at its formation the earth wasn't the watery plannet that it is today. The earth was slowly seeded with water by icy meteroids, and even then likely most of it didn't end up liquid at first. The oceans formed roughly 3,800 years ago, pretty close. That means while the planet has know life for over 80% of its existence, the earth's oceans have known life for at least 97% of their existance. That is, there is evidence for life at roughly 100 million years after the oceans first formed. It might very well have formed even sooner. Or, and this is a subject we will touch on later in more depth, it might not have actualy have formed here, but have taken root soon after circumstances allowed it to.
Remember this. As soon as the earth was seeded with oceans, life was, on the scale of the age of the solar system, almost instantanious.
Complex 'thinking' life on earth
The first known life on earth were microbes. Single celular organisms. We know there must have been life-like replicators before cells, but things get interesting with cells. When we talk about aliens, we obviously aren't talking about microbes. We are talking about sentient, thinking aliens. So what is the first requirement for life to be able to develope sentience? A central nervous system.
So what do we know about the appearance of animals with a central nervous system on our planet? What we know now, the first animals with something of an actual brain are planarian. A type of worm like creature that evolved around 550 million years ago. So if we don't want to speculate too much, we could say that complex thinking life has existed for abour 14% of the existance of our oceans.
The great uncomformity and snowball earth
Now it's time for some speculative stuff. Not because we want to speculate, but because there simply is a lot of time missing from the geological records of the earth.
The great uncomformity refers to a period of time spanning a whopping 1,200 million years from what there are simply no rocks. 1,200 million years of earhts history errased. A period that ended just before the so called Cambrian explosion, the time when we see our first complex thinking life arrise. Or did we?
Before the Cambrian explosion, earth was completely covered with ice, ice that as it seems erased 1,200 million years of earths history with its glaciers.
We don't actually know how life survived the millions of years of snowball earth, and you could speculate it didn't and it just formed once more, but let assume it did. It is reasonable to assume that much of complex life might have parished in this ultra ice-age. It's 1,200 million years erased from our evidence records where animals with a central nervous system might or might not have existed.
So to sum things up, we know that in the history of earths oceans, animals with a central nervous system heve existed for at least 14% and at most 46% of the age of our oceans.
New species and extinction
We know that how long species survive depends greatly on their complexity. The average invertibrate goes extinct in roughly 10 million years. The average mamal in one million years.
Given this information it is rather silly to look at evolution of inteligent species as somehow linear. Animals go extinct, often leaving a niche that others can than fill. New species have a chance to develop inteligence, and most that do will still go extinct because like other evolutionary properties, what works changes with time and one era's advantage might be another era's handicap.
The important thing to take from this is inteligence is no more a guarantee to not go extinct in a million years or so than any other evolutionary road taken. When we look at visible evolutionary trades, we see that evolution often repeats itself in completely different genetic lines.
Remember this when we get to aliens.
Do we have non-religious reasons to believe that humans are somehow more inteligent than all the many extinct animals that have ever walked this planet? We really don't.
Inteligence plus opposable tumbs
It may come as a shock to some of you, but humans aren't actually the most inteligent species on the planet. While actually meeasuring inteligence in animals is hard and subjective, animals like the beluga whale are incredably inteligent, possibly more inteligent than humans even.
One big question to ask when thinking about what we would consider sentient aliens is: is sentience and inteligence enough?
If there exists another planet with inteligent life and that intelegent life is more like a beluga whale, an octopus or a wolfe than like us, do we consider it as a sentient alien species?
I think though that here is another spec other than sentience and inteligence that people often assume when talking about aliens: the opposable thumb. Without an opposable thumb, inteligence isn't likely going to lead to complex tool usage, and without complex tools there can be no potential for technological societies like the one humans have right now.
And is potential enough? Would the ewuivalent of stone age humans count as what people consider sentient alien lifeforms, or do we need to wait and see if a species builds cities, agriculture, combustion engines, computers, space shipt before we count them in as part of the inteligent lifeform family?
We can thus divide alien life forms up in a few classes:
- Life
- Life with a cental nervous system
- Sentient life
- Sentient inteligent life
- Sentient inteligent tool using life
- Sentient inteligent civilication building life
- Technological society
We were looking at earth so far. And so far we came up to numbert two, establishing that 14% up to 46% of the time since oceans formed on earth that life with a central nervous system existed.
let's extend on this with some conservative guestimates for the other ones.
Sentient marine vertibrates developed about 25 million after planarian, meaning that if we consider the post snowbal earth era to be representative, the earth has had sentient life 95% of the time that it had life with a central nervous system.
From sentient to inteligent
So far, while having a bit of a spread, the numbers have been reasonable close to eachother. We have established that our planet has housed sentient life in one form or the other for more than 10% of the age of our star, possibly up to one third.
But how much of the time has it had inteligent life of close to human potential or more? We know that right now, there are multiple species that rival humans in terms of base inteligence (as far as it is measurable) or number of cortex neurons. As we know about extinction roughly happens in a million years, there are multiple species right now with comparable neuron counts and there is nothing evolutionary special about right now compared to say 100 million years ago, it is reasoanable to assume that the amount of time that there has been inteligent sentient life on earth is the same or close to the same as the amount of time that there has been sentient life. But lets be pesimistic and say its only been half that. Now our bottom estimate for inteligent life is at 5% of the age of our solar system.
Tool usage.
Humans, just like multiple other primates, some bird species, octopusses, some groups of dolphins, and elephants all use tools. All of these are reasoanbly inteligent, though most not at the level of humans or close. Roughly a third of all the species with neuron counts close to or exceeding that of humans use tools. So again we are in a time where we are not the only inteligent toolusing spacies on our own planet. 100% would not be a far fethed hypothesis, but lets tone it down a little and say that not 5% but 3% of the age of our solar system tool using inteligent beings have inhabited our planet.
civilications
Now things get really fuzzy. The step from tool using to civilication building is a huge one and one where it is quite impossible to make a usefull lower bound estimate.
We have established that our planet has likely had inteligent tool using life for 3% up to 40% of the age of the solar system, but how much of that life had the potential to build societies. Could elephants build cities? Could beluga whales develop agriculture? Most likely not. We are a sub species of primates are currently the only tool using species with a body, or rather thumbs capable of extending simple tool usage to more complex tools needed for building cities and developing agriculture, at least as far as we know. It is quite impossible to make a usefull estimate based on the current existence of one.
Is the absence of geological evidence for other species developing cities or agriculture proof that humans are the first tool users to get this far? Far from it. In fact, ice ages are so much of an anti-forenhsics scrubbing tool that we don't even know for sure if there haven't been human civilications predating the holocene. If there was an advanced civilication of hyper inteligent velociraptors that lasted for thousands of years, there is no way for us to ever find out.
But note what just happened! We mixed two concepts in our uncertainty. The uncertainty about species being bioligically capable of eveloping a civilication, and a biologically capable species actually doing so.
We also recognize the consept of pseudo extinction when it comes to civilications. Things like natural disasters and ice ages have the potential to make a civilication go extinct without making the species do the same. We have evidence from antiquity, and more recently looking at the eurpean middle ages that civilications can and do colapse and then reboot. This puts time pressure on higher level civilications developing into technological societies.
But let's take a step back. We wanted to look at sentient aliens and the probablility of their existance. There is going to be a lot of speculation later on already, so lets for now just accept that inteligent tool using creatures that don't live on our planet count as sentient inteligent aliens and work from there.
The rare earth hypothesis
So far our planet has been looking pretty friendly for life. Almost as soon as the oceans formed live arose, a sizeable portion of the time life existed it has included complex life with a central nervour system, an a big chunk of that there have likely been inteligent tool using creatures on the planet.
But when it comes to aliens, the big question is how hospicable are other planets and moons in other star systems?
The rare-earth hypothesis tells us that our friendly little earth might be very exceptional. We have:
- A relatively big iron core
- A moon that is very big for the size of our planet.
- Plate tectonics.
- A place in the galaxy with low supernova activity.
And there are a few more such exceptional circumstances that some speculate are esential for giving life and evolution enough stability. The rare earth hypothesis states that the combination of all these factors may be bery rare, and thus the development and prolonged existance of complex life on other planets may be rare too.
So how rare is rare? Jupyter has at least one moon that like earth also has a large iron core that could protect an earth with a magnetic field. Pluto, a reclasified planet has a moon that in proportion to its own mass is even more massive than our large moon. One of the moons of Jupyter has its own plate tectonics. So looking at our solar system and the number of planets and moons, it seems fair to weigh each of these factors pesimisticly at about 1%. That is, an earth like planet or moon in a stars goldylock zone has a one in 1,000,000 of being truly earth like. And given the lower bound of 5% for the presence of inteligent tool using life, one in 20,000,000 such moons or planets would have alien life.
But we are not there yet. Super novas can mess up the plans for life prematurely in the wrong gaslaxy neignourhood. There are more galactic factors to consider. In short, the galaxy has its own goldylock zone, and roughly 20% of all the stars in the galaxy are in that zone.
So taking this into account, we are down to a lower limit of one in 100,000,000 planets and moons of the earth like planets and moons that are in the stars goldylock zone that are going to have inteligent tool using aliens.
Earth like planets per star
Current estimates for the number of planets per star that could support life is 0.4. So now we are ready to look at our own galaxy. The low estimate for the number of stars in our glaxy is about one hundred billion. That means there should be roughy 40 billion planets that could potentially support life. If we devide that by one hundred million, the number of planets (or moons) with inteligent tool using life in our galaxy should be at least 400.
This is not a big number considering the hige number of stars in our galaxy, and it means that it could be very likely that we are the only technological species in our galaxy right now, but the number is going to go up, way up. And while we could be the only technological species in our galaxy, our planet is not the only one with inteligent toool using creatures.
From galaxy to universe
The observable universe is huge. And what is more, it is only a small fraction of the entire universe. The lowest estimate is that its less than half a percentage of the real univers. The observable universe alone consists of about 200 billion galaxies, meaning the whole universe is tens of trillions of galaxies in size, each with 400 or more alien planets.
Imagine if we are one in a billion planets with inteligent tool using life that ends up actually making it to a technological civilication, that would mean the probability per galaxy to have a tecnological civilication would be one in 2.5 million.
The probability of a single galaxy not having a technological civilication would be 0.9999996.
Now lets look haw that scales to larger numbers
| galaxies | probability of no technological civilication |
|---|---|
| 1 | 0.9999996 |
| 1,000 | 0.9996 |
| 1,000,000 | 0.67 |
| 1,000,000,000 | 1.915016e-174 |
After this, even at the scale of our observable universe standard calculators give up and just say 0
Aliens exist. We are not alone in our galaxy, though we might be alone as technological civilication, there are hundreds of planets at least with inteligent tool using life at this very moment. And even if we are alone as technological civilication in our galaxy, we absolutely ate not alone in the universe. Not even just the observable universe.