Listen and Subscribe
Podcast Transcript
In 1950, physicist Enrico Fermi asked a very simple yet important question: If the universe has intelligent civilizations, as so many people believe, where are they?
This became known as the Fermi Paradox, and it has been one of the foremost philosophical and scientific questions of our age.
One of the most straightforward answers to the Fermi Paradox is also one that many people simply don’t want to believe.
Learn more about the Rare Earth Hypothesis and how we might actually be alone on this episode of Everything Everywhere Daily.
The origin of this episode goes back to a book I first read about twenty-five years ago by astronomers Peter Ward and Donald E. Brownlee titled Rare Earth: Why Complex Life Is Uncommon in the Universe. They are the originators of the theory that I’ll be covering in this episode, and it is one that I tend to lean towards.
Before I get into the theory itself, I want to restate the problem. Enrico Fermi proposed what became known as the Fermi Paradox.
The Fermi Paradox arose from several values. The first of which is that the universe is very old. Current estimates are that the universe is about 13.8 billion years old.
The second is that the universe contains a lot of stars. Our galaxy alone, the Milky Way, has between 100 and 400 billion stars. Since Fermi suggested the paradox in 1950, our knowledge of the universe has exploded.
There are now an estimated 2 trillion galaxies in the universe, which means there might be around 200 sextillion stars.
Given these enormous numbers, the general consensus was that even if the odds of intelligent life arising were incredibly small, we should still see many intelligent civilizations in the universe.
Frank Drake created the Drake Equation in 1961 to quantify that number. I previously did an episode on the Drake Equation.
The variables in the Drake Equation include:
the rate of star formation in the galaxy, the fraction of stars that have planets, the average number of habitable planets per planetary system, the fraction of habitable planets where life actually develops, the fraction of life-bearing planets where intelligent life evolves, the fraction of intelligent civilizations that develop detectable technology, and the length of time such civilizations remain detectable.
Most of the variables in the Drake Equation are totally unknown. Since it was developed, we have learned a lot about the universe, especially about exoplanets. As of the recording of this episode, over 6,000 planets have been discovered orbiting other stars.
We have also made discoveries providing evidence of amino acids in deep space and of organic molecules in our own solar system. The significance is that the chemistry needed for life appears to be widespread throughout the universe.
Many people just assumed that starting life is the hard part. Once simple single-cell life takes hold, then you are off to the races. It is just a matter of time before more complex forms of life will arise, and eventually intelligence will emerge.
However, we only have one data point. There is only one planet we know of where life exists. While we have only started the search for life in our own solar system, so far we’ve come up empty-handed.
The Rare Earth Hypothesis is the idea that while simple life may be common in the universe, complex multicellular life, especially intelligent life, may be extremely rare.
It argues that Earth’s history was not just a matter of life appearing once conditions were right, but the result of a long chain of highly unlikely events that all had to occur in the correct sequence. If anything in that chain were different, then humanity might never have arisen.
The Rare Earth Hypothesis is the counter to the idea that if there are many stars and planets, then surely the odds are there must be intelligent life somewhere. It says that there were a series of highly improbable, perhaps events that had lottery-level odds; all had to work. So even if there was a twin planet to Earth somewhere, there is no guarantee the results would be the same.
So, consider everything that happened on Earth to get where we are: a global civilization that is listening to podcasts.
Let’s start with our sun. Complex life took roughly 4 billion years to evolve on Earth. A star must therefore be stable for a very long time.
Large stars are poor candidates because they burn through their fuel quickly, emit intense radiation, and often end their lives in supernovae.
A star like our Sun is near the ideal range as it is stable with a life of about 10 billion years, has relatively low radiation output, and enough heavy elements to form rocky planets.
Red dwarf stars are extremely common, making up perhaps 70% of stars in the Milky Way. However, Rare Earth advocates argue they may be problematic because their habitable zones are very close to the star, planets there may become tidally locked, always pointing the same direction, and solar flares can strip atmospheres.
So, we basically have a perfect star.
Next, the planet must orbit at the right distance from its star.
If it is too close, water evaporates, and oceans may be lost permanently. If it’s too far, water freezes permanently. Earth sits in the Sun’s habitable zone, sometimes called the Goldilocks Zone.
Next, a planet has to be the right size. A planet must be large enough to retain an atmosphere, maintain internal heat, and sustain geological activity. But not so large that it becomes a gas giant.
Another overlooked feature of the Earth is that it has plate tectonics, which many who support the Rare Earth Hypothesis think is necessary for complex life.
The carbon cycle depends heavily on tectonics. Volcanoes release carbon dioxide; rocks absorb CO? through weathering; plate subduction carries carbon back underground; and Volcanism releases it again.
This acts as a planetary thermostat over millions of years. Without it, planets may experience permanent ice ages or a runaway greenhouse effect. Just look at Venus and Mars.
The Moon may be one of Earth’s rare advantages. A large moon helps stabilize Earth’s axial tilt. Earth’s tilt varies only slightly, around 22.1 to 24.5 degrees. This produces relatively stable seasons.
The moon also serves as a giant shield that protects the Earth from meteor impacts. Jupiter also serves a similar function, causing many bodies in the outer solar system to collide into it, rather than us.
Earth’s magnetic field protects life from solar radiation. It is generated by the movement of liquid iron in Earth’s outer core. The field helps prevent atmospheric erosion by the solar wind and excessive radiation exposure. Mars appears to have lost its magnetic field early in its history, contributing to the loss of its atmosphere.
So, that is a long checklist of things that the Earth has going for it that make the world we live in possible. Take any one of them away, and the results you get would be totally different.
These are the type of things that the Drake Equation considers. A planet with all of these features would be exceedingly rare.
However, that isn’t even what constitutes the real low-probability aspect of this. Once you have a perfect planet, you need a series of highly improbable events to take place.
The very first life appeared on Earth approximately 3.5 to 3.8 billion years ago. This is one reason it is believed that simple life might be quite common in the universe.
These first life forms were simple, single-celled creatures without a nucleus, known as prokaryotes. For about 1.5 to 2 billion years, that was it. Life didn’t really get more complex than that. That isn’t to say these creatures didn’t evolve, but they remained simple.
Chlorophyll appeared about 3 billion years ago and probably arose from a single evolutionary event. This eventually led to the Great Oxygenation Event about 2.4 billion years ago, when oxygen began accumulating in Earth’s atmosphere.
Then, around 1.6 to 2.1 billion years ago, eukaryotes appeared. These were single-cell organisms with a nucleus. The transition to eukaryotes was a major evolutionary bottleneck because eukaryotic cells are fundamentally more complex. They contain a nucleus holding DNA, mitochondria, which provide much more efficient energy production, and internal membranes and specialized structures.
This event appears to have happened only once in Earth’s history, which is why it is considered one of the strongest candidates for a “Rare Earth” bottleneck. For billions of years, life existed successfully without eukaryotes, but once they appeared, they opened the door to large multicellular organisms.
Then, approximately 541 to 530 million years ago, the Cambrian Explosion occurred. It was a relatively rapid period in evolutionary terms when most major groups of complex animals first appeared in the fossil record. Before this event, life was dominated by simple organisms, including microbes, algae, and relatively simple multicellular forms.
One theory holds that the Cambrian Explosion might have been spurred by an event known as Snowball Earth, which took place between 720 and 635 million years ago during the Cryogenian Period. The entire planet was covered by ice, save for the deep ocean. CO2 was removed from the atmosphere by early plant life and was only returned through volcanism.
Then there were several extinction events, which, if they hadn’t happened, wouldn’t have resulted in the life that came afterward.
For tens of millions of years, dinosaurs ruled the Earth, and they probably would have kept on going if it wasn’t for the rare event of the Chicxulub impact in the Yucatan Peninsula about 66 million years ago. That impact allowed mammals to become dominant.
Go back a few hundred thousand years from today, and there was a genetic bottleneck in which our early human ancestors were down to only about a few thousand individuals. If they had been hit by a natural disaster, humanity could have ended right there.
Then consider that for thousands of years, humanity’s level of technical advancement was extremely static. Then, around the Renaissance, the Enlightenment, and the Industrial Revolution, we saw an explosion in science and technology. Without that, there would be no radio, no computers, and no spaceflight.
So, it is entirely possible that, given the prevalence of organic molecules, there are many places in the universe, and maybe even our own solar system, where simple life forms arose. However, it might be nothing but a massive sea of algae, and then it never gets more complicated than that.
Or maybe there is a world that gets as far as we did with something like dinosaurs, but they never have a meteor impact. Or maybe they even develop something as advanced as early humans, but they get wiped out in a freak event.
The point of all this is that a host of extremely unlikely events led to the development of humans. Even if lifeforms on some other planet were very different than us, it would probably require a similar series of very unlikely events for intelligent civilization to develop on a world with a bunch of very rare attributes.
There are a host of theories to explain the Fermi Paradox, and this is the one that I personally think makes the most sense. It doesn’t require theorizing about the motives of alien civilizations that may or may not exist. It is based on what we know about ourselves, our planet, and the universe.
It is simple and straightforward. We haven’t found any other intelligent life in our galaxy, because there just isn’t any. We are it. If there is another intelligent civilization, it is probably very far away, and there is a good chance we’ll never find it.
Every solution to the Fermi Paradox is ultimately guesswork, and the Rare Earth Hypothesis is no exception. At the end of the day, we can’t prove anything. The Rare Earth Hypothesis is also not very satisfying because many of us would really like to think that we could contact an alien civilization someday.
If it is true, that means we are very, very lucky and that the universe is very, very empty.