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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's 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 actually might be alone on this episode of Everything Everywhere Daily. This episode is sponsored by Mint Mobile. When we were kids, summer meant freedom. No homework, no teachers, no school. And as adults, summer can still mean freedom, thanks to Mint Mobile's summer sale. Right now, all of Mint's plans are $15 a month, even unlimited. So free yourself from Big Wireless and their gimmicks and gotchas and switch to Mint Mobile for high speed data and coverage on T Mobile's 5G network. That's why I recommend Mint Mobile. 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That's what Square does for businesses. It brings payments, point of sale, inventory, staffing and online sales together in one system. Its AI powered tools can handle routine tasks, while real insights help owners make smarter decisions. Square can work for someone opening their first location or a business expanding into several. With Free plus and Premium plans, transparent pricing and no long term contracts, Square helps you run your business more smoothly, bringing payments, operations and insights together in one place so you're ready for whatever next. Right now, listeners can get up to $200 off square hardware when you sign up at square.com godaily that's sq U-A-R-E.com godaily get started with square and build a setup that works the way you do. The origin of this episode goes back to a book I first read about 25 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 the one I tend to lean towards. But 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 billion years old. And the second is that the universe has a lot of stars. Our galaxy alone, the Milky Way, has somewhere between 100 and 400 billion stars. And since Fermi suggested the paradox in 1950, our knowledge of the universe has only exploded. There are now an estimated 2 trillion galaxies in the universe, which means that there might be somewhere around 200,000 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. Frank Drake created the Drake equation in 1961 to quantify that number, and I previously did an episode on the Drake Equation. The commonly used variables in the Drake Equation are as the rate of star formation in a 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 finally, 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 exoplanets have been discovered orbiting other stars. We've 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 assume that just starting life is the hard part. Once single cell life takes hold somewhere, then you're off to the races. It's 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, and that's our planet earth. While we've only just 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 may 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 that there must be intelligent life somewhere. It counters and says that there were a series of highly improbable, perhaps, events that had lottery level odds that all had to work. So even if there was a twin planet to earth somewhere else, there's no guarantee that the results would be the same. So consider everything that happened on earth to get to where we are today, A global civilization that can listen to podcasts. So 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 a supernova. A star like our sun Is near the ideal range as it's 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 the stars in the milky way. However, rare earth advocates argue they may be problematic because their habitable zones Would be too close to the star. Planets there may become tidally locked, Always pointing in the same direction, and solar flares could strip atmospheres. So we basically have a perfect star. Next, the planet's orbit has to be the right distance from the star. If it's 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 just the right size. A planet has to 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 plate Tectonics. Volcanoes release carbon dioxide. Rocks absorb CO2 through weathering. Plate subduction carries carbon back underground, and volcanism releases it again. This acts as a planetary thermostat over the course of millions of years. Without it, planets may experience permanent ice ages or, or runaway greenhouse effects. Just look at Mars and Venus. The Moon may be one of the Earth's rare advantages. A large moon helps stabilize the Earth's axial tilt. The Earth's tilt varies only slightly, around 22.1 to 24.5 degrees, and this produces relatively stable seasons. The Moon also serves as a gigantic 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. The Earth's magnetic field protects life from solar radiation. It's generated by the movement of liquid iron in the 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 its loss of atmosphere. So that's a long checklist of things that the Earth has going for it that makes the world we live in possible. Take any one of them away and the results you would get would be totally different. And 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 all of this. Once you have the perfect planet, then 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, right after the Earth was formed. This is one of the reasons it's believed that simple life might be quite common in the universe. These first lifeforms 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 get more complex than that. That isn't to say these creatures didn't have some form of evolution, but they remained very 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, where oxygen began accumulating in the Earth's atmosphere. Then, around 1.6 to 2.1 billion years ago, eukaryotes appeared. These were single celled 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 provides much more efficient energy production, and internal membranes and specialized structures. This event appears to have only happened once in Earth's history, which is why it's 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 may have been spurred by an event known as Snowball Earth, which took place between 720 and 635 million years ago during the Cryogean period. It says the entire planet was covered by ice, save for the deep oceans. CO2 had been removed from the atmosphere by early plant life and then was only returned through volcanism. There were then several extinction events, which, if they hadn't happened, wouldn't have resulted in the life that came afterwards. For tens of millions of years, dinosaurs ruled the Earth. And they probably would have kept on going if it wasn't for the ultra rare event of the Chicxulub impact in the Yucatan Peninsula about 6, 66 million years ago. It was that impact that 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, sometime around the Renaissance, the Enlightenment, and the Industrial Revolution, we saw an explosion in science and technology. And without that, there would have been no radio, no computers, and no spaceflight. So it's entirely possible that, given the prevalence of organic molecules, there are many places in the universe, and maybe even in our own solar system, where simple life forms arose. However, it might be a planet that consists of nothing but a massive sea of algae. And it never gets more complicated than that. Or maybe there's 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 of this is that a host of extremely unlikely events led to the development of humanity. Even if lifeforms on some other planet were very different than usual, it would probably still require a series of similar 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 an alien civilization that may or may not exist. It's based on what we know about ourselves, our planet, and the universe. It's simple. And 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, very far away, and there's a good chance that we will 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 to many people, because many of us would really like to think that we could contact an alien civilization someday. And if the Rare Earth Hypothesis is true, it means that we are very, very lucky and that the universe is very, very. The executive producer of Everything Everywhere Daily is Charles Daniel. The associate producers are Austin Otkin and Cameron Kiefer. My big thanks go to everyone who supports the show over on Patreon. Your support helps make this podcast possible, and I also want to remind everyone about the community groups on Facebook and Discord. That's where everything happens that's outside the podcast, and links to those are available in the show notes. As always, if you leave a review on any major podcast app or in the above community groups, you too can have it read on the show.