Where are they?

Ever since Man has had eyes with which to look, he has gazed at the stars and wondered about their nature. Where we once wondered how someone could have possibly managed to get up quite so high to light all those campfires, we now wonder how someone could possibly not be there. Even disregarding the numerous UFO sightings by people on Earth, there have been many curious phenomena that may very well point to the presence of intelligent life elsewhere in the universe. But this raises an important question: where is everyone? Surely, if there are indeed other civilizations out there, even if they were only as technologically advanced as us, we would have seen a more definitive sign by now... right? Many theories have been proposed to answer this question, with some being a bit more sci-fi than others, but I would like to propose an additional option. In order to properly introduce the Human Phase Hypothesis, we must first briefly explore some existing proposals.

The Usual Suspects

If you have ever approached this topic before, you have likely familiarized yourself with the Fermi Paradox. In a nutshell, it asks the following: "Since there are billions of sun-like stars in our galaxy, with a sizable percentage of them likely playing host to Earth-like planets, then wouldn't life have sprung up on a few of those planets at least a couple of times?" Effectively, it is an expanded version of the question posed in the introductory paragraph. Despite being a relatively simple question, it hardly has a simple answer.

In an attempt to set the basis for the Human Phase Hypothesis, we will briefly cover two possible explanations to the Fermi Paradox. (If you feel as though you are already familiar enough with them, feel free to skip to the next section.) The first explanation is the Rare Earth Hypothesis. The originators of the hypothesis, Peter Ward and Donald E. Brownlee, assert that there are essential criteria for life. Some of these criteria include a terrestrial planet with plate tectonics and oxygen, a large moon, a magnetic field, a Jupiter-like gas giant in order to divert potential astronomical threats, and a stable orbit in the habitable zone of the right kind of star. (The book goes into much further detail and is well worth the read.) At its core, the Rare Earth Hypothesis claims that life is scant throughout the universe simply due to the hyper-specific environment needed to foster its growth. While this is perhaps not universally true, it is certainly easier for us to consider life developing on a safe and resource-rich planet like our own than on a barren rock like Mercury. (This isn't to say the latter is incapable of supporting some kind of life, but our present concern is not microbes living on otherwise inhospitable worlds.)

Another attempt to find the answer to the Fermi Paradox is a rather intriguing idea known as the Firstborn Hypothesis. Playing somewhat off the Rare Earth Hypothesis, we are now introduced to the idea that mankind might simply be first. While this seems intuitively incorrect, picking at our natural sense of scale, it is not an impossibility. We are presently aware of life going back as far as 3.7 billion years on this planet, and it might have started up to 600 million years sooner. This means that only a fraction of the Earth's current lifespan had passed before the conditions to form life were met. The issue posed by the Firstborn Hypothesis is that we simply do not know if that is a relatively fast timeline or an abysmally slow one. Given the pace of human technology, imagine what we might be capable of in another 600 years, let alone 600 million. If we survive that long, there is literally no telling what our limits might be.

In that same vein, assuming an even loosely similar story of life and technological progress on an Earth-like planet, were they to have arrived an extra few million years earlier than us, we should equally expect to see some evidence of their wide-reaching powers. Since we do not currently see evidence of any such thing, there are only two possibilities. If lifeforms exist on other planets, they are either only several years behind our current technology, or they have not even begun to approach it. Given how phenomenally unlikely it would be for two intelligent life forms to be right on each other's heels technologically, despite having evolved billions of miles apart and having no ability to contact one another, we can only realistically assume that the second possibility is true: if there is intelligent life out there, they are probably not as technologically capable as us. (The Na'vi from Avatar would be a good sci-fi example.)

What then could be another solution to this problem? While the two preceding hypotheses are both intriguing, does either truly capture the reason why we have yet to discover evidence of alien life? I propose that not only do both the Rare Earth Hypothesis and the Firstborn Hypothesis work together in concert, but that there is a third piece to this puzzle. When all three are considered together, we will find that the likelihood of our being alone in this galaxy is far greater than might previously have been assumed. Allow me to, at length, walk you through my Human Phase Hypothesis.

Pulls and Endpoints

Gravity is not a thinking agent. It is a fundamental force of nature, acting on all matter, pulling objects towards each other. Despite this action, gravity does not have a conscious will or a predetermined goal. It doesn't desire to pull things together; it simply does. If left unchecked, gravity would draw all matter into a single point at the center of the universe. But this isn't because gravity 'wants' this outcome. This is simply the natural consequence of gravity's properties. In a similar vein, I propose that the process of evolution on Earth-like planets might have its own "pull," so to speak. Not in the sense of a conscious desire or predetermined outcome, but as a natural consequence of the conditions and events that shape life's development. In short, for every question there is an answer.

(This idea is not without precedent. On Earth, we see numerous examples of convergent evolution, where distinct species independently evolve similar traits in response to similar environmental pressures. From the streamlined bodies of dolphins and sharks, to the flight mechanisms of bats and birds, to the camouflage abilities of chameleons and octopuses, life on Earth has repeatedly converged on similar solutions to survival. It might even be said that there could exist optimal or even perfect solutions to specific problems.)

Assuming this is true, what then might be some of evolution's most "favored" traits? Taking a cursory glance toward the animal kingdom, we can clearly see that two of the most important (although not the only important) traits are strength and speed. For predators, having a strength advantage could make the difference between securing a meal and starving to death. In direct contrast, strength can help potential prey in escaping or fighting off predators. Rival males also tend to engage in contests of strength in order to win in competitions for mates, as seen in many animal species. Speed can also be a life or death factor. Where predators may need speed to catch their prey, that same prey may very well need great speed in order to escape. Even for non-predatory species, speed can be useful for quickly reaching safe hiding spots or food sources.

It would then make sense that, were evolution allowed to go on unchecked over a long enough timeline, the rise of animals that best utilize these traits (among others) would be inevitable. This is not a matter of speculation, we know this to be true. If you want proof of this, you can simply go to your local museum and take a look for yourself. For 165 million years, the planet was dominated by dinosaurs. With their size, strength, and in some cases, speed, dinosaurs can be said to be the pinnacle of physical prowess, perfectly adapted to their resource-rich environment. What animal alive today could go toe-to-toe (without weapons or tools) against a Tyrannosaurus? When it comes to pure physical ability, it's hard to argue that the long term success of the dinosaurs betrays anything other than incredible optimization. (Of course, over their millions of years on Earth, dinosaurs did see some changes—even the introduction of entirely new species. When compared to the animal kingdom of today, however, it can be said that these changes were significantly smaller.)

But what if the success of dinosaurs also represents a kind of evolutionary "endpoint"? By this, I don't mean an ultimate goal or universal destination, as evolution is not a process with a predetermined outcome. Rather, I'm suggesting that the conditions on Earth during the Mesozoic Era, combined with the inherent properties of life itself, might have unavoidably led to the emergence and dominance of large, physically robust creatures like dinosaurs. After enough time, such as 160 million years, it may very well have been the case that dinosaurs had either reached or were approaching a point where they simply could not have become any better suited to their environment.

Consider the environmental conditions of the Mesozoic Era. The climate was warm, there was plenty of land and vegetation, and there were no land-dwelling creatures capable of challenging the dinosaurs' dominance. At the same time, the cognitive demands on dinosaurs were relatively low. They didn't need to invent tools, build shelters, or develop complex social structures. Their survival didn't depend on their ability to understand and manipulate their environment in the way that human survival does. As a result, there was little evolutionary pressure for dinosaurs to become anything more than large, strong, and vicious animals. In this sense, dinosaurs might represent a sort of natural endpoint for evolution on Earth-like planets—a stage where life has become as physically formidable as it can be, given the constraints of its environment and biology. This is not to say that dinosaurs were literally perfect, or that evolution had entirely stagnated, rather that the dinosaurs represent evolution's "pull" toward one possible endpoint—where physicality rules over all.

A Second Chance

Clearly, the above scenario would only be sustainable in an environment that was relatively unchanging. Fortunately for life at the time, the Mesozoic Era was warm, moist, and relatively stable. It wouldn't be until the the Chicxulub impactor graced the Earth with its presence that things would experience a significant shakeup. Suddenly, it was no longer advantageous to be a big, powerful, and fast-moving carnivore that needs a constant source of food in order to survive. Instead, it was suddenly much better to be a small, clever, and furry little mouse. Where our ancestors had once been forced to live in the dirt, burrowing tunnels and persisting on scraps, they were suddenly given the opportunity to emerge from the shadows and live without the constant threat of being snatched up by some monstrous figure too large to even understand.

Evolution (which again, I acknowledge is not an intelligent agent with wants and desires) was now posed with a new problem: fitting the only animals left alive to their new environment as perfectly as possible. Indeed, over the next 65 million years, those small mousey creatures were formed into Homo sapiens, which you might find yourself intimately familiar with. In contrast to the monsters that ruled the Earth all that time ago, humans are quite different. While we are clearly not the most physically inclined creatures on this planet, we are the only ones to have set foot on the moon. In this sense, it may very well be the case that evolution has once again done its good work and molded life into a form that is best suited to survive in its environment. In this hypothesis, just as the dinosaurs represented an endpoint for physical prowess, humans represent an endpoint for mental prowess. (While some of you might have an immediate reaction to this claim, suspecting AI might some day prove to be the true endpoint of mental prowess, it's worth pointing out that any manmade creation is foundationally different from those forged by evolution—as we are thinking beings with the ability to design things from scratch in order to achieve a specific goal. Evolution does not have this convenience. Since this hypothesis is focused on natural life, we will ignore AI.)

I acknowledge it might be possible that human beings were originally destined only to be another stepping stone along the path to the actual endpoint. However, even if this were true, it would hardly matter when our intelligence was already great enough to facilitate a global takeover in only a few hundred thousand years. Whether or not there might have been another version of natural life somewhere down the line that could have done a better job, the fact of the matter is that mankind was already good enough to establish itself as the dominant force on this planet—making us a de facto endpoint.

A Tale of Two Phases

While this might be an overly simplistic representation of the path life has taken on Earth, it is my assertion that we can essentially break it up into two significant phases:

1. The Dinosaur Phase
2. The Human Phase

While the names are obviously anthropocentric, they serve an important narrative purpose. The first phase represents the endpoint where physical abilities are favored over all others, while the second phase represents the period where intelligence is dominant. (This is not a comprehensive list of all possible phases. There could also be said to be a Microbial Phase that predated the dinosaurs, for example. However, since the ultimate purpose of this hypothesis is focused on the development of intelligent life, it's not especially worth discussing every single phase at length. While there may have been phases that came and went prior to the dinosaurs, it is clear that none of them enjoyed the same level of dominance as what came later. In fact, it might be said that each successive phase has seen an increase in the level of control exerted over the world by the respective endpoint species, but this could be the product of coincidence.)

There are several issues with these two phases. The first and most obvious is how does a transition between them occur? While there could be any number of causes, such as a sufficiently large volcanic eruption or a widespread virus, the one cause that we know for sure can facilitate the transition is a large meteor impact. This raises the question: how common is it for an Earth-like planet to experience such a large impact? While it's tricky to know exactly how common this is, the current mainstream belief is that something on this scale might occur roughly once every 100 million years, and that's only if the gravitational pulls of Saturn and Jupiter don't have anything to say about it.

The Phase Equation

Let's try to create our own equation that might be used to determine the number of Earth-Like planets that have escaped the Dinosaur Phase and have entered the Human Phase. Before we can attempt a calculation, we will need to establish some basic things that need to be accounted for. This has its roots in the Drake Equation, but our purposes are slightly different than that of the Drake Equation.

1. Total Number of Planets in Our Galaxy $\left(N_{\text{p}}\right)$: A realistic estimation for the number of planets in the Milky Way galaxy is tricky simply due to the vastness of the galaxy and the limitations of our current technology. However, it is currently estimated that there are at least 100 billion stars in the Milky Way. It is also estimated that each star has, at the very least, one orbiting planet. Therefore, there are likely to be at least 100 billion planets in our galaxy. However, since our own solar system contains 8 planets (9 if you're old-school), and it is highly unlikely that this is a unique situation, we will assume that the average star has 3 orbiting planets. This brings the total number of planets up to 300 billion, although this might still be too conservative of a number. It's possible that the true number may very well be in the trillions.

2. Habitability Rate $\left(R_{\text{h}}\right)$: This is the fraction of planets that are capable of supporting life. As we are discussing Earth-like planets, we are only going to consider planets in the habitable zone of their stars, where conditions might be right for liquid water to exist. It is currently estimated that 22% of planets existing in habitable zones. Unfortunately, as explained by the Rare Earth Hypothesis, there is far more to consider than merely a planet's location relative to its host star. As a large amount of the values in the Rare Earth equation are nearly impossible to do anything other than guess at (Ward and Brownlee suggest that the math might work out to show that there is only 1 planet in our galaxy capable of supporting life—our own), we are going to make an incredibly forgiving estimate and assume that 25% of all planets in habitable zones further meet all the requirements posed by the Rare Earth Hypothesis.

3. Probability of Life $\left(P_{\text{l}}\right)$: As we only have one Earth-like planet to observe, it might seem as though this is an impossible number to calculate with any degree of accuracy. However, seeing as the planets we are taking into consideration reside in the habitable zone, where they are far more likely to have liquid water and plant life, we could assume (similar to the Drake Equation) that the probability for the development of life might essentially be 100% for any Earth-like planet. However, in an effort to be conservative, we will relegate this figure to just 50%. 

4. Probability of Complex Life $\left(P_{\text{c}}\right)$: While we previously discussed the natural "pull" of evolution, it is not necessarily the case that all life survives for any long amount of time. Another way to ask the question "What is the probability of complex life?" can be asked in what might be a much more useful way: "How often does life move past the single cell?" Considering it took nearly 3 billion years for life on Earth to become multicellular, we can assume that whatever process forces that change to occur must be vanishingly rare. There is no evolutionary change throughout the entire history of life on Earth that took longer than the transition to multicellular life. As such, we cannot be sure that it is a guaranteed occurrence. It might be true that 99% of all life throughout the universe is single cell. However, what we can assume is that once life becomes multicellular, it begins being "pulled" by evolution. Despite this likely being too high, in an attempt to be fair, we will assume that there is a 10% chance of life surviving long enough to become multicellular.

5. Probability of Phase Change $\left(P_{\text{x}}\right)$: Of all the values in this equation, $P_{\text{x}}$ is easily the most difficult to estimate. As stated earlier, an event comparable to the Chicxulub impact (which, I should point out, necessarily scales with the size of the planet in question. The Chicxulub impact would not have been sufficient to achieve similar results on an Earth-like planet the size of Jupiter, for example. For a planet the size of Mercury, however, a much smaller impact would suffice) might occur only once every 100 million years. It is also worth noting that volcanoes have also been a cause of mass extinction in the past, but none of them have ever contributed to such a significant change in the environment as the Chicxulub impact. As a result, no phase change can be said to have ever occurred at the hands of a volcano. Without further data to study, we can only assume that this holds true across most Earth-like planets. As such, the value that will be used here is 0.000001%. (Again, I understand that there are other possible causes for a phase shift, but anything else would likely have an even smaller chance of occurring. It may very well be the case that calculating for a meteor impact is the conservative option.)

6. Number of Planets in Human Phase $\left(N\right)$: This is the figure we are attempting to calculate. Once all the preceding numbers are accounted for, this should tell us how many planets we could expect to see in our galaxy that have escaped their respective Dinosaur Phase and have either started down the path to their Human Phase or have already arrived.

Putting it all together, the general form of the phase equation is:

$N=N_{\text{p}}\times R_{\text{h}}\times P_{\text{l}}\times P_{\text{c}}\times P_{\text{x}}$

Plugging our numbers into this equation, we get $N=8.25$ as our final result. This means that out of the 300 billion planets we assumed to exist in our galaxy, there are likely only a handful that have been fortunate enough to make it to the Human Phase. It is worth noting that this number is the product of using very generous math. The true number, using only the equation above, may be closer to only 2 or 3 planets. Even then, there are several things that were decidedly unaccounted for in the calculation.

One notable omission is, as touched upon earlier, the potential for additional phases. There is no guarantee that, once the Dinosaur Phase ends, the Human Phases necessarily follows. There could just as easily be any number of phases in-between—each being pulled by evolution toward biological perfection in their respective domains. There is also nothing in the current equation that deals with the likelihood that the extinction event that ends the Dinosaur Phase doesn't simultaneously wipe out all life on the planet. Had our own Dinosaur Phase been ended by an impact caused by a meteor the size of our moon, for example, it is unlikely there would have been a single living organism left. While these factors, among others, were not directly addressed in the equation, they still pose even further challenge to life entering the Human Phase.

Final Thoughts

While there are many questions about the possibility of alien life, none have quite grasped our collective curiosity like the Fermi Paradox. I propose that the answer to this paradox, in concordance with the Rare Earth Hypothesis, is that the reason we do not see any evidence of alien life is simply because it is so incredibly rare. The myriad challenges facing the mere existence of life, let alone the advancement to Human-level intelligence, are so numerous that they might only be overcome a couple of times throughout an entire galaxy. Furthermore, it might very well be the case that any other lifeforms lucky enough to have mirrored our evolutionary path, entering their own Human Phase, simply have not had enough time to progress technologically to the point where they could even be detected. The Firstborn Hypothesis makes far more intuitive sense when the number of potential competitors can be counted on one hand. If there were tens or hundreds of thousands of potential Human Phase lifeforms in the galaxy, it would be unlikely that none had technologically surpassed us.

Ultimately, there is no concrete way to determine if the Human Phase Hypothesis is correct. I am neither a seasoned astrophysicist nor a veteran biologist. I am by no means qualified to propose my own solution to the Fermi Paradox, let alone one that attempts to rope in two other widely known and respected hypotheses. However, I still feel as though there is some merit to all this. While researching this, I was unable to find any discussion, debates, or evidence that discouraged me or made me realize I was wrong. Even if the Human Phase Hypothesis is completely wrong-headed, I hope it made you think differently about the nature of life in our universe.