By any rational approximation, Earth is not the only planet in our galaxy capable of supporting life.

For starters, the Milky Way is home to roughly 250 billion stars, many of which have their own cornucopia of planets, satellites, asteroids, etc.

Then, consider those requirements for life as we know it:

  • A medium sized star, not so different from the sun, burning in its main sequence;
  • A medium sized planet, large enough to hold an atmosphere and not so large as to be a gas giant, orbiting at a distance from its star and with a period of rotation that warms the surface without overheating or exposing it to excessive radiation;
  • The presence of water on that planet, as well as other elements, none of which ought to be particularly rare;
  • And other factors.

All of which, even after accounting for liberal exclusions, still leave 500 million or more such planets in our galaxy capable of supporting life.

All that is to say that these planets could support life, not that they do. However…

Once you have a planet that can support life, you also have a planet that quite likely will. Allowing billions of years, it’s probable that some simple amino acids will develop and that they will tend to progress toward life; admittedly life of the very rudimentary kind.

Such life is also likely to start in oceans, as we did, which provide moderate, protective conditions from weather and gravity, as well as an abundance of the necessary chemical components.

Now, when we speak of life out in the universe, I doubt many people are hoping for protozoa. Rather, they are speaking of intelligent life and specifically sentient, technologically advanced civilizations; which is itself unlikely to be common. Regardless, intelligence is not a spontaneous process, and so any life should start somewhat as it did on Earth, from single cells onward.

Objectively, we should consider another possibility. Maybe you are certain there is something special about Earth or of humankind and that we are the only meaningful life in the universe. Admittedly, although there is strong evidence to suggest that extraterrestrial life ought to exist, we haven’t encountered it and so can’t yet speak definitively. Still, I think it’s more fun to consider life as abundant.

So, given this beginning of life on a foreign planet and that we have no reason to believe evolution as a phenomenon unique to Earth, perhaps more complex life would eventually evolve; developing a bundle of nerves toward the head, specialized cells, sensory organs and in some cases vertebrate.

Assuming our life harbouring planet is not entirely ocean, then eventually some of the vertebrates will find themselves on shore, perhaps because they are seeking new sources of food. Given enough time, some of these creatures may learn to survive on shore and even thrive.

It’s not unreasonable to think that the characteristics of such vertebrates would be similar to those on Earth, albeit allowing for great variation in the specifics. These creatures, for example, will likely be amphibious.

Now, regarding a food chain, there is a strong correlation between strength and position. The lion is higher than the coyote is higher than the gazelle is higher than the rutabaga. One exception to this ladder of strength is humans, who as individuals are pathetically equipped to wrestle a large cat, but do have the advantage of intelligence, which is worth something.

Absent intelligence, brawn will prevail; which means that to survive, either you need to be large enough to be a successful predator, large enough to defend yourself from one or small and/or fast enough to evade capture. The middle ground doesn’t win.

Given this advantage of size, evolution is likely to favour some of our small, coastal amphibians to become larger amphibians, and then larger reptiles, and then something not dissimilar from an Earthly dinosaur.

What would these astrosaurs be like? We have no reason to believe extraterrestrial dinosaurs would develop human level intelligence, regardless of the progression of time, because the brain-mass ratio is not conducive to any meaningful IQ. Dinosaurs had relatively small brains in proportion to their bodies, which means their potential for intelligence may be limited to that of a whale or an elephant; smart, but unlikely to pass the LSAT. One exception could be dinosaurs of roughly human size and with roughly human proportions of brain and body.

Further, would it only be dinosaurs that develop, or something mammalian as well? Extraterrestrial mammals are probable, though unlikely to thrive while dinosaurs rule. Perhaps great grandpa tree shrew could do better if his dinosaur cousins went extinct, yet would that happen? There was certainly an extinction event on Earth, though we are uncertain of the specific cause.

Mass catastrophes, those that would extinguish all life on a planet, are rare. Catastrophes of the sort that have stricken Earth, only extinguishing some life, are more common and also more likely survived. Ice ages are escapable by migration, and even a large collision or enormous volcanic eruption are not decidedly Armageddon. Again, there is no reason to believe that those events that occurred on Earth would necessarily be duplicated elsewhere.

And yet, something could happen, and given my arguments on the number of planets and billions of years of opportunity then surely something will. In some cases there will be a complete extinguishing of life, while in others perhaps the mammal-like species and therefore something human-like could eventually rise from the ashes.

Regardless, there is as yet no evidence that intelligent life will survive any longer than the dinosaurs did. In fact, I would argue that we have the odds stacked against us. Without specific indicators otherwise, the chance of external forces capable of producing extinction is similar. Granted, a species sufficiently advanced may evade such forces by becoming multi-planetary, deflecting a forthcoming asteroid, or similar, and so slightly improve their odds.

However, life of the sort intelligent enough to evade extinction also has the capacity for complete self destruction: either via nuclear war, environmental impact or otherwise. Who is to say that our extinction, followed by millions of years of random process, would not create something dinosaur like again?

But barring an odds-defying number of catastrophes, the dinosaurs having evolved first would continue on. Which is all a long way of saying I believe dinosaurs rule the galaxy.

And that’s just our galaxy. What about the billions of others and their countless planets? Jurassic Park.

This essay is dedicated to Isaac Asimov, perhaps the best science writer of all time.

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