How likely is the evolution of advanced aliens, really?

Could you even fill a cantina?

February 12, 2022

Every February, to help celebrate Darwin Day, the Science section of AIPT cranks up the critical thinking for SKEPTICISM MONTH! Skepticism is an approach to evaluating claims that emphasizes evidence and applies the tools of science. All month we’ll be highlighting skepticism in pop culture, and skepticism *OF* pop culture.

For Darwin Day, retired spacecraft designer Rob Palmer wonders just how likely evolution is to produce advanced alien civilizations on other planets.

Extraterrestrial civilizations exist … in fiction, anyway. It seems that every M-type planet visited by a starship of the United Federation of Planets has one. In fact, the UFP’s mission statement is to seek out new life and new civilizations. Most every planet in that galaxy of long ago and far, far, away is home to at least one native, intelligent species. Many DC heroes and villains are extraterrestrials. The Marvel Cinematic Universe is so full of civilizations competing for limited resources, that Thanos believed it was his duty to rectify the overpopulation.

Marvel Comics

Fiction generally reflects the popular beliefs of the time in which its written. In ancient civilizations like Greece, Rome, and in Scandinavia, the superheroes and villains weren’t from other planets. They were immortal gods, fundamentally distinct from the natural world inhabited by humans. Earth was the only place with mortal beings — thoughts of extra-solar planets and extraterrestrials just wasn’t in the public consciousness.

But as time marched on, thanks to science, our understanding of the universe and our place in it was totally transformed. We learned that Earth is not at the center of the solar system, nor is the solar system at the center of our galaxy. Then we discovered that our galaxy is not the entire universe, but is just one of 200 billion or more galaxies, each with hundreds of billions of stars.

And most recently (beginning in 1992), we learned that not only do some stars have planets, but that most all stars have them (as of January 1, the verified count of known exoplanets stands at 4,905). If we consider just the Earth-sized ones, and extrapolate that number (which were discovered in just the small portion of our galaxy searched to date) to the volume of the entire Milky Way, the math suggests there are likely over 40 billion Earth-sized exoplanets in our galaxy alone, to say nothing of other types of planets and moons.

Is it any wonder that when people contemplate these facts, for the most part, their assumption is that we just can’t be alone? To quote Dr. Ellie Arroway from the film adaptation of Carl Sagan and Ann Druyan’s book Contact, “I’ll tell you one thing about the universe, though. The universe is a pretty big place. It’s bigger than anything anyone has ever dreamed of before. So, if it’s just us … seems like an awful waste of space.”

But does the scientific evidence actually support this point of view? Is it necessarily true that because of the huge number of planets in existence, civilizations are likely to have arisen elsewhere? The unstated assumption is that humanity emerging on Earth was itself likely, or even a sure thing. What if the odds of humanity emerging and surviving on Earth were actually low, and maybe even infinitesimally small? What if we are just very, very, very lucky to be here? If we owe our existence to a fortunate and extremely rare set of circumstances, then no matter how many exoplanets there are, all bets are off on hordes of ETs existing outside of fiction. How can we determine which it is?

In the latter part of the 20th century, astronomer Carl Sagan and others argued that the characteristics of the Earth – such as the composition of the planet itself, and the conditions in our solar system and galaxy – were likely typical for a great number of planets, no matter where they were. From the principle of mediocrity they concluded that the evolution of life on Earth, resulting in intelligent beings, was also likely typical. Therefore, this line of reasoning goes, the universe is probably full of complex life, presumably including other beings with technology.

However, more recently, some scientist analyzing the topic have argued that perhaps the opposite is true. They suggest that some unusual characteristics of the Earth and its environs were instrumental to the development of life, and this set of conditions may not be typical for other planets. In fact, they argue that Earth’s set of advantageous circumstances may be extremely rare.

These perhaps necessary conditions to produce life may include any or all of the following: a favorable location in its galaxy (not too close to the dangerous galactic core and other stars), a favorable and stable orbital distance from a certain type of star, a favorable arrangement of orbits of the other planets in its system, being a terrestrial planet of a certain size, having plate tectonics, having an atmosphere (one favorable to life).

On top of all this, assuming that a planet has sufficient favorable conditions to allow abiogenesis to occur, there is still a long way to go to produce intelligent, technology-capable lifeforms. Let’s take a look at the only example of success in that regard we know of: humans on Earth.

Numerous evolutionary steps led to our species, including all of these: self-replicating molecules formed and thrived. Simple (prokaryotic) single-cell life emerged and thrived, followed by complex (eukaryotic) single-cell life. Sexual reproduction developed. Multicellular life arose, creating ever more diverse and complex lifeforms. This long chain of events resulted in over five billion species developing on our planet, all of which had to constantly struggle for survival against the elements and their competition. Of all the species that there ever were, 99% are now extinct, including many other hominids such as our cousins, Australopithecus africanus, Homo habilis, and Neandertals.

The sequence of species that led to us was no sure thing. Life took a billion or so years to get started on our 4.5-billion-year-old planet. It took another 3 billion for something as complex as worms to evolve in the seas. And that wasn’t a certainty! The evolution of Earth’s life was influenced by gradual changes to the environment, as well as by a number of cataclysmic events. One line of evidence points to a human population bottleneck occurring about 70,000 years ago caused by a supervolcano, reducing our species down to a precariously low number, perhaps just 10,000 people. That’s how close it might have come to there being no humans on Earth.

In fact, the planet’s history is punctuated by catastrophic events which most species did not survive. Around 650 million years ago, the Earth likely froze over (like in Snowpiercer, but without the train). We have documented many global extinctions, including at 517, 502, and 488 million years ago. The Cretaceous–Paleogene extinction event of 66 million years ago not only killed all non-avian dinosaurs, but over 75% of all animal species on Earth. A slight change to any of these catastrophes, or the addition or subtraction of such events, would have resulted in a very different biosphere on our planet now. Collectively, the impediments to life and a civilization emerging on a planet are referred to as Great Filters.

Snowball Earth, a "Great Filter" of evolution

If time were re-run, and events on Earth were started over from the beginning, and anything significant was changed, would Homo sapiens still emerge? Would life have happened at all? This scenario was explored in the series finale of Star Trek: The Next Generation, “All Good Things.” The omnipotent being Q brought Picard back in time to the primordial Earth at the exact moment life had originally started. Picard bears witness to a new timeline in which a puddle of slime, about to spawn the first life on Earth, is interfered with, and fails to do so. Thus, humanity will never exist.

Since our lineage survived all the Great Filters of the past, it may seem that humans are home free. But it’s not certain that we’re out of the woods. There may be Great Filters in our future, and any other civilization elsewhere in the universe will have to overcome these, also. To mention just a few: we must survive a looming climate crisis, prepare for possible pandemics even worse than COVID-19, and proactively thwart any death-from-the-sky global extinction events.

So enjoy those tales of close encounters and galactic empires. They may contain the only aliens that human beings will ever encounter.

AIPT Science is co-presented by AIPT and the New York City Skeptics.