Marc Defant gave a TEDx talk on the improbable events that had to happen in our planet’s history for us to eventually evolve, along with the implications for other intelligent life in the galaxy.
I find a lot to agree with in Defant’s remarks, although there are a couple points I’d quibble with. The first, and I’m sure a lot of SETI (Search for Extraterrestrial Intelligence) enthusiasts will quickly point this out, is that we shouldn’t necessarily use the current lack of results from SETI as a data point. It’s a big galaxy, and within the conceptual space where SETI could ever pay off, we shouldn’t necessarily expect it to have done so yet.
My other quibble is that Defant seems to present the formation of our solar system as a low probability event, or maybe he means a solar system with our current metallicity. I can’t really see the case for either being unlikely. There are hundreds of billions of stars in our galaxy, most with some sort of attending solar system. So I’m not sure where he’s coming from on that one.
My own starting point for this isn’t SETI, but the fact that we have zero evidence for Earth having ever been colonized. If the higher estimated numbers of civilizations in the galaxy are correct, the older ones should be billions of years older than we are. They’ve had plenty of time to have colonized the entire galaxy many times over, even if 1% of lightspeed is the best propagation rate.
The usual response is that maybe they’re not interested in colonizing the galaxy, not even with their robotic progeny. That might hold if there is one other civilization, but if there are thousands, hundreds, even a few dozen? Across billions of years? The idea that every other civilization wouldn’t be interested in sending its probes out throughout the galaxy seems remote, at least to me.
But to Defant’s broader point about the probability of intelligent life evolving, there are many events in our own evolutionary history that, if we were to rewind things, might never happen again.
Life seems to have gotten an early start on Earth. Earth is roughly 4.54 billion years old, and the earliest fossils date to 3.7 billion years ago. With the caveat that we’re unavoidably drawing conclusions from a sample of one planet’s history, the early start of life here seems promising for its likelihood under the right conditions.
But there are many other developments that seem far less certain.
One crucial step was the evolution of photosynthesis, at least 2.5 billion years ago. The development of photosynthesis gave life a much more reliable energy source than what was available before, converting sunlight, water, and carbon dioxide into sugars.
And its waste product, oxygen, started the process of oxygenation, increasing the levels of oxygen in Earth’s atmosphere, which would be very important as time went on. The early atmosphere didn’t have much oxygen. Indeed, the rise of oxygen levels may have originally been a serious problem for the life that existed at the time. But life adapted and eventually used oxygen as a catalyst for quicker access to free energy.
The good news with photosynthesis is that there are multiple chemical pathways for it, and it’s possible it evolved multiple times, making it an example of convergent evolution. That means photosynthesis might be a reasonably probable development. Still, oxygen producing photosynthesis doesn’t seem to have arisen until the Earth was more than halfway through its current history, which doesn’t make it seem very inevitable.
The rise of eukaryotes may be a more remote probability. The earliest life were simple prokaryotes. Eukaryotes, cells with organelles, complex specialization compartments, arose 1.6-2.1 billion years ago. All animal and plant cells are eukaryotes, making this development a crucial building block for later complex life.
Eukaryotes are thought to have been the result of one organism attempting to consume another, but somehow instead of consuming it, the consuming organism entered into a symbiotic relationship with the consumed organism. This low probability accident may have happened only once, although no one knows for sure.
Yet another crucial development was sexual reproduction, arising 1-1.2 billion years ago, or when Earth was 73% of its current age. Sexual reproduction tremendously increased the amount of variation in offspring, which arguably accelerated evolution. Who knows how long subsequent developments might have taken without it?
Oxygen had been introduced with the rise of certain types of photosynthesis, but due to geological factors, oxygen levels remained relatively low by current standards until 800 million or so years ago, when it began to rise substantially, just in time for the development of complex life. The Cambrian explosion, the sudden appearance of a wide variety of animal life 540-500 million years ago, would not have been possible without these higher oxygen levels.
Complex life (animals and plants) arose in the last 600-700 million years, after the Earth had reached 84% of its current age. When you consider how contingent complex life is on all the milestones above, it’s development looks far from certain. Life may be pervasive in the universe, but complex life is probably relatively rare.
Okay, but once complex life developed, how likely is intelligent life? There are many more low probability events even within the history of animal life.
Earth’s environment just so happens to be mostly aquatic, providing a place for life to begin, but with enough exposed land to allow the development of land animals. In general, land animals are more intelligent than marine ones. (Land animals can see much further than marine ones, increasingly the adaptive benefits of being able to plan ahead.) A 100% water planet may have limited opportunities for intelligence to develop. For example, mastering fire requires being in the atmosphere, not underwater.
Defant mentions the asteroid that took out the dinosaurs and gave mammals a chance to expand their ecological niche. Without an asteroid strike of just the right size, mammals might not have ascended to their current role in the biosphere. We might still be small scurrying animals hiding from the dinosaurs if that asteroid had never struck.
Of course, there have been a number of intelligent species that have evolved, not just among mammals but also among some bird species, the surviving descendants of dinosaurs. Does this mean that, given the rise of complex life, human level intelligence is inevitable? Not really. While there are many intelligent species (dolphins, whales, elephants, crows, etc), the number of intelligent species that can manipulate the environment is much smaller, pretty much limited to the primates.
(Cephalopods, including octopusses, can manipulate their environment, but their short lives and marine environment appear to be obstacles for developing a civilization.)
Had our early primate ancestors not evolved to live in trees, developing a body plan to climb and swing among branches, we wouldn’t have the dexterity that we have, nor 3D vision, or the metacognitive ability to assess our confidence in making a particular jump or other move. And had environmental changes not driven our later great ape ancestors to live in grasslands, forcing them to walk upright, and freeing their hands to carry things or manipulate the environment, a civilization building species may never have developed.
None of this is to say that another civilization producing species can’t develop using an utterly different chain of evolutionary events. The point is that our own chain is a series of many low probability events. In the 4.54 billion years of Earth’s history, only one species, among the billions that evolved, ever developed the capability of symbolic thought, the ability to have language, art, mathematics, and all the other tools necessary for civilization.
Considering all of this, it seems like we can reach the following conclusions. Microscopic single celled life is likely fairly pervasive in the universe. A substantial subset of this life probably uses some form of photosynthesis. But complex life is probably rare. How rare we can’t really say with our sample of one, but much rarer than photosynthesis.
And intelligent life capable of symbolic thought, of building civilizations? I think the data is telling us that this type of life is probably profoundly rare. So rare that there’s likely not another example in our galaxy, possibly not even in the local group, or conceivably not even in the local Laniakea supercluster. The nearest other civilization may be hundreds of millions of light years away.
Alternatively, it’s possible that our sample size of one is utterly misleading us and there actually are hundreds or even thousands of civilizations in the galaxy. If so, then given the fact that they’re not here, interstellar exploration, even using robots, may be impossible, or so monstrously difficult that hardly anyone bothers. This is actually the scenario that SETI is banking on to a large extent. If true, our best bet is to continue searching with SETI, since electromagnetic communication may be the only method we’ll ever have to interact with them.
What do you think? Is there another scenario I’m missing here?