Much human synaptic activity has been activated by contemplation of the possibility that there are people out there whom we might someday encounter. I have come to believe that this is extremely unlikely.
Salient in this phenomenon is the search for planets that maintain a distance from their sun that permits them to have liquid water. This is believed to be an important factor. I’ll agree that it’s a highly desirable factor, but it’s nowhere near sufficient. There are more important obstacles.
The inspiration for this essay was a consideration of the major steps between the appearance of life on earth and the attainment of a society capable of interstellar influence of some sort. It is now generally agreed that the odds against the development of biochemical reactions are astronomical, but that the number of trials that took place on earth were equally astronomical. In other words, it took a gazillion-to-one shot to get the pieces working together, but there were gazillions of shots.
Some people seem to think that, once we had living cells, it was just a hop, skip, and a jump to get people and technology. That’s way wrong; there were a number of critical steps in the process, steps that were not at all likely.
Eukaryotes
The first life took the form of prokaryotes: simple cells that were not terribly efficient. It took another few billion years for these things to form groupings called eukaryotes, which are single cells containing internal units — organs, you might say — specialized to particular tasks. Eukaryotes are more efficient than prokaryotes, and therefore were able to move on to the next step. The regular prokaryotes continued on, but never amounted to anything more than bacteria. Eurkaryotes, on the other hand, had the flexibility and efficiency to make the next leap.
I find the development of eukaryotes entirely likely, because of the gazillions of prokaryotes hanging around on the surface of the earth and the billions of years available for the step to take place. [Laura Mixon found an excellent web page explaining all this in great detail.]
Multi-cellular creatures
It took roughly a billion years for eukaryotes to group together to form multicelluar organisms. This organization permitted even greater specialization, which in turn increased overall efficiency. This too strikes me as a reasonably likely step.
Eyes
The next big step was the development of vision in animals. This step took only a few hundred million years, and was the trigger for the Cambrian Explosion of life around 650 million years ago. The suddeness with which this happened suggests that it was a lucky break. However, vision has independently arisen numerous times, which strongly suggests that it is a likely development.
Colonization of the land
This was another crucial step, because the terrestrial environment is more complex than the marine environment and therefore is more amenable to the next step: the development of intelligence. However, I see nothing particularly odd about the rise of terrestrial creatures.
Now we get weird: the Permian extinction.
The story has now reached a point about 300 million years ago. Up to this point there was nothing particularly unlikely; all previous steps are entirely plausible advances. But then some strange and highly unlikely things happened. The first was the Permian extinction about 250 million years ago. There are a number of competing hypotheses about the cause of the extinction, but the extinction itself is certain. 96% of all marine species and 70% of all terrestial species were wiped out. Insects were completely annihilated. This was one very serious event.
The important point here is that this is a truly unique event: it happened only once in the half-billion years that we have data about. That in itself suggests that it was extremely unlikely. The Permian extinction was not part of any plausible course of development of life. It was an extreme oddity, something that we could not expect for any other life-bearing planet.
You might think that the improbability of the Permian extinction is a good thing; another planet that did not undergo anything like our Permian extinction would have been that much better off. That’s not true; the Permian extinction led to a big advance in the complexity of life on this planet. Life was nearing an equilibrium point, a kind of evolutionary stasis. The biosphere as a whole had reached something of an “adaptive optimum” in which each species was well-adapted to its environment — which included all the other species. Changes in a species were not likely to be advantageous, because they were already close to optimal, so things were slowing down.
I’m not saying that evolutionary processes had stopped before the Permian extinction. Continents were still drifting, the earth’s orbit was still shifting around, and other long-term processes were still at work. I’m sure that, had there been no Permian extinction, the biosphere would have continued changing. But the change would have been slow and tentative. Nothing radically new would have emerged.
The Permian extinction wiped the board clean and forced the biosphere to start almost from scratch, allowing it to develop in new directions. The mammals arose after — because of? — the Permian extinction. Another even more important group arose: the dinosaurs.
Dinosaurs and the Cretacious extinction
The next 190 million years saw the dinosaurs move to the top of the biosphere. We see the same process of optimization that ruled prior to the Permian extinction. During the Triassic period, many new forms of animals developed, including mammals and the first flying animals. However, after only about 50 million years, another mass extinction, much smaller than the Permian extinction, rattled the cage again.
This was followed by the Jurassic and Cretacious periods, during which time the dinosaurs took over. The central motif of these two periods was “success through size”. The dinosaurs had developed the biological systems necessary to support very large sizes, and so they developed a carnivory pyramid based on size, with bigger animals eating those a bit smaller, all the way down to tiny mammalian rodents eating bugs. The basic evolutionary process was asymptotically approaching an equilibrium point; we see far less change from Jurassic to Cretacious than we see from Triassic to Jurassic. The biosphere was once again settling down to near-stasis.
But then the meteor hit, triggering the Cretaceous extinction that wiped out the dinosaurs. The fact that the extinction was so severe attests to the optimization of the biosphere. In a near-optimal biosphere, the dependency linkages between all the species have become so complete that the extinction of a single species can lead to extinctions of other species.
An aside: the collapse of complex systems
This point is not readily appreciated, so I’ll take a moment to explain it. Imagine a remote volcanic island that suffers a violent eruption that destroys all life on the island. For decades the island is nothing but bare rock. Even if other animals happen upon the island, they don’t survive because there’s nothing to eat. However, eventually wind-borne seeds will establish some vegetation, and with time the vegetation will expand to cover the island.
Consider the evolutionary forces acting on these plant communities. The initial colonists are affected by only three factors: sun, soil, and water. They must optimize themselves for that environment. Later on, as other plant species arrive, each plant species finds itself in competition with every other plant species, so the number of factors working on the evolution of each species increases with every new species. That is, each species must be able to compete with every other species. This tends to force each species into smaller ecological niches: they specialize. The first colonists were ecological generalists, but as the biome increases its diversity, each species must specialize more narrowly.
Eventually the amount of plant life on the island is sufficient to sustain some vegetarian bugs. This creates a new selective force for the plants: now they must develop a means to deal with the leaf-chomping bugs. Each plant now finds itself dealing with even more factors. Its interaction with its environment grows more complex.
Next the leaf-eating bugs attract bug-eating animals, which attract larger carnivores, and so on until the biome reaches climax: its stable condition in which every species is biologically interacting with every other species, however indirect that interaction. Bugs and birds are pollinating plants and carrying their seeds to distant locations; diseases of one species jump the species barrier and infect other species; one species eats the species that’s eating a third species, keeping their population in check. The whole thing is a complex web of interactions that we call an ecosystem.
Now let’s perturb this system; let’s exterminate one of the species selected at random. This creates a perturbation in the ecosystem: the species that it fed on experience a population boom, while the populations that fed on it contract. The effects of these big population changes will reverberate through the ecosystem, but it should be able to get back into balance after some time.
But what happens if we exterminate, say, three species? That will trigger a lot of dramatic changes, which will throw the ecosystem much further out of balance. It will take longer to restabilize itself.
What happens if we exterminate, say, 20% of all species? This would be considered a small mass extinction, but consider this: just about every species on that island will be thrown way out of balance. Many species will be deprived of their food sources and will die back. This in turn will rob other species of their food sources and they too will die back. The process is a vicious circle, with its negative consequences spreading further and further through the ecosystem. That vicious circle will eventually reach its natural limits and the destruction will stop, leaving a much-reduced ecosystem with fewer species and less overall biomass.
Here’s the key point: the amplification of the destruction is concomitant with the optimization of the ecosystem. The closer it is to optimum, the more intricate the interactions among species, and the more sensitive the system becomes to large perturbations.
The fact that one meteor was able to wreak vast global destruction demonstrates that the late Cretacious ecosystem had stabilized at something close to its optimum.
On to the Cenozoic Era
The biosphere took a while to reconstitute itself, and even then it was nowhere near equilibrium for many millions of years. Mammals took over as the dominant terrestrial fauna. Where the evolutionary motif for dinosaurs was size, for mammals it was intelligence. Mammals had a higher brain-to-weight ratio than dinosaurs. There was nothing intrinsically superior about intelligence; after all, the dinosaurs were very successful for nearly 200 million years without much in the way of intelligence. I suspect that mammals were saved by their underground life styles rather than their intelligence. The advance of intelligence was most likely due to dumb luck.
OK, so the mammals spread out all over the globe and into all sorts of new ecological niches. This process took tens of millions of years, but signs of stabilization were beginning to show after 30 million years. I believe (but am not certain) that all of the existing orders of mammals were in place by about 25 million years ago.
The hominoidea (apes) first showed up about 30 million years ago; the hominids (gorillas, chimpanzees, and humans) first appeared about 15 million years ago; and our own genus came around 6 million years ago. This evolutionary sequence is certainly one of increasing intelligence, and the fact that intelligence steadily increased demonstrates that intelligence per se was adaptively advantageous. However, the emphasis on increasing intelligence seems to be confined to the apes; other mammals showed no proclivity towards increasing intelligence.
The crucial process that led to human intelligence was a fluke; it is explained well in Lowly Origin, a book by a scientist of human evolution. Basically, the last few million years saw an oscillation in climate that alternately confined early humans to river valleys in east Africa, then liberated them to mix together and share genes. Each confinement reduced human populations, winnowing out the dumbest individuals; each liberation allowed the sharing of genes to give the population greater resilience.
The flukey nature of the evolution of human intelligence is also demonstrated by the small area in which human evolution took place. There were at least three waves of human colonization of the earth. In each case, humans spread out from Africa into Eurasia. The first of these waves began almost 2 million years ago with Homo Erectus. There was considerable debate over two basic hypotheses (“Candelabra” and “Out of Africa”) and various combinations of these two. The Candelabra hypothesis argued that modern humans evolved from Homo Erectus simultaneously all over Eurasia, with considerable gene flow between the different populations. This hypothesis is no longer in favor; most opinion holds for the “Out of Africa” hypothesis, which posits that most human evolutionary progress took place in the special environment of Africa, which new, improved versions expanding out of Africa to replace their predecessors.
The “Out of Africa” hypothesis suggests that the evolution of human intelligence was a very flukey thing, relying entirely on the very special conditions in a small region of east Africa. Early humans outside of that area were not able to evolve greater intelligence as quickly as their African cousins, which is why they were wiped out by African colonists.
Thus, the evolution of human intelligence was a huge fluke, something not likely to have happened, either on earth or anywhere else.
From intelligence to technology
There’s another quantum leap we took that was a fluke: the development of science and technology. Most people assume that the rise of science and technology was a natural, inevitable process. It most certainly was not; indeed, it was perhaps the biggest fluke of them all.
It would at this point edify you to read my long hyperbook A History of Thinking. It’s rather lengthy, but it explains in detail exactly how unlikely the development of science and technology was. Here’s another indicator of just how flukey it was: for most of human history, Chinese civilization was more advanced in almost every regard than any other civilization. They invented new technologies before anybody else. But there’s a crucial consideration: the Chinese never used science to develop technology. They relied on completely different approaches, as explained in my essay How Technology Advances. Their technologies were created without any science, and they never developed any science from their technologies. I maintain that, had Europe never existed, and China had remained the leading civilization on the planet, we’d still be about where they were 200 years ago. Chinese technology developed slowly, tentatively, accident by accident, rather than deliberately as was the case in Europe.
There is nothing in Chinese history to suggest that they would ever have developed science. Which means that humanity would never achieve interstellar capabilities had the fluke that was Greece not happened.
The same probabilities would apply to any other planet. The likelihood of stumbling onto rationalism is too low, and the number of trials (planets with intelligent species) is too low to overcome that obstacle.
Sustainability
An even more serious argument concerns the likelihood of any technological society lasting long enough to achieve interstellar travel. Imagine our remote planet with its intelligent species. Assume that this intelligent species somehow develops science and technology. More science and technology causes expansion of population and economic output. Expansion of population and economic output causes more science and technology. And so the wheel turns faster and faster. But this wheel spins out problems as well as benefits. The speed with which an intelligent species can respond to new problems is finite; at some point, the problems will come faster than the species’ ability to adapt.
Here on earth, our technology is altering the environment to our benefit — for the most part. Until just recently, the negative side effects of our technology were small enough that we could adapt to them, although the crunch is getting tighter. Nuclear weapons challenged our ability to control our warlike impulses. We squeaked through that crunch between technology and wisdom (so far). But then pollution arose as a new challenge — and once again we squeaked through that crunch (although China has not yet caught up with the rest of the developed world in this regard). Now we are facing climate change due to our carbon emissions, and this time our ability to adapt to this new environmental challenge is inadequate. We’ll get around to it, and maybe we’ll squeak past this one, too. But in each case, our response has been less and less adequate to deal with the problem. What if the greenhouse effect were ten times stronger and we faced climate change moving ten times faster? We’d be in deep trouble. What will the next challenge be? Will we be able to cope? At some point, something will hit us faster than we have the intelligence to cope with.
Conclusions
The standard argument in favor of extraterrestrial life is probabilistic: there are so many planets out there that, even if the probability of life arising on one is low, the number of planets is high enough to make the overall probability high. I believe that this argument works right up the rise of complex multicellular life. But the three obstacles that I find to be deal-killers are:
1. The low probability of the right number of mass extinctions: not so high as to wipe out life, but not so low as to permit it to stagnate.
2. The low probability of intelligence developing.
3. The low probability of an intelligent species developing science and technology.
To get past all three obstacles seems to me highly improbable. We earthlings are truly a rare breed.
