Are we alone within the universe? It comes down as to whether intelligence is a possible final result of pure choice, or an inconceivable fluke. By definition, possible occasions happen continuously, inconceivable occasions happen not often—or as soon as. Our evolutionary historical past reveals that many key diversifications—not simply intelligence, however advanced animals, advanced cells, photosynthesis, and life itself—had been distinctive, one-off occasions, and due to this fact extremely inconceivable. Our evolution might have been like successful the lottery…solely far much less seemingly.
The universe is astonishingly huge. The Milky Manner has greater than 100 billion stars, and there are over a trillion galaxies within the seen universe, the tiny fraction of the universe we will see. Even when liveable worlds are uncommon, their sheer quantity—there are as many planets as stars, possibly extra—suggests numerous life is on the market. So the place is everybody? That is the Fermi paradox. The universe is massive, and previous, with time and room for intelligence to evolve, however there’s no proof of it.
May intelligence merely be unlikely to evolve? Sadly, we will’t examine extraterrestrial life to reply this query. However we will examine some 4.5 billion years of Earth’s historical past, taking a look at the place evolution repeats itself, or doesn’t.
Evolution generally repeats, with totally different species independently converging on comparable outcomes. If evolution continuously repeats itself, then our evolution may be possible, even inevitable.
And hanging examples of convergent evolution do exist. Australia’s extinct, marsupial thylacine had a kangaroo-like pouch however in any other case seemed like a wolf, regardless of evolving from a special mammal lineage. There are additionally marsupial moles, marsupial anteaters, and marsupial flying squirrels. Remarkably, Australia’s total evolutionary historical past, with mammals diversifying after the dinosaur extinction, parallels different continents.
Different hanging circumstances of convergence embrace dolphins and extinct ichthyosaurs, which advanced comparable shapes to glide by means of the water, and birds, bats, and pterosaurs, which convergently advanced flight.
We additionally see convergence in particular person organs. Eyes advanced not simply in vertebrates, however in arthropods, octopi, worms, and jellyfish. Vertebrates, arthropods, octopi, and worms independently invented jaws. Legs advanced convergently within the arthropods, octopi, and 4 sorts of fish (tetrapods, frogfish, skates, mudskippers).
Right here’s the catch. All this convergence occurred inside one lineage, the Eumetazoa. Eumetazoans are advanced animals with symmetry, mouths, guts, muscular tissues, a nervous system. Totally different eumetazoans advanced comparable options to comparable issues, however the advanced physique plan that made all of it attainable is exclusive. Advanced animals advanced as soon as in life’s historical past, suggesting they’re inconceivable.
Surprisingly, many important occasions in our evolutionary historical past are distinctive and, in all probability, inconceivable. One is the bony skeleton of vertebrates, which let massive animals transfer onto land. The advanced, eukaryotic cells that each one animals and vegetation are constructed from, containing nuclei and mitochondria, advanced solely as soon as. Intercourse advanced simply as soon as. Photosynthesis, which elevated the vitality out there to life and produced oxygen, is a one-off. For that matter, so is human-level intelligence. There are marsupial wolves and moles, however no marsupial people.
There are locations the place evolution repeats, and locations the place it doesn’t. If we solely search for convergence, it creates affirmation bias. Convergence appears to be the rule, and our evolution seems possible. However whenever you search for non-convergence, it’s in all places, and critically, advanced diversifications appear to be the least repeatable, and due to this fact inconceivable.
What’s extra, these occasions trusted each other. People couldn’t evolve till fish advanced bones that allow them crawl onto land. Bones couldn’t evolve till advanced animals appeared. Advanced animals wanted advanced cells, and sophisticated cells wanted oxygen, made by photosynthesis. None of this occurs with out the evolution of life, a singular occasion amongst singular occasions. All organisms come from a single ancestor; so far as we will inform, life solely occurred as soon as.
Curiously, all this takes a surprisingly very long time. Photosynthesis advanced 1.5 billion years after the Earth’s formation, advanced cells after 2.7 billion years, advanced animals after 4 billion years, and human intelligence 4.5 billion years after the Earth shaped. That these improvements are so helpful however took so lengthy to evolve implies that they’re exceedingly inconceivable.
An Unlikely Collection of Occasions
These one-off improvements, important flukes, might create a series of evolutionary bottlenecks or filters. If that’s the case, our evolution wasn’t like successful the lottery. It was like successful the lottery once more, and once more, and once more. On different worlds, these important diversifications might need advanced too late for intelligence to emerge earlier than their suns went nova, or by no means.
Think about that intelligence is determined by a series of seven unlikely improvements—the origin of life, photosynthesis, advanced cells, intercourse, advanced animals, skeletons, and intelligence itself—every with a ten p.c probability of evolving. The percentages of evolving intelligence grow to be one in 10 million.
However advanced diversifications may be even much less seemingly. Photosynthesis required a sequence of diversifications in proteins, pigments, and membranes. Eumetazoan animals required a number of anatomical improvements (nerves, muscular tissues, mouths, and so forth). So possibly every of those seven key improvements evolve simply 1 p.c of the time. If that’s the case, intelligence will evolve on simply 1 in 100 trillion liveable worlds. If liveable worlds are uncommon, then we may be the one clever life within the galaxy, and even the seen universe.
And but, we’re right here. That should depend for one thing, proper? If evolution will get fortunate one in 100 trillion instances, what are the chances we occur to be on a planet the place it occurred? Really, the chances of being on that inconceivable world are 100%, as a result of we couldn’t have this dialog on a world the place photosynthesis, advanced cells, or animals didn’t evolve. That’s the anthropic precept: Earth’s historical past should have allowed clever life to evolve, or we wouldn’t be right here to ponder it.
Intelligence appears to rely upon a series of inconceivable occasions. However given the huge variety of planets, then like an infinite variety of monkeys pounding on an infinite variety of typewriters to jot down Hamlet, it’s certain to evolve someplace. The inconceivable consequence was us.