https://theconversation.com/evolution-tells-us-we-might-be-the-only-intelligent-life-in-the-universe-124706?utm_medium=ampfacebook&utm_source=facebook&fbclid=IwAR1-for5sFLH-0UdQTvUNU8iN8L0a9zCKo8Mj0ENKQ1LZZG5HaD23bXP7Bw
Are we alone in the universe? It
comes down to whether intelligence is a probable outcome of natural selection,
or an improbable fluke. By definition, probable events occur frequently,
improbable events occur rarely – or once. Our evolutionary history shows that
many key adaptations – not just intelligence, but complex animals, complex
cells, photosynthesis, and life itself – were unique, one-off events, and
therefore highly improbable. Our evolution may have been like winning the
lottery … only far less likely.
The universe is astonishingly vast.
The Milky Way has more
than 100 billion stars, and there are over a
trillion galaxies in the visible universe, the tiny fraction of the
universe we can see. Even if habitable worlds are rare, their sheer number –
there are as many
planets as stars, maybe more – suggests lots of life is out there. So where
is everyone? This is the Fermi
paradox. The universe is large, and old, with time and room for
intelligence to evolve, but there’s no evidence of it.
Could intelligence simply be unlikely
to evolve? Unfortunately, we can’t study extraterrestrial life to answer this
question. But we can study some 4.5 billion years of Earth’s history, looking
at where evolution repeats itself, or doesn’t.
Evolution sometimes repeats, with
different species independently converging on similar
outcomes. If evolution frequently repeats itself, then our evolution might
be probable,
even inevitable.
The wolf-like thylacine. Wikipedia
And striking examples of convergent
evolution do exist. Australia’s extinct, marsupial thylacine had
a kangaroo-like pouch but otherwise looked like a wolf, despite evolving from a
different mammal lineage. There are also marsupial moles, marsupial anteaters
and marsupial flying squirrels. Remarkably, Australia’s entire evolutionary
history, with mammals
diversifying after the dinosaur extinction, parallels other
continents.
Other striking cases of convergence
include dolphins and extinct ichthyosaurs, which evolved similar shapes to
glide through the water, and birds, bats and pterosaurs, which convergently
evolved flight.
Squid eye. PLoS Biology
We also see convergence in individual
organs. Eyes evolved not just in vertebrates, but in arthropods, octopi, worms
and jellyfish. Vertebrates, arthropods, octopi and worms independently invented
jaws. Legs evolved convergently in the arthropods, octopi and four kinds of
fish (tetrapods, frogfish, skates, mudskippers).
Here’s the catch. All this
convergence happened within one lineage, the Eumetazoa. Eumetazoans are complex
animals with symmetry, mouths, guts, muscles, a nervous system. Different
eumetazoans evolved similar solutions to similar problems, but the complex body
plan that made it all possible is unique. Complex animals evolved
once in life’s history, suggesting they’re improbable.
Surprisingly, many critical events in
our evolutionary history are unique and, probably, improbable. One is the bony
skeleton of vertebrates, which let large animals move onto land. The complex,
eukaryotic cells that all animals and plants are built from, containing nuclei
and mitochondria, evolved only once. Sex evolved just once. Photosynthesis,
which increased the energy available to life and produced oxygen, is a
one-off. For that matter, so is human-level intelligence. There are
marsupial wolves and moles, but no marsupial humans.
The vertebrate skeleton is unique. Smithsonian
Institution
There are places where evolution
repeats, and places where it doesn’t. If we only look for convergence, it
creates confirmation bias. Convergence seems to be the rule, and our evolution
looks probable. But when you look for non-convergence, it’s everywhere, and
critical, complex adaptations seem to be the least repeatable, and therefore
improbable.
What’s more, these events depended on
one another. Humans couldn’t evolve until fish evolved bones that let them
crawl onto land. Bones couldn’t evolve until complex animals appeared. Complex
animals needed complex cells, and complex cells needed oxygen, made by
photosynthesis. None of this happens without the evolution of life, a singular
event among singular events. All organisms come from a single ancestor; as far
as we can tell, life
only happened once.
Curiously, all this takes a
surprisingly long time. Photosynthesis evolved 1.5 billion years after the
Earth’s formation, complex cells after 2.7
billion years, complex animals after 4 billion years, and human
intelligence 4.5
billion years after the Earth formed. That these innovations are so
useful but took so long to evolve implies that they’re exceedingly improbable.
An unlikely series of events
These one-off innovations, critical
flukes, may create a chain of evolutionary bottlenecks or filters. If so, our
evolution wasn’t like winning the lottery. It was like winning the lottery
again, and again, and again. On other worlds, these critical adaptations might
have evolved too late for intelligence to emerge before their suns went nova,
or not at all.
Imagine that intelligence depends on
a chain of seven unlikely innovations – the origin of life, photosynthesis,
complex cells, sex, complex animals, skeletons and intelligence itself – each
with a 10% chance of evolving. The odds of evolving intelligence become one in
10 million.
Photosynthesis, another unique adaptation. Nick Longrich
But complex adaptations might be even
less likely. Photosynthesis required a series of adaptations in proteins,
pigments and membranes. Eumetazoan animals required multiple anatomical
innovations (nerves, muscles, mouths and so on). So maybe each of these seven
key innovations evolve just 1% of the time. If so, intelligence will evolve on
just 1 in 100 trillion habitable worlds. If habitable worlds are rare, then we
might be the only intelligent life in the galaxy, or even the visible universe.
And yet, we’re here. That must count
for something, right? If evolution gets lucky one in 100 trillion times, what
are the odds we happen to be on a planet where it happened? Actually, the odds
of being on that improbable world are 100%, because we couldn’t have this
conversation on a world where photosynthesis, complex cells, or animals didn’t
evolve. That’s the anthropic
principle: Earth’s history must have allowed intelligent life to evolve, or
we wouldn’t be here to ponder it.
Intelligence seems to depend on a
chain of improbable events. But given the vast number of planets, then like
an infinite
number of monkeys pounding on an infinite number of typewriters to
write Hamlet, it’s bound to evolve somewhere. The improbable result was us.
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