

Original article @ evolution-tells-us-we-might-be-the-only-intelligent-life-in-the-universe-124706

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.



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.


An unlikely series of events


Photosynthesis, another unique adaptation. Nick Longrich



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