Humans May Be the Only Intelligent Life in the Universe, If Evolution Has Anything to Say.

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▼ 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 billionstars, and there are over a trillion  galaxiesin 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.
And striking examples of convergent evolution do exist. Australia's extinct, marsupial thylacinehad 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  diversifyingafter 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.
We also see convergence in individual organs. Eyes evolved not just in (▪ ▪ ▪)

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