Are we alone in the universe? It comes down to whether intelligence is a probable outcome of natural selection, or an improbable fluke. 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. 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 .On other worlds, these critical adaptations might have evolved too late for intelligence to emerge before their suns went nova, or not at all.