"Here on Earth, at least 3.8 billion years ago and perhaps even earlier in our planet's history, life emerged on our world, and has persisted ever since. We've had photosynthetic life for at least the last 2.7 billion years. We've had eukaryotic life, with differentiated organelles inside its cells, for more than 2 billion years. Multicellular life and sexual reproduction have been around for over a billion years."
"How many other worlds in our Solar System had life arise on them? How many inhabited planets are there in our galaxy right now? How many planets are teeming with life the way Earth is? How many intelligent alien species are out there, and how many are more technologically advanced than we are? And, perhaps most fundamentally, are we alone in the Universe?"
Life on Earth began at least 3.8 billion years ago and has continued ever since. Photosynthesis has existed for at least 2.7 billion years, and eukaryotic cells with organelles for over 2 billion years. Multicellular organisms and sexual reproduction have been present for more than a billion years, while plants, animals, and fungi emerged over 500 million years ago. Modern humans developed technology and initiated spaceflight in the 20th century. The chemical ingredients for life are widespread across the galaxy and observable Universe. That ubiquity raises questions about the number of inhabited worlds, Earth-like biospheres, and intelligent civilizations, and calls for caution against overgeneralizing from single, fictional examples.
Read at Big Think
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