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In the simplest telling, it held that our Milky Way galaxy came together nearly 14 billion years ago when enormous clouds of gas and dust coalesced under the force of gravity.
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The Big Bang was the moment 13.8 billion years ago when the universe began as a tiny, dense, fireball that exploded. Most astronomers use the Big Bang theory to explain how the universe began. But what caused this explosion in the first place is still a mystery.
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The Sun formed about 4.6 billion years ago in a giant, spinning cloud of gas and dust called the solar nebula. As the nebula collapsed under its own gravity, it spun faster and flattened into a disk.
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When did the Moon form? Scientists believe the moon formed during a giant impact about 60-175 million years after the solar system was born. To arrive at this estimate, they can use rocks from Earth.
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Formation. When the solar system settled into its current layout about 4.5 billion years ago, Earth formed when gravity pulled swirling gas and dust in to become the third planet from the Sun.
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It remains unclear how and when life first originated on Earth, but we know that the first unicellular organism emerged between 3.6 billion and 2.7 billion years ago, giving rise to bacteria and archaea, which have no nucleus or other sub-cellular organelles.
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Large, multicellular life forms may have appeared on Earth one billion years earlier than was previously thought. Macroscopic multicellular life had been dated to around 600 million years ago, but new fossils suggest that centimetres-long multicellular organisms existed as early as 1.56 billion years ago.
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Triassic Period (252 to 201 million years ago)
Unlike today, there were no polar ice caps. It was in this environment that the reptiles known as dinosaurs first evolved. -
The first human ancestors appeared between five million and seven million years ago, probably when some apelike creatures in Africa began to walk habitually on two legs. They were flaking crude stone tools by 2.5 million years ago. Then some of th