Because all the physical conditions are exactly right for it! There’s lots of water and oxygen around, and we’re about the right distance from the Sun, making the Earth a warm and comfortable place to live. There’s also an abundant supply of all the chemicals needed to construct living cells. A massive number of plant and animal species has evolved and adapted to the environment. The atmosphere plays an important role, shielding the Earth’s surface from the scorching rays of the Sun and providing the right temperature on Earth for water to exist as a liquid. Scientists have been surprised at the abundance of life on Earth, finding evidence that life exists in some fairly unexpected places and extreme conditions.

One theory of how life came to exist on Earth is that it evolved around hydrothermal vents deep in the oceans. Another idea comes from clues in the moon dust found by the Apollo mission. Tiny glass beads in the dust are evidence that the Moon was bombarded by meteorites about 500 million years ago, and this might well also have happened on Earth. So life as we know it might have been triggered by a massive meteorite shower half a billion years ago!