One theory holds that life originated somewhere in space and came to Earth. Another theory suggests asteroids crashing into Earth have changed the course of life on Earth.
The most famous such incident is the impact that likely wiped out the dinosaurs, opening the way for the rise of mammals. There seems to be correlations between other mass extinctions in the fossil record and major impact events, as well-- also between such impacts and climate change. Scientists are now looking at trace elements of chemicals in ancient rock fragments to try to support the theory.
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