1) Life was created. Some people believe that life on Earth was created by a supreme being or spirit. These ideas have been around for a very long time and differ among different cultures and religions. We can't use science to test these ideas, so we leave it to each person to decide for him- or herself.
2) Life began somewhere else in the universe, then arrived on Earth later. This idea got some new attention with the recent discovery of a meteorite from Mars. The meteorite showed some signs that ancient forms of life may have existed on Mars a long time ago, but more research is needed before we can reach any conclusions.
3) Life began on Earth by chance. Many scientists believe that life began as the result of complex chemical reactions that took place in the Earth's atmosphere 3.5 billion years ago. When chemicals in the atmosphere interacted with the electricity from lightning, certain substances formed that were necessary for early forms of life. In the 1950's, two biochemists conducted an experiment which showed how this could happen in the laboratory.
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