Although life had begun about 3000 million years ago, it was still rather primitive. Towards the end of the Precambrian era, multi-celled organisms such as annelids and jellyfish also developed. One big step in the history of life was the development of sexual reproduction. When different gametes combine, a new result is formed every time. This greatly increases the diversity of organisms.
Still, life existed only in the oceans - the land was void.
The picture above shows fossilised tracks made by crawling annelids more than 540 million years ago.
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