The Precambrian world, where the land was completely barren The first life on Earth appeared not on land but in the oceans. There was not enough oxygen in the air to breathe and ultraviolet radiation, lethal to life forms, was still at dangerous levels. Neither problem affected life underwater. Life probably arose about 3.8 billion years ago. The span of time from the origin of the Earth, about 4.6 billion years ago, to the time when a wide variety of life forms first appeared, about 541 million years ago, is called the Precambrian Period. The Cambrian Period lasted from 541 to 485 million years ago.
Beginnings of life
No one knows how life began, but shallow, warm-water pools at the edges of the oceans might have been the kind of environment suitable for the formation of chemicals that would eventually become the building blocks of life. The sea contain all kinds of salts, minerals and other elements. Certain vital chemical reactions could have been triggered by lightning or the shock waves of a meteorite or asteroid collision.
The Cambrian Period is named after Cambria, the Latin name for Wales, where a large expanse of Cambrian rocks are exposed at the surface.
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