![]() ![]() ![]() Modern geologic time scales include the Hadean Eon, which is an informal interval that extends from about 4.6 billion years ago to 4.0 billion years ago. ![]() ![]() The formal geologic time begins at the start of the Archean Eon (4 to 2.5 billion years ago) and continues to the present day.The oxygen-rich ozone layer was also established, protecting the Earth’s surface from harmful solar radiation. About 800 million years ago, oxygen levels reached about 21% and began to breathe life into more complex organisms. Many things have happened in that time.The earliest living organisms were microscopic bacteria that showed up in fossils as early as 3.4 billion years ago. Earth is 4.5 billion years old, but humans have only walked on the planet for 190,000 years.See the fact file below for more information on the Prehistoric Period or alternatively, you can download our 24-page Prehistoric Period worksheet pack to utilize within the classroom or home environment. More broadly, it can refer to all the time preceding human existence and the invention of writing. It refers to the period of human existence before the availability of those written records with which recorded history begins. Prehistory, meaning before history, is the span of time before recorded history or the invention of writing systems. Download the Prehistoric Period Facts & Worksheets. ![]()
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