Precambrian The Start of Life on Earth
Precambrian Objectives
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Describe how researchers study early earth, including stratigraphy and geochronology.
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Provide the approximate time span, geologic changes, and representative organisms of the Precambrian Supereon.
Geologic time is so expansive it is difficult to picture, requiring a variety of models to encapsulate events.
Since humans were not alive for most of Earth’s 4.6 billion year history, different forms of indirect evidence have to be used to piece together early environmental conditions and forms of life.
Stratigraphy is a field in geology related to studying layers of rock, also called strata. Organic and inorganic materials deposit in layers over time, and stratigraphy explains these processes.
The deepest rock layers are generally the oldest. Earthquakes, floods, and glacial melt can fold and bend the Earth, leaving behind evidence of Earth’s major changes above and below water.
This geological park in Gansu province, China shows long swaths of time and mineral deposition.
Biostratigraphy uses fossils to determine the age range of rock layers. Certain species or groups of species only existed at one point in time, and their presence indicates that general time period. For example, the brachiopod pictured existed approximately 350-415 mya (million years ago).
Geochronology is using radioactive isotope measurements to determine the age of sediments and the fossils they contain. Constant radioactive decay rates can be used to establish the age of source materials. There are now dozens of different isotopes used to determine age over different periods of time.
“Geologic Time” is broken into Eons, Eras, and Periods, with some recognizable names like the Jurassic Period.
This video is a scan of the chart without voice. There is an additional look at geologic time in the resource section.
Precambrian Supereon
The rest of this webpage and the next are a tour of four billion years of life on Earth, including the origins of animals. Note the order, approximate time, and major events, not just for the quiz, but also for this Guide’s media piece that is introduced at the bottom of the next Phanerozoic webpage.
Geologic time is so expansive it is difficult to picture, requiring a variety of models to encapsulate events.
Billions of years of Earth’s past pre-dated animals, but established the conditions for animal life. Animals need oxygen for cellular respiration and other organisms to eat. The Precambrian Supereon set up the conditions for animal life.
This video introduces the major features of the Precambrian Supereon.
Remnants of prehistoric cyanobacteria activity are found in stromatolites and date back as far as 3.6 billion years ago.
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These stromatolites are from western Australia. Cyanobacteria secrete layers of chemicals, including calcium. Fossilized stromatolites date as far back as 3.4 to 3.6 bya. Descendent cyanobacteria still form stromatolites in Australia to this day.
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By the end of the Precambrian Supereon, all life still existed in the oceans.
The next section continues the story of life, including the origins of vertebrates, with the Phanerozoic Eon.
Check your knowledge. Can you:
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describe how researchers study early earth, including stratigraphy and geochronology?
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provide the approximate time span, geologic changes, and representative organisms of the Precambrian Supereon?
Go back to the Geologic Time Guide Overview
Go forward to the Phanerozoic Page
Geologic Time Guide Contents
Complete all four of these sections before taking the quiz and making your media piece.
Back to Evolution
Module 7
This Guide
7A: Geologic Time
Products Information
7A: Quiz & Media
