Defining Earth’s history has long been important to scientists, but the current geological timeline paleontologists, geologists, and anthropologists use exists in thanks to many types of people throughout history. From early 16th century miners (for whom it was necessary to distinguish different types of rocks) to a early 19th century builder, surveyor, and amateur geologist William Smith, the current geological period knowledge base is due to a long history of contributors.
How is the Geological Timeline Organized?
The geological timeline is organized quite simply and effectively. The basic history of the earth is broken up into a set of divisions, decreasing in time as they are listed: eons, eras, periods, epochs, and ages.
Precambrian Time
“Precambrian time” is an informal title given to several eons before the current one. Starting 4.5 billion years ago when the Earth was born, so to speak, the Precambrian stretches out until about 543 million years ago, marking the beginning of the next and current eon. The period of time implied by “Precambrian” encompasses three eons: Hadean, Archaean, and Proterozoic, the former being the oldest.
While there were few living entities during the Precambrian, which may make it seem less interesting on the surface, this unimaginably vast stretch of time is where many of Earth’s major significant events took place: the first life arose, as did the first tectonic plates (and they began to move). The first eukaryotic cells evolved during this time, and the Earth’s atmosphere became enriched with oxygen, making living conditions much more desirable.
During the Hadean eon (which some argue isn’t really a defined eon at all), the Earth was just beginning. It is difficult for researchers to say much about this so-called eon, as it isn’t really geological in that there are no Earth rocks that date back as far as 4.5 billion years ago. The only rocks uncovered that date back to the Hadean eon are lunar rocks and meteorites.
The Archaean eon, which is where Earth’s true geologic record begins, saw the first signs of life. This is where bacterial microfossils likely arose, and also where tectonic plates are thought to have formed and arose.
The Proterozoic eon, the last eon during the Precambrian time, begun about 2.5 billion years ago and ended about 543 million years ago (mya). This period of time (which, like the other eons, is divided further) saw the growth of eukaryotic cells and an abundance of microfossils.
The Phanerozoic Eon
As the Proterozoic eon ended, it ushered in the geologic and paleontological eon of today: the Phanerozoic eon. It is divided into three eras, starting with the Paleozoic, then Mesozoic, and finally the Cenozoic.
The Paleozoic era (543 mya-248 mya) took up just over half of the Phanerozoic eon, and it hosted some of the most dramatic changes life would ever see. Not only did the existing organisms undergo a massive diversification, but the vast majority of marine life (numbers upwards of 90 percent) underwent a mass extinction. Neither of these events has been sufficiently explained by modern paleontology.
The Mesozoic era (248 mya-65 mya) is by far the most popularly known, as it was during this period of time that the Earth saw the emergence of the dinosaurs, which also went extinct before the beginning of the next era. Thanks to popular culture, the Mesozoic is more immediately recognized by its three time periods: the Triassic, Jurassic, and Cretaceous.
Finally, the Cenozoic era (65 mya-present) witnessed another great diversification of life. While the dinosaurs were likely gone at the outset of the Cenozoic, mammals, flowering plants, insects, birds, and fish flourished, as did, of course, humans and their civilizations, much later into the Cenozoic.
This is a very simplified breakdown of the geological timeline, especially noted by the lack of inclusion for periods, epochs, and ages. For a more thorough explanation of these, along with the previously noted eons and eras, visit the University of California Berkley’s excellent Geographical Time Machine.
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