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A Brief History of Life
by Lexi Krock

For most of us, the Pyramids symbolize the distant past. After all, they're more than 4,500 years old. But for geologists and paleontologists, they might as well have been built yesterday. That's because these biographers of the primordial deal in increments of millions and even billions of years as they struggle to reconstruct our planet's life history, which is so vast that those 4,500 years would have to be replicated a million times over to reach back to Earth's beginnings 4.5 billion years ago. Looked at another way, if one were to measure our planet's age as a single 24-hour day, the first human civilizations would appear less than a second before midnight.

The geological time scale, established by scientists in the mid-19th century and agreed upon internationally, breaks down the eternity of our planet's history into more manageable units than years. The scale functions as a massive calendar, dividing the history of life into eras, periods, and epochs based on fossil evidence. In this feature, explore the history of life on Earth as we know it today, from the earliest bacteria to the first modern humans.

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Lexi Krock is editorial assistant of NOVA Online.

Image credits

Sources
The Oxford Encyclopedia of Evolution by Mark Pagel, ed. New York: Oxford University Press, 2002.

The Book of Life: An Illustrated History of the Evolution of Life on Earth by Stephen Jay Gould, ed. New York: W.W. Norton, 1993.

Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth by Lynn Margulis et al. New York: W.H. Freeman, 1998.


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