![]() The Cenozoic started about 65 million years ago. Last but not least is the Cenozoic (Sen-oh-ZOE-ik) Era, the one in which we live. After the Precambrian come the Paleozoic Era and Mesozoic Era. It is divided into Eons known as the Hadean (HAY-dee-un), Archean (Ar-KEY-un) and Proterozoic (Pro-tur-oh-ZOE-ik). The oldest - and by far the longest - is called the Precambrian. It divides Earth’s entire 4.6 billion years into four major time periods. How to track such a long, complex history? Using dazzling detective skills, geologists created a calendar of geologic time. As species evolve or go extinct, the fossils trapped in the rock layers reflect these shifts. Sandstone might once have been an ancient desert, where early land animals scurried. These rocks contain traces of life that existed in those oceans over time. Limestone or shale, for example, may be the remains of long-gone oceans. It also marks when they thrived - and when, over millions of years, most of them went extinct. Put together, the rock records the long saga of life on Earth. Like chapters in a very, very thick book, layers of rock chronicle Earth’s history. They didn’t show up until just 12 minutes before midnight on New Year’s Eve.Īlmost as mind-boggling is how geologists figured this all out. The first modern humans - Homo sapiens - were real late-comers. Dinosaurs stomped around from December 16 until December 26. Fish first swam onto the scene in late November. If Earth formed on January 1, the earliest primitive life (think algae) wouldn’t appear until March. To grasp just how old Earth is, imagine fitting its entire history into one calendar year. That’s why, in fact, it’s known as geologic time. And to measure it, scientists use special terms, most of which focus on the planet’s changing geology. That’s how old the Earth is - a mind-boggling length of time. Imagine the nearly unimaginable: 4.6 billion years.
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