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The way the planet has changed through geologic time, and life on it, the account of the Earth, is the topic of this and the next three chapters, starting in this chapter with the Precambrian Supereon. The overarching principles of geologic time, plate tectonics, and evolution worked dynamically to create the biography of the planet. This chapter traces back to the recesses of the geologic record and early Earth, from its birth and the formation of the Moon through seven-eighths of its existence, a huge span of time. Early life forms emerged during this supereon in the Archean Eon and had a profound influence on other Earth systems. Life interacted and changed the chemistry of the atmosphere through photosynthesis, so much so that the changes are thought to have sent planetary systems over an edge into multiple “Snowball Earth” episodes when most of the planet froze over. In addition to the beginning of organic life and climate, the emergence and configuration of the continents during the Precambrian are covered. Events of this supereon set the stage for the burgeoning of life forms in the next eon, the Phanerozoic.

The story of the Phanerozoic Eon continues in this chapter with the Mesozoic Era. The first period in the Mesozoic, the Triassic, was bookended by two extinction events, the one at the beginning, discussed in the prior chapter at the end of the Permian Period, the Great Dying, and then another at the end of the period, related to the further breakup of Pangea. Dinosaurs evolved and diversified during the Mesozoic to occupy nearly each and every ecological niche on the planet, with large dinosaurs and small dinosaurs, ones that flew, those that ate vegetation, and those that preyed upon the herbivores—making this time a dino-dominated age. In the late Jurassic Period, small mammals, many of them insectivores, were starting to become prevalent. The era ended with a “big bang” of a different type than is theorized as the start of the universe—with the Chicxulub asteroid impact 66 million years ago that ended the lives of most of the dinosaurs, the non-avian lines, and opened up new ecological niches for the next “masters of the universe,” the mammals.

This chapter focuses on the first era of the Phanerozoic Eon (“visible life”), the Paleozoic Era, when life burgeoned after a rapid shift from a frozen setting during Snowball Earth times to dramatic warming related to greenhouse conditions. The chapter includes stories, such as that of the finding of the unique creatures of the Burgess Shale in Canada, that represent the diversity of life in the early part of the Paleozoic, the Cambrian Period. Several climatic shifts happened in the Paleozoic with resulting extinction events, the first occurring at the end of the Ordovician Period, the second at the end of the Devonian, and a third massive extinction at the end of the Permian Period. The latter was the largest extinction event ever recorded when 95 percent of all genera were killed as the supercontinent Pangea began to be split apart at the end of the Paleozoic.

The last era in the Phanerozoic Eon, the Cenozoic Era, is detailed in this chapter. The rise and radiation of the mammals occurred during Cenozoic after the devastation wrought by the Chicxulub Asteroid impact at the end of the Mesozoic Era. Ecological resources and niches vacated by the dinosaurs because of the mass extinction were filled by the mammals with concurrent developments in plants. Changes in climate and the mid-Miocene warming happened mid-era, then drying out and opening of grasslands followed by a plunge into ice ages and the Pleistocene extinction event. The late Cenozoic witnessed the development of humankind as the great ice sheets from the Pleistocene started to melt and the climate warm. The planet started to look similar to how it appears to humans today, and the current age of the Earth is the Cenozoic Era, Quaternary Period, Holocene Epoch, Meghalayan Age.