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Our planet is 4.5 billion years old, and life existed at least 3.8 billion years ago. The earliest life forms were bacteria, and for at least 3.5 billion years of Earth history, cyanobacteria—photosynthetic bacteria—have been changing the nature of the atmosphere from a reducing one to the oxidizing one we have today. The accumulated bodies of cyanobacteria have likewise over the years been transformed into the oil and natural-gas deposits that humans have been using, with coal, to power their industrial processes for more than 200 years. By about 1.5 billion years ago, the first eukaryotic cells (cells with nuclei) had appeared, in part as a result of processes of serial symbioses that provided the basis for their intracellular complexity. Eukaryotic cells had aggregated to form multicellular organisms by about 700 million years ago, and these multicellular organisms—the ancestors of terrestrial vertebrates, arthropods, fungi, and plants—invaded the land, first becoming terrestrial about 430 million years ago.

On land, with its greater array of distinct habitats, organisms proliferated greatly. Today, some 85% of all living species occur on land, even given the much greater fundamental diversity of marine organisms. As terrestrial organisms evolved in part into larger and more complex forms, forests came into existence, by at least 300 million years ago; the masses of decaying vegetation from the forests, under suitable circumstances, became coal. In the forests and other vegetation types that characterized the world of the Mesozoic Era (65–245 million years ago), biological diversity increased greatly. The Mesozoic Era began with the most extensive extinction event recorded, the great majority of all living species disappearing forever, and ended with the most recent extinction event, that at the end of the Cretaceous Period, the third and final geological period into which the Mesozoic is divided.

About 65 million years ago, it is estimated that two-thirds of all terrestrial organisms disappeared; the character of life changed permanently. Perhaps 500,000 kinds of organisms survived the extinction event at the end of the Cretaceous, and they have gradually given rise to what has conservatively been estimated as 7 million kinds of living eukaryotic organisms today and an unknown number of kinds of prokaryotic ones (cells without nuclei). This great elaboration of life has resulted not only in the elaboration of species and the forms of individual organisms, but also in the development of increasingly complex biological communities, particularly at low latitudes.

We are now participating in the sixth great extinction event; again, an estimated two-thirds of the kinds of terrestrial organisms are threatened with extinction in the near future. The extinction event that closed the Mesozoic Era seems almost certainly to have resulted from the collision of an asteroid Earth somewhere off the end of what is now the Yucatan Peninsula, but humans are the active force driving the wholesale, massacre of living things that is taking place today. How has that come to be?

Our genus, Homo, evolved from Australopithecus in Africa some 2 million years ago but existed at relatively low population densities until quite recently, geologically speaking. At the time when our ancestors were developing crop agriculture, starting about 10,000 years ago, the human population of Earth numbered several million fewer than visit the Smithsonian museums each year—far fewer than



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