Forty-two million years ago, the Earth lived inside a warm breath. The planet was in the Eocene Epoch, a time when ice had not yet claimed the poles and the world’s weather felt softer, wetter, more generous. There were no white caps on Antarctica, no permanent Arctic ice. Instead, forests reached toward the high latitudes as if they had forgotten what cold could do. The coastline sat farther inland than today, because the seas were higher, swollen by warmth, spilling into shallow continental shelves and low valleys. If you stood on a hill above an Eocene riverplain, you would not see grasslands rolling into the distance. You would see trees. Trees in layers. Dense canopy. Tangled understory. Swamps and oxbow lakes where the air hung thick with insects. The world was, in many places, a green architecture of trunks and leaves, humming with life that moved mostly in shade. The continents were already familiar in outline, but they were not yet finished with their long argument. India had crashed into Asia and refused to stop, beginning the slow uplift of the Himalayas. Australia was peeling away from Antarctica and drifting north, carrying its future forests and animals toward warmer seas. The Atlantic was opening wider, but ocean currents were still rearranging themselves, still learning how to circulate heat around the globe. Earth’s geography was a work in progress, and the climate was part of the negotiation. In that warm world, mammals were experimenting boldly. The dinosaurs were long gone—twenty-four million years in the past—and their absence had left space like an empty theatre waiting for new actors. Forests were filled with small horses, dog-sized creatures browsing on leaves. Early relatives of elephants moved through African woodlands. Strange carnivorous mammals hunted along riverbanks, wearing combinations of teeth and limbs that would later vanish from the evolutionary catalogue. And above, in the branches, primates watched. They were not human ancestors in a straight line, but they were the kind of beings from which such ancestors could one day emerge: quick-eyed, grasping-handed, tuned to social life and fruiting trees. Their world was vertical. They lived in the three-dimensional maze of forest space, where food ripened, danger climbed, and every jump mattered. Out in the oceans, another great transformation was underway. Whales—once walking creatures—were committing themselves to water. The sea had become a new homeland. Some forms still carried hints of the shoreline in their bodies, while others were beginning to look truly ocean-born. It was an age of transitions, where the boundaries between land and sea were being renegotiated by evolution itself. The Eocene Earth was not peaceful. Storms still tore through forests. Volcanoes still reshaped horizons. Predators still fed, and prey still fled. But the overall mood of the planet was abundance. Warmth made water circulate; water made forests; forests made niches; niches made diversity. Life multiplied into forms that would later become familiar, and forms that would never be seen again. Yet this world was also standing near a threshold. The warmth of the Eocene would not last forever. Over millions of years, the planet would begin to cool. Antarctica would drift into a position that allowed cold currents to isolate it. Carbon dioxide would fall. Ice would arrive. Forests would retreat. Grasslands would spread. The open, windswept Earth that would eventually shape apes, hominins, and humans was still far ahead—but the long pivot toward it had already begun. Forty-two million years ago, Earth was a planet wrapped in green, breathing warmth into every coastline and canopy. It was a world that felt like summer without end—lush, humid, alive—and quietly, inevitably, on its way to becoming something harsher, clearer, and more open.
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