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A hundred million years from now, when we're all dead and gone, a team of geologists will be digging in a field somewhere ...
... and they will discover, buried in the rocks below, a thin layer of sediment — very thin, about the width of a cigarette paper, . That skinny strip, when they look close, will send what's called a "biostratigraphic signal" that something enormous happened back in our era, something life-changing, planet-reorganizing, even Earth-shaping. The evidence, when they look closely, will be visible in that same skinny layer all over the world. In her , The Sixth Extinction, Elizabeth Kolbert describes what they'll find.
For starters, Kolbert says, below this layer, geologists will see fossil remnants of all kinds of large animals: elephants, buffalo, rhinos, lions, tigers, whales, giant turtles (and deeper down, even earlier — saber-toothed tigers, mammoths and giant sloths). Their big bones will litter those older rocks. But above this layer — after our era — they disappear. Something killed off Earth's megafauna.
During this same time, they will discover that animals and plants that used to be in one place — gingko trees in China, tulips in Asia, starlings in Europe — suddenly moved all over the world. Grasses found on one continent now strangely appear on four continents. Flowering plants, rats, goats, pigeons, kudzu, ants, inexplicably spread their territories across enormous oceans, climates, time zones. Specific life forms — chickens, cattle, roses, wheat, rice — turn up everywhere. Something moved them, though they may not know who or how. ...
Also at this time, bits of air trapped in the rock will show that the concentration of carbon dioxide in the atmosphere jumped sharply — to about 500 parts per million, higher than at any other point in the previous 800,000 years. So in this era, the chemical composition of the atmosphere changed, and changed very suddenly. ...
And down in the soil where supplies of nitrogen had been relatively rare, coming from existing populations of plants and animals — something changed, too. Out of nowhere, tons and tons of extra nitrogen appear. The supply jumps feverishly — feeding plants as never before. What happened?
Digging on six continents, geologists will discover that almost all of the Earth's major rivers, instead of winding and meandering across the planet's surface, were altered — blocked, re-routed or straightened. In some cases those rivers were dammed and pointed to new destinations. Enormous lakes were starved of water, and disappeared.
And surveying the continents, they will discover that the land on Earth that's free of ice had been significantly altered (yes, half) to provide space for crops, reservoirs, mining, logging, quarrying, housing, commerce or transportation. Wild spaces continued to exist, of course, mostly as rain forests, deserts, tundra and the higher mountain ranges, but they were a smaller and smaller proportion of the planet, sometimes crisscrossed by pipelines and affected by climate change. The densely built spaces, meanwhile ...
