If Aliens Exist, Here's How We'll Find Them - Issue 97: Wonder

Suppose aliens existed, and imagine that some of them had been watching our planet for its entire four and a half billion years. What would they have seen? Over most of that vast timespan, Earth's appearance altered slowly and gradually. Continents drifted; ice cover waxed and waned; successive species emerged, evolved, with many of them becoming extinct. But in just a tiny sliver of Earth's history--the last hundred centuries--the patterns of vegetation altered much faster than before. This signaled the start of agriculture--and later urbanization. The changes accelerated as the human population increased. Then came even faster changes.