1.3 billion years ago, two black holes were locked together in a death spiral. Rotating around each other at two thirds the speed of light, separated only by the distance between London and Paris, they quickly and violently merged into a single black hole. Its mass was less than the combined mass of the original ones; the leftover energy was blasted out in all directions in the form of a gravitational wave.
Meanwhile, on Earth, the Rodinia supercontinent was home to nothing more than single-celled organisms. Life slowly grew in complexity; the continents separated; the dinosaurs arose; the continents recombined; the dinosaurs died out. When the gravitational waves had traveled 99.985% of the way to Earth humans appeared. Over hundreds of millenia we developed language, agriculture, philosophy, mathematics, and science.
Mystical explanations for the natural world slowly gave ground to empiricism. Newton developed a theory of gravity and Einstein later refined it, predicting that some moving objects would radiate waves of gravitational energy. Almost a century later, the LIGO experiment began to look for these waves. After seeing nothing for five years the detectors were taken offline and upgraded; they saw nothing again and were taken offline and upgraded again.
They were brought back online last February and in September they detected the gravitational waves from the merging black holes. The waves had been traveling for 1,300,000,000 years — since before humans existed — and they passed through our detector seven months after we turned it on.