For context [says Brannen], let’s compare the eventual geological legacy of humanity (somewhat unfairly) to that of the dinosaurs, whose reign spanned many epochs and lasted a functionally eternal 180 million years—36,000 times as long as recorded human history so far. But you would never know this near-endless age was so thoroughly dominated by the terrible reptiles by looking to the rock record of the entire eastern half of North America. Here, dinosaurs scarcely left behind a record at all. And not because they weren’t here the entire time—with millions of generations of untold dinosaurs living, hunting, mating, dying, foraging, migrating, evolving, and enduring throughout, up and down the continent, in great herds and in solitary ambushes. But the number of sites within that entire yawning span, and over these thousands of square miles, where they could have been preserved—or that weren’t destroyed by later erosion, or that happen to be exposed at the surface today—was vanishingly small.
[Our] most enduring geological legacy ... will be the extinctions we cause. The first wave of human-driven extinctions, and the largest hit to terrestrial megafauna since the extinction of the dinosaurs, began tens of thousands of years ago, as people began to spread out into new continents and islands, wiping out everything we tend to think of as “Ice Age” fauna—mammoths, mastodons, giant wombats, giant ground sloths, giant armadillos, woolly rhinoceroses, giant beavers, etc. This early, staggered, human-driven extinction event is as reasonable a starting date as any for the Anthropocene and one that has, in fact, been proposed. However, a few thousand years—or even a few tens of thousands of years—will be virtually indistinguishable in the rocks a hundred million years hence. That is, it would not be obvious to the geologists of the far future that these prehistoric human-caused extinctions were not simultaneous with our own modern-day depredations on the environment. The clear-cutting of the rain forest to build roads and palm-oil plantations, the plowing of the seabed on a continental scale, the rapid changes to the ocean and atmosphere’s chemistry, and all the rest would appear simultaneous with the extinction of the woolly mammoth. To future geologists, the modern debate about whether the Anthropocene started 10 minutes ago or 10,000 years ago will be a bit like arguing with your spouse on your 50th wedding anniversary about which nanosecond you got married.