“I don’t hear birdsongs in the morning, like I did when I was a kid,” said Jim Stone, executive director at Walk San Diego. “They’re slowly going away. Things in our lives change, but because they change in slow increments over a long period of time, we become accustomed to what’s new.”

Stone and I weren’t actually talking about birds. We were talking about walking and walkability in America. About how our access to places on foot alters subtly from one generation to the next, almost imperceptibly. “What we witness and what we encounter becomes the new status quo, the new benchmark in how we make assessments,” Stone said. “If people would remember a time when they could walk freely, they could make a comparison. The problem is, that time is getting further away from us.”

Cars are the primary predators of the modern urban ecosystem. They roam at will, and kill some 400,000 pedestrians worldwide every year — about 4,500 annually in the United States.

Faced with evolutionary pressure, pedestrians will do what other species have done over many millennia: evolve and adapt, such that the fittest will survive. This notion is at least a century old. “What is the future of the pedestrian, anyway?” the New York Times asked in 1908. “Darwin might tell us if he were here, but he is not here and we must look elsewhere for enlightenment.”

Wayne Curtis is a contributing editor to The Atlantic and the author of And a Bottle of Rum: A History of the New World in Ten Cocktails.