Emotions. They're hallmarks of being human (and, as recent studies suggest, they may also be shared among other species as well!). We're connected to one another through shared experiences of sadness, happiness, excitement, fear, and other nuanced feelings. Yet through the centuries, root triggers for our emotions have shifted considerably.
Let's say, for instance, that you lived a million years ago in the Paleolithic era. What may have caused you anxiety or fear ten thousand years ago would have had nothing to do with traffic, street crime, or that constant stream of terror-inducing news we're all privy to. Back then, if you were foraging for berries along the river with your young child, you might keep one eye on the thick brush in case a predator happened to be watching.
And if a leopard did suddenly spring for your child, that "fight or flight" response (a.k.a. acute stress response) would instantly flip on. Your heart rate would quicken. Adrenaline and other hormones would flood your system. Your liver would release glucose, honing your focus and response time. As a result, you'd make a series of split-second decisions around whether to run or to stay and fight. And once the threat was over, your body would return to normal.