You are in the oceans of Canada, North America, 508 million years ago.

You—donned with specialized, full-body diving equipment to handle the intolerable atmospheric pressure—rested on the silty bottom of the shallow sea, the water bright with filtered sunlight. Just ahead, a trilobite, barely larger than your palm, scuttled purposefully. It moved on dozens of spindly legs, its seven segmented plates flexing as it navigated past soft-bodied sponges.