land, marked still by foam and the wet mark of the recent high tide. The tidal change here is at least 15 meters of vertical distance between the high-tide line and the ocean’s level, a tidal change that can be found only in the Bay of Fundy, Canada, in our own world. We look up at a noticeably dimmer sun, dimmer because all stars grow more energetic through time, and to the east we see a half moon just risen, dim as well in the afternoon sunlight but far greater in size than it will be in our time. The moon is clearly closer to Earth in this long ago time, and that would explain the great tidal range.

We walk to an outcrop of hard basalt that makes one of the headlands of this broad sandy beach, and even in areas of the outcrop where the low tide still washes up, there are few of the familiar features of our world. There is no upper tide assemblage of Littorina (the common seashore periwinkle snails), or barnacles beneath the snails, or lines of mussels beneath the barnacles—no intertidal zonation at all, in fact. But it is not dead, this rock. There is a variety of algae clinging to it, reds and browns that wash in the rushing current, not all that different from the kelps of our world, except for the fact that no animals live amid them.

It is time to look at what lives in the sea. This being a thought experiment, we suit up with appropriate diving gear, clean our masks, don flippers, and dive downward. We are not the first to take this trip. In his 1998 book, The Crucible of Creation, Simon Conway Morris took readers on a submersible trip into the Cambrian Ocean at the site of what would become the Burgess Shale and in so doing described the vast assemblage of organisms that would become the fossil finds of that most important of all fossil deposits, our best view into the world at the height of the Cambrian Explosion. But the Burgess Shale was deposited some 510 million years ago. Here we are in the world of 522 million years ago, almost 12 million years before the Burgess. Twelve million years of evolution is a long time for animals to diversify. We are at the beginning of things, and the place we dive into will come to be known as Chengjiang, China, and, like the Burgess Shale (if not so famously), it will yield fossils with soft parts, giving us a window into the start of the Cambrian Explosion.

We descend through the surf zone and make our way offshore into



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