Even now, as Fortey says, it can be startling to go to the right formation of rocks and to work your way upward through the eons finding no visible life at all, and then suddenly "a whole Profallotaspis or Elenellus as big as a crab will pop into your waiting hands." These were creatures with limbs, gills, nervous systems, probing antennae, "a brain of sorts," in Fortey's words, and the strangest eyes ever seen. Made of calcite rods, the same stuff that forms limestone, they constituted the earliest visual systems known. More than this, the earliest trilobites didn't consist of just one venturesome species but dozens, and didn't appear in one or two locations but all over. Many thinking people in the nineteenth century saw this as proof of God's handiwork and refutation of Darwin's evolutionary ideals. If evolution proceeded slowly, they asked, then how did he account for this sudden appearance of complex, fully formed creatures? The fact is, he couldn't.

And so matters seemed destined to remain forever until one day in 1909, three months shy of the fiftieth anniversary of the publication of Darwin's On the Origin of Species, when a paleontologist named Charles Doolittle Walcott made an extraordinary find in the Canadian Rockies.