Sean B. Carroll in The New York Times:
In 1909, Charles Walcott, a paleontologist and secretary of the Smithsonian Institution, discovered one of the greatest and most famous fossil troves high in the Canadian Rockies on Burgess Pass in British Columbia. The slabs of Burgess Shale that Walcott excavated contained the earliest known examples at the time of many major animal groups in the fossil record, in rocks that were about 505 million years old. Walcott’s discovery was further evidence of the so-called Cambrian Explosion — the apparently abrupt appearance of complex animals in the fossil record within the Cambrian Period, from about 542 to 490 million years ago. Although not seen before on the scale documented in the Burgess Shale, the emergence of trilobites and other animals in the Cambrian was familiar to paleontologists, and had troubled Charles Darwin a great deal.
The difficulty posed by the Cambrian Explosion was that in Darwin’s day (and for many years after), no fossils were known in the enormous, older rock formations below those of the Cambrian. This was an extremely unsettling fact for his theory of evolution because complex animals should have been preceded in the fossil record by simpler forms. In “On the Origin of Species,” Darwin posited that “during these vast, yet quite unknown, periods of time, the world swarmed with living creatures.” But he admitted candidly, “To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer.”