The first exploration of the Cambrian strata began in the later half of the 19th Cent. The lowest known stratum of fossils was then found in Wales. The leading scientist there was Roderick Murchinson. Five years before Darwin's Origin of Species was published Murchinson had already called attention to the fact that this earliest strata of fossils (which he called Silurian, after an ancient Welsh people) evidenced a sudden appearance of complex designs. No one then, of course, understood the enormous significance of this observation. In fact, this fact was already known to Darwin because during his student days nearly twenty years before he had gone for field studies with his professor at Cambridge, Adam Sedgwick to the rock field of northern Wales, to study the very same strata Murchinson was to speak about. Sedgewick had named the strata Cambrian (the Latin name of ancient Wales), a name that would eventually replace Silurian. With this memory in mind, Sedgewich wrote Darwin upon reading Darwin's Origin of Species that "You have deserted—after a start in that tram-road of all solid physical truth—the true method of induction." Darwin found no support from a mentor who knew what he knew.
This sudden eruption of so many and complex life forms in the earliest fossil strata became even more evidently complex when, by a serendipitous accident in 1909, Charles Doolittle Walcott, director of the Smithsonian Institution, discovered a fossil field (now called the Burgess Shale up high in the Canadian Rockies) so rich that, over several years, he managed to cart back to his museum in the US some 65,000 extremely well preserved specimens. (The Canadians had to mount their own expedition—but led by a British team—in the 1960s to acquire their own set of fossils from the Burgess Shale.) The discovery would stump everyone with fossil organisms so strange and complex that, even until today, no one knows what they were and how to classify them. One particularly famous one was called Hallucigenia sparsa and was thought to 'walked' on the row of appendages on one side while another row of 'spikes' protruded from the other. Years later it was proposed that they got things wrong and that it probably 'walked' on what were the spikes and the 'legs' were the spikes.
By 1984 Chinese paleontologists were beginning to discover their own Cambrian fossils in Kunming in the Yunnan Province (these are often referred to as the Chengjiang fossils). They soon discovered specimens so fine that even the embryos in their early stages of division were preserved, and that the scale of the explosion was far greater than previously thought. Furthermore, the Chengjiang fossils suggest that the darwinian explanation now face another major problem. Darwinism envisages a bottom-up pattern of evolution, i.e., the emergence of unique species that then gave rise to other forms that make up a genus, from which evolved the family and so on until the different phyla are formed. The Chengjiang fossils show that evolution probably took place top-down instead: the complex body-plan was laid down first, from which the representative species then evolved. (China is one of the most exciting locations for paleontological studies today.)
All in all, then the Cambrian Explosion remains to be understood, and many questions remain unanswered. For those with an interest in evolution, it is one of the most significant discoveries.
Further Reading:
Simon Conway Morris, The Crucible of Greation: The Burgess Shale and the Rise of Animals (Oxford: OUP, 1998). Morris is considered the 'Cambrian expert' today.