information:extraction
History and significance

The seemingly rapid appearance of fossils in the "Primordial Strata" was noted by William Buckland in the 1840s, and in his 1859 book On the Origin of Species, Charles Darwin discussed the then-inexplicable lack of earlier fossils as one of the main difficulties for his theory of descent with slow modification through natural selection. The long-running puzzlement about the seemingly-sudden appearance of the Cambrian fauna without evident precursor(s) centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period during the early Cambrian, what might have caused such rapid change, and what it would imply about the origin of animal life. Interpretation is difficult, owing to a limited supply of evidence based mainly on an incomplete fossil record and chemical signatures remaining in Cambrian rocks.

The first discovered Cambrian fossils were trilobites, described by Edward Lhuyd, the curator of Oxford Museum, in 1698. Although their evolutionary importance was not known, on the basis of their old age, William Buckland (1784–1856) realized that a dramatic step-change in the fossil record had occurred around the base of what we now call the Cambrian. Nineteenth-century geologists such as Adam Sedgwick and Roderick Murchison used the fossils for dating rock strata, specifically for establishing the Cambrian and Silurian periods. By 1859, leading geologists including Roderick Murchison were convinced that what was then called the lowest Silurian stratum showed the origin of life on Earth, though others, including Charles Lyell, differed. In On the Origin of Species, Darwin considered this sudden appearance of a solitary group of trilobites, with no apparent antecedents, and absent other fossils, to be "undoubtedly of the gravest nature" among the difficulties in his theory of natural selection. He reasoned that earlier seas had swarmed with living creatures, but that their fossils had not been found because of the imperfections of the fossil record. In the sixth edition of his book, he stressed his problem further as:

To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer.

American paleontologist Charles Walcott, who studied the Burgess Shale fauna, proposed that an interval of time, the "Lipalian", was not represented in the fossil record or did not preserve fossils, and that the ancestors of the Cambrian animals evolved during this time.

Earlier fossil evidence has since been found. The earliest claim is that the history of life on Earth goes back 3,850 million years: Rocks of that age at Warrawoona, Australia, were claimed to contain fossil stromatolites, stubby pillars formed by colonies of microorganisms. Fossils (Grypania) of more complex eukaryotic cells, from which all animals, plants, and fungi are built, have been found in rocks from 1,400 million years ago, in China and Montana. Rocks dating from 580 to 543 million years ago contain fossils of the Ediacara biota, organisms so large that they are likely multicelled, but very unlike any modern organism. In 1948, Preston Cloud argued that a period of "eruptive" evolution occurred in the Early Cambrian, but as recently as the 1970s, no sign was seen of how the 'relatively' modern-looking organisms of the Middle and Late Cambrian arose.

The intense modern interest in this "Cambrian explosion" was sparked by the work of Harry B. Whittington and colleagues, who, in the 1970s, reanalysed many fossils from the Burgess Shale and concluded that several were as complex as, but different from, any living animals. The most common organism, Marrella, was clearly an arthropod, but not a member of any known arthropod class. Organisms such as the five-eyed Opabinia and spiny slug-like Wiwaxia were so different from anything else known that Whittington's team assumed they must represent different phyla, seemingly unrelated to anything known today. Stephen Jay Gould 's popular 1989 account of this work, Wonderful Life, brought the matter into the public eye and raised questions about what the explosion represented. While differing significantly in details, both Whittington and Gould proposed that all modern animal phyla had appeared almost simultaneously in a rather short span of geological period. This view led to the modernization of Darwin's tree of life and the theory of punctuated equilibrium, which Eldredge and Gould developed in the early 1970s and which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change.

Other analyses, some more recent and some dating back to the 1970s, argue that complex animals similar to modern types evolved well before the start of the Cambrian.