Cambrian explosion

The Cambrian explosion is the first appearance in the fossil record, beginning about 539 million years ago, of essentially all the animal phyla.^[1] The International Commission on Stratigraphy dates the start of the Cambrian Period to 538.8 million years ago and its end to 485.4 million years ago.^[2] A 2011 summary has diverse bilaterian clades emerging "apparently within a few million years during the early Cambrian", and a 2006 review calls the episode "complex and somewhat protracted".^[3][1]

Genetic and developmental evidence points to animal lineages that are older than their fossils. The authors of the 2011 summary place the establishment of the animals' developmental toolkits in the Cryogenian, which they date to 850 to 635 million years ago, and the later ecological success of animals in the Ediacaran and Cambrian. If that is right, the Cambrian fossils record the success of animals that already existed, and the question becomes why that success came when it did.^[3] Charles Darwin posed the problem long before the best Cambrian fossil beds were found. In the sixth edition of the Origin of Species (1872) he wrote of the missing earlier record that "I can give no satisfactory answer".^[4]

Adam Sedgwick named the Cambrian after Cambria, the Latin name of Wales, for rocks in Wales that lay below Roderick Murchison's Silurian. The two men presented a joint paper in 1835, "On the Silurian and Cambrian Systems, exhibiting the order in which the older sedimentary strata succeed each other in England and Wales". Sedgwick's upper Cambrian overlapped the lower part of Murchison's Silurian, and the overlap became one of the most heated geological controversies of its day.^[5]

In chapter 10 of the sixth edition, under the heading "On the sudden Appearance of Groups of allied Species in the lowest known Fossiliferous Strata", Darwin noted that species of several main divisions of the animal kingdom appear suddenly in the lowest fossil-bearing rocks. The Cambrian trilobites, for instance, descend from an ancestor that "must have lived long before the Cambrian age, and which probably differed greatly from any known animal". If his theory was true, he concluded, "it is indisputable that before the lowest Cambrian stratum was deposited long periods elapsed, as long as, or probably far longer than, the whole interval from the Cambrian age to the present day; and that during these vast periods the world swarmed with living creatures."^[4]

Then he looked at the clock. William Thomson allowed no more than four hundred million years since the Earth's crust consolidated, and James Croll put the interval since the Cambrian at about sixty million years. Darwin wrote that the 140 million years before that "can hardly be considered as sufficient" for the varied forms of life that already existed in the Cambrian.^[4] The ICS now puts the base of the Cambrian at 538.8 million years ago, almost nine times Croll's figure.^[2]

A trace fossil, a burrow, defines the base of the Cambrian. Its boundary stratotype is a coastal section at Fortune Head, near Fortune, on the Burin Peninsula of southeastern Newfoundland. The level lies 2.4 m above the base of Member 2 of the Chapel Island Formation, at the first occurrence of Trichophycus pedum, and it also marks the base of the Phanerozoic Eon and the Paleozoic Era.^[6]

Charles Walcott first visited the Mount Stephen area of British Columbia in 1907 for its trilobite beds, and on August 30, 1909 he "stumbled upon" the original Burgess Shale site. Its fossils are from the middle of the Cambrian, roughly 505 to 510 million years old, and about 150 species have been identified from Walcott's quarry.^[7] UNESCO dates the origin of the Burgess Shale to soon after "the rapid unfolding of animal life about 540 million years ago" and says the fossils record a marine community dominated by soft-bodied organisms.^[8]

Chengjiang, in Yunnan, China, is from the early Cambrian. UNESCO counts at least sixteen phyla and about 196 species there, preserved down to the gut of the arthropod Naraoia and the gills of Yunnanozoon, and says its sediments hold what are currently the oldest known fossil chordates, the phylum that includes all vertebrates. Its description places the diversification at 530 million years ago, "when almost all of today's major animal groups emerged".^[9]

Stephen Jay Gould read the Burgess Shale in Wonderful Life (1989) as evidence that animal history is contingent and would turn out differently if replayed. Simon Conway Morris, who had worked on the same fossils, argued the opposite case from convergence in The Crucible of Creation (1998).^[10][11]

Marshall's review of 2006 sorted the explanations into environmental, developmental and ecological groups. He concluded that the time of onset is constrained by the evolution of the environment and the duration mainly by the rate of developmental innovation. The rise in the number of body plans and in diversity, he wrote, is best understood as the result of the bilaterian developmental system meeting the growing number of needs that complex ecological interactions created.^[1]

Darwin and others read the Great Unconformity, the surface across which Cambrian shallow-marine sediments rest directly on much older crystalline basement, as a failure of the geological record. Shanan Peters and Robert Gaines argued in 2012 that the gap is a cause. Marine sediments of 540 to 480 million years ago record both an expansion of shallow seas and signs of increased ocean alkalinity and more intense chemical weathering of continental crust, and they proposed that the formation of the unconformity may have been an environmental trigger for the evolution of biomineralization.^[12]

The most discussed ecological candidate is vision. Andrew Parker proposed in In the Blink of an Eye (2003) that the evolution of eyes drove the explosion; the idea is treated in Light switch hypothesis.^[13] Fossils show that sharp vision existed by the middle of the event. In 2011 John Paterson and colleagues described paired eyes 2 to 3 cm across from the Emu Bay Shale of South Australia, about 515 million years old, assigned to the predator Anomalocaris. A single eye had at least 16,000 hexagonally packed lenses, which the authors compared with the most acute compound eyes of modern arthropods. They concluded that the compound eye evolved before features such as a hardened exoskeleton, and that a large predator with sharp vision "probably helped to accelerate the escalatory 'arms race' that began over half a billion years ago."^[14] The longer history of the organ is in Evolution of the eye.

• Light switch hypothesis
• Evolution of the eye
• The Seeing Eye
• Everybody's racing to build the brain
• Brain lab