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Concept
Cambrian explosion
Summary
The Cambrian explosion, Cambrian radiation, Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately in the Cambrian Period of early Paleozoic when there was a sudden radiation of complex life and practically all major animal phyla started appearing in the fossil record. It lasted for about 13 – 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification in other groups of organisms as well.
Before early Cambrian diversification, most organisms were relatively simple, composed of individual cells, or small multicellular organisms, occasionally organized into colonies. As the rate of diversification subsequently accelerated, the variety of life became much more complex, and began to resemble that of today. Almost all present-day animal phyla appeared during this period, including the earliest chordates.
A 2019 paper suggests that the timing should be expanded back to include the late Ediacaran, where another diverse soft-bodied biota existed and possibly persisted into the Cambrian, rather than just the narrower timeframe of the "Cambrian Explosion" event visible in the fossil record, based on analysis of chemicals that would have laid the building blocks for a progression of transitional radiations starting with the Ediacaran period and continuing at a similar rate into the Cambrian.
History of life
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.
Official source
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