The appearance of the first abundant skeletal organisms in the earliest Cambrian was a quantum leap in the evolutionary history of life. This paper demonstrates that this evolutionary process resulted from a combination of environmental and biological factors. The author maintains that the appearance of the oldest shelly fossils is a reasonable criterion for defining the Precambrian-Cambrian boundary. This arises not only from the viewpoint of taking the Cambrian Period as the first period in the Paleozoic Era, but is also a logical extension of regarding the history of life as being divisible into a series of developmental stages.--Modified journal abstract.

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