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The Cambrian Explosion
The geologic timescale is marked by significant geologic and biological events, including the origin of Earth about billion years ago, the origin of life about billion years ago, the origin of eukaryotic life-forms (living things that have cells with true nuclei) about billion years ago, and the origin of animals about billion years ago. The last event marks the beginning of the Cambrian period. Animals originated relatively late in the history of Earth—in only the last 10 percent of Earth’s history. During a geologically brief 100-million-year period, all modern animal groups (along with other animals that are now extinct) evolved. This rapid origin and diversification of animals is often referred to as “the Cambrian explosion.”
Scientists have asked important questions about this explosion for more than a century. Why did it occur so late in the history of Earth? The origin of multicellular forms of life seems a relatively simple pared to the origin of life itself. Why does the fossil record not document the series of evolutionary changes during the evolution of animals? Why did animal life evolve so quickly? Paleontologists continue to search the fossil record for answers to these questions.
One interpretation regarding the absence of fos