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Overview: Usually, extinction operates at a fairly constant rate, culling some species while speciation generates new ones. However, at a few points in life's history the humdrum of regular extinction has been interrupted by mass extinctions. Learn more about these biodiversity crises. Author/Source: UC Museum of Paleontology Grade level: 13-16 Time: 30 min Concepts: Correspondence to the Next Generation Science Standards is indicated in parentheses after each relevant concept. See our conceptual framework for details. During the course of evolution, only a small percentage of species have survived until today. Background extinctions are a normal occurrence. Mass extinctions occur. Rates of extinction vary. Extinction can result from environmental change. Human influence may be causing a modern mass extinction. Extinctions may create opportunities for further evolution in other lineages to occur. Scientists use fossils (including sequences of fossils showing gradual change over time) to learn about past life. Scientists use physical, chemical, and geological evidence and comparative anatomy to establish the age of fossils. Teacher background: Geologic Time History of Life

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