Chapter Summary

• The dates of evolutionary events can be determined by the dating of fossils and the strata of sedimentary rocks in which the fossils are found.

• Radiometric dating techniques use a variety of radioisotopes with different half-lives to date events at different times in the remote past. Review Figure 24.1

• Geologists divide the history of life into eons, eras, and periods. These divisions are based largely on major differences in the fossil assemblages found in successive strata. Review Table 24.1

• Earth’s crust consists of solid lithospheric plates that float on the viscous mantle. Continental drift is caused by convection currents in the mantle, which move the plates and the continents that lie on top of them. Review Focus: Key Figure 24.3, Animation 24.1

• Five episodes of mass extinction events punctuated the history of life in the Paleozoic and Mesozoic eras. Review Figure 24.2

• Major physical events on Earth, such as continental collisions and volcanic eruptions, have affected Earth’s surface, climate, atmosphere, and sea levels. In addition, extraterrestrial events such as meteorite strikes have created sudden and dramatic environmental shifts. All of these changes affected the history of life. Review Figure 24.4

• Oxygen-generating cyanobacteria liberated enough O₂ to open the door to oxidation reactions in metabolic pathways. Aerobic prokaryotes were able to harvest more energy than anaerobic organisms and began to proliferate. Increases in atmospheric O₂ concentrations supported the evolution of large eukaryotic cells and, eventually, multicellular organisms. Review Figure 24.8, Investigating Life: The Relationship between Atmospheric Oxygen Concentration and Body Size in Insects

• Extinctions of species happen throughout evolutionary history, but major, rapid environmental changes can produce mass extinctions of many species simultaneously. Review Figure 24.10

• Paleontologists use fossils and evidence of geological changes to determine what Earth and its biota may have looked like at different times. Review Figure 24.12

• Before the Phanerozoic, life was almost completely confined to the oceans. Multicellular life diversified extensively during the Cambrian explosion, an example of an evolutionary radiation. Review Figure 24.13

• The periods of the Paleozoic era were each characterized by the diversification of specific groups of organisms. The Paleozoic ended with the most drastic mass extinction in Earth’s history. During the Mesozoic era, distinct terrestrial biotas evolved on each continent. Review Figure 24.14

• Earth’s flora has been dominated by flowering plants since the Cenozoic era began.

• The tree of life can be used to reconstruct the timing of evolutionary events.