Summary

Concept 18.1: Events in Earth’s History Can Be Dated

• The relative ages of organisms can be determined by the dating of fossils and the strata of sedimentary rocks in which they are found.
• Radiometric dating techniques use a variety of radioisotopes with different half-lives to date events in the remote past. Review Figure 18.1
• Geologists divide the history of life into eons, eras, and periods, based on major differences in the fossil assemblages found in successive strata. Review Table 18.1

Concept 18.2: Changes in Earth’s Physical Environment Have Affected the Evolution of Life

• Earth’s crust consists of solid plates that float on fluid magma. Continental drift is caused by convection currents in the magma, which move the plates and the continents that lie on top of them. Review Figure 18.2 and ANIMATED TUTORIAL 18.1
• Major physical events on Earth, such as continental collisions and volcanic eruptions, have affected Earth’s 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 18.3 and Table 18.1
• 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 Figures 18.7 and 18.8

Concept 18.3: Major Events in the Evolution of Life Can Be Read in the Fossil Record

• Paleontologists use fossils and evidence of geological changes to determine what Earth and its biota may have looked like at different times. Review Figures 18.11 and 18.12
• Before the Phanerozoic, life was almost completely confined to the oceans. Multicellular life diversified extensively during the Cambrian explosion, a prime example of an evolutionary radiation.
• The periods of the Paleozoic era were each characterized by the diversification of specific groups of organisms. During the Mesozoic era, distinct terrestrial biotas evolved on each continent.
• Five episodes of mass extinction punctuated the history of life in the Paleozoic and Mesozoic eras.
• The Cenozoic era is divided into the Tertiary and the Quaternary periods, which in turn are subdivided into epochs. This era saw the emergence of the modern biotas as mammals radiated extensively and flowering plants became dominant. Review Table 18.2
• The tree of life can be used to reconstruct the timing of evolutionary events.

See ACTIVITY 18.1 for a concept review of this chapter.