19.1 The Fossil Record

From the fossil record, paleontologists learn about the structure of ancient organisms, their environment, and the ways in which they lived.

Relative dating allows paleontologists to determine whether a fossil is older or younger than other fossils. Radiometric dating uses the proportion of radioactive to stable isotopes to calculate the age of a sample.

The geologic time scale is based on both relative and absolute dating. The major divisions of the geologic time scale are eons, eras, and periods.

Building mountains, opening coastlines, changing climates, and geological forces have altered habitats of living organisms repeatedly throughout Earth history. In turn, the actions of living organisms over time have changed conditions in the land, water, and atmosphere of planet Earth.

• extinct (538)

• half-life (540)

• paleontologist (539)

• geologic time scale (542)

• relative dating (540)

• era (543)

• index fossil (540)

• period (543)

• radiometric dating (540)

• plate tectonics (544)

19.2 Patterns and Processes of Evolution

If the rate of speciation in a clade is equal to or greater than the rate of extinction, the clade will continue to exist. If the rate of extinction in a clade is greater than the rate of speciation, the clade will eventually become extinct.

Evidence shows that evolution has often proceeded at different rates for different organisms at different times over the long history of life on Earth.

Two important patterns of macroevolution are adaptive radiation and convergent evolution. Adaptive radiation occurs when a single species or a small group of species evolves over a relatively short time into several different forms that live in different ways. Convergent evolution occurs when unrelated organisms evolve into similar forms.

The relationship between two coevolving organisms often becomes so specific that neither organism can survive without the other. Thus, an evolutionary change in one organism is usually followed by a change in the other organism.

• macroevolutionary patterns (546)

• background extinction (548)

• mass extinction (548)

• gradualism (549)

• punctuated equilibrium (549)

• adaptive radiation (550)

• convergent evolution (551)

• coevolution (551)

19.3 Earth's Early History

Earth's early atmosphere contained little or no oxygen. It was principally composed of carbon dioxide, water vapor, and nitrogen, with lesser amounts of carbon monoxide, hydrogen sulfide, and hydrogen cyanide.

Miller and Urey's experiment suggested how mixtures of the organic compounds necessary for life could have arisen from simpler compounds on a primitive Earth.

The “RNA world” hypothesis proposes that RNA existed by itself before DNA. From this simple RNA-based system, several steps could have led to DNA-directed protein synthesis.

The endosymbiotic theory proposes that a symbiotic relationship evolved over time between primitive eukaryotic cells and the prokaryotic cells within them.

The development of sexual reproduction sped up evolutionary change because sexual reproduction increases genetic variation.

• endosymbiotic theory (556)

Think Visually Construct a table comparing the Paleozoic, Mesozoic, and Cenozoic eras. Include the approximate time periods for each era and identify the characteristic organisms.