Section 2

Earth's History

• Scientists think that Earth formed by the gravitational accumulation of dust and debris moving through space.

• Isotopes are atoms with varying numbers of neutrons. The ages of rocks and other materials can be determined by measuring the amount of radioactive decay that has occurred in radioactive isotopes found in samples of those materials. An isotope's half-life is the time that one-half of a sample of the isotope takes to decay.

• The first simple organic compounds on early Earth may have formed under conditions of high energy and in an atmosphere very different from that of today's Earth.

• Meteorites may have brought organic compounds to Earth.

• Further chemical reactions may have converted simple organic compounds into the macromolecules important to life. Lightning, ultraviolet radiation, or heat from within the Earth could have provided the energy for these reactions. These conditions have been experimentally modeled.

• Cell-like structures, including microspheres and coacervates, form spontaneously in certain kinds of solutions. These structures could have been a step in the formation of modern cells but lack hereditary material.

• Scientists continue to investigate many hypotheses about the origins of organic molecules and cells in Earth's history.

Vocabulary radiometric dating (p. 282) isotope (p. 282)

mass number (p. 283) radioactive decay (p. 283)

radioactive isotope (p. 283) half-life (p. 283)

microsphere (p. 286) coacervate (p. 286)

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