What Is Life?
• All organisms on the earth share several fundamental properties of life. The foremost of these properties is heredity; others include cellular organization, sensitivity, growth, development, reproduction, regulation, and homeostasis. (p. 63)
4.2 There is considerable disagreement about the origin of life.
Hypotheses About the Origin of Life
• In principle, the three possible explanations for the origin of life are special creation, extraterrestrial origin, and spontaneous origin. The scientific viewpoint can only address the second and third possibilities because they permit testable hypotheses. (p. 64)
Scientists Disagree About Where Life Started
• Many scientists believe the earth's early atmosphere was a reducing atmosphere of carbon dioxide and nitrogen gas, with very little oxygen. (p. 65)
• Other locations where life may have originated include at the ocean's edge, under frozen seas, deep in the earth's crust, within clay, and at deep-sea vents. (p. 65)
The Miller-Urey Experiment
• Miller and Urey's classic experiment attempted to reproduce a model of earth's possible reducing atmosphere and produce complex organic molecules. (p. 66)
• Subsequent chemical evolution on the earth is argued among many groups, including the RNA-first group, the protein-first group, and a peptide-nucleic acid group. (pp. 66—67)
4.3 There are many hypotheses about the origin of cells.
Ideas About the Origin of Cells
• Many different versions of bubble hypotheses have been championed by numerous scientists. (p. 68)
• Current hypotheses encompass chemical evolution within bubbles, but whether early bubbles were lipid or protein remains unsolved. (p. 69)
4.4 Cells became progressively more complex as they evolved.
The Earliest Cells
• Documented microfossils have been found that are as old as 2.5 billion years. (p. 70)
• Currently, prokaryotes are either archaebacteria or bacteria. (pp. 70—71)
The First Eukaryotic Cells
• Eukaryotes seemed to appear about 1.5 billion years ago. They seem to be much larger than prokaryotes and contain internal membranes and thicker cell walls. (p. 72)
• Endosymbiotic theory suggests that energy-producing bacteria may have come to reside within larger bacteria, eventually evolving into mitochondria. (pp. 72—73)
• Sexual reproduction allows for frequent genetic recombination and serves as the raw material for evolution. (p. 73)
• Multicellularity has arisen many times among the eukaryotes, and has promoted the development of diversity. (p. 73)
4.5 Scientists are beginning to take the possibility of extraterrestrial life seriously.
Has Life Evolved Elsewhere?
• Because at least 10% of stars are thought to have planetary systems, it is theoretically possible that life may have evolved many times. (p. 76)
• Currently, the Jupiter moon Europa is the most promising candidate to hold life because satellite images have revealed liquid oceans under the ice. (p. 76)