Learning Objectives
22.1 Origin of Life on Earth
• Outline the four overlapping stages that are hypothesized to result in cellular life as we currently understand it.
• Describe the process of chemical selection, and its relevance in the theory of the RNA world.
22.2 Fossils
• Describe how fossils are formed.
• Explain the importance of the discovery of transitional fossils.
• Explain the various biases that affect our interpretation of the fossil record.
22.3 History of Life on earth
• Place the appearance of prokaryotic and eukaryotic organisms within the four eons of geologic time.
• List several environmental influences on the emergence and disappearances of species.
• Trace the evolution of single-celled and multicellular eukaryotes through the different eras and periods in the geological time scale.
Chapter Synopsis
Many of the properties that we associate with life do not sufficiently describe only life. The seven properties that best do so are: cellular organization, sensitivity, growth, development, reproduction, regulation, and homeostasis. Finally, all organisms possess heredity, a mechanism for the preservation of improvement and the driving force of evolution. There are three probable explanations for the presence of life on earth. Panspermia and spontaneous origin are the only testable hypotheses currently available.
There is great scientific controversy as to the conditions on the early earth and the likely locations that spurred the origin of life on earth. The early earth was physically quite different from the world of today, perhaps possessing an atmosphere with some of the same gases, but notably devoid of oxygen. That, as well as the presence of various hydrogen compounds, constitutes its description as a reducing atmosphere. The reactions that occurred billions of years ago could not occur today partly because of the lack of that unique atmosphere. Experiments simulating these early conditions produced various organic compounds ultimately including the amino acids and nucleic acids required for life. The first protocells possessed elements of the properties of life and became true cells with the development of heredity. The current explanations of the origin of cells derive from Oparin’s bubble theory.
The earliest fossil cells closely resemble present day eubacteria. A few unique archaebacteria have survived unchanged through billions of years and are relics of those very diverse first life forms. Archaebacteria are prokaryotic in structure, but are significantly different from eubacteria in their cell wall and membrane structure. The membrane structures of modern eukaryotes probably evolved from invaginations of the membranes of early prokaryotes. The theory of endosymbiosis explains how certain organelles of eukaryotes likely evolved from a variety of prokaryotes. The first evidence of eukaryotes, cells with distinct nuclei, appeared after 1.5 billion years. All living organisms other than bacteria evolved from these forms.
The existence of life on other worlds is a mathematical certainty. Recent examination of Mars meteorites provides supporting evidence for the existence of life there. Future investigation of other celestial objects is necessary to substantiate the presence of life elsewhere.
Chapter Materials
22.1 Origin of Life on Earth
22.2 Fossils
22.3 History of Life on earth
PowerPoint Presentations (click link to download)
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