Laboratory simulations of early Earth conditions have produced organic polymers The forming of complex organic molecules, or polymers, from simpler building-block molecules may have been inevitable on the primitive Earth. Polymers = Chains of similar building blocks or monomers D. Protobionts can form by self-assembly Living cells may have been preceded by protobionts. Protobionts = Aggregates of abiotically produced molecules able to maintain an internal environment different from their surroundings and exhibiting some life properties such as metabolism and excitability There is experimental evidence for the spontaneous formation of protobionts: - When mixed with cool water, proteinoids self-assemble into microspheres surrounded by a selectively permeable protein membrane. E. RNA was probably the first genetic material Today's cells transcribe DNA into RNA, which is then translated into proteins. This chain of command must have evolved from a simpler mechanism of heritable control. - One hypothesis is that before DNA, there existed a primitive mechanism for aligning amino acids along RNA molecules, which were the first genes. - Once this simple machinery for replication and translation of genetic information became sequestered into membrane-bound protobionts, molecular cooperation could be refined as natural selection acted on the level of the entire protobiont. Initially, RNA could have provided the template to produce DNA. -Because it is more stable, DNA would have replaced RNA as the store of genetic information. RNA's role would change as it became an intermediate in translation. G. Debate about the origin of life abounds No one knows how life actually began on Earth. The chemical evolution described and supporting lab simulations indicate key steps that could have occurred. Several other alternatives have been proposed. III. The Major Lineages of Life A. Arranging the diversity of life into the highest taxa is a work in progress Systematists have traditionally considered the kingdom to be the highest, most inclusive taxonomic category. - The two kingdom system (animals and plants) long prevailed, but was not suitable as biologists learned more about the structures and life histories of different organisms. - The five kingdom system was proposed by Robert H. Whittaker (1969) and modified by Lynn Margulis. Classifying living systems is a work in progress that reflects our increased understanding of the phylogeny of living organisms. - Using the tools of molecular systematics, biologists have gathered new data that leads them to challenge the traditional five-kingdom system. - This new information has reopened issues of biological diversity at the highest taxonomic levels. These alternative classification systems may replace the five kinddom system.
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