Chapter 25 The History of Life on Earth 1 Chapter 25 The History of Life on Earth Lost World invertebrates above the species level Fossils reveal that 500 million years ago the ocean waters surround Antarctica were warm and teeming with tropical The sweeping changes in life on earth as revealed by fossils illustrate macroevolution the broad pattern of evolution Macroevolutionary changes include the emergence of terrestrial vertebrates though a series of speciation events the impact of mass extinctions on the diversity of life Concept 25 1 Conditions on early earth made the origin of life possible Direct evidence of life on earth comes from fossils of micro organisms that are about 3 5 billion years old They hypothesize that chemical and physical processes on earth aided by emerging force of natural selection could have produced very simple cells through a sequence of four main stages The abiotic nonliving synthesis of small organic molecules such as amino acids and nitrogenous bases The joining of these small molecules into macromolecules such as proteins and nucleic acids The packaging of these molecules into protocells droplets with membranes that maintained an internal chemistry different from that of their surroundings The origin of self replicating molecules that eventually made inheritance possible Synthesis of Organic Compounds on Early Earth There is scientific evidence that earth and the other planets of the solar system formed about 4 6 billion years ago condensing from a vast cloud of dust and rocks that surrounded the young sun The phase in which the rock and ice left over from the formation of the solar system ended about 4 2 The first atmosphere was probably thick with water vapor along with N O CO2 CH4 NH3 H As earth cooled the water vapors condense into oceans and much of the hydrogen escaped into 3 9 billion years ago space Miller Urey type experiments demonstrate that the abiotic synthesis of organic molecules is possible under various assumptions about the composition of the early atmosphere A second source of organic molecules may have been meteorites Abiotic Synthesis of Macromolecules Every cell has a vast assortment of macromolecules including enzymes and other proteins and the nucleic acids that are essential for self replication The abiotic synthesis of RNA monomers can occur spontaneously from simple precursor molecules Protocells All organisms must be able to carry out reproduction and energy processing metabolism This suggests that self replicating molecules and a metabolism like source of the building blocks may have appeared together in early protocells The necessary conditions may have been met in vesicles fluid filled compartments bounded by a membrane like structure Chapter 25 The History of Life on Earth 2 Abiotically produced vesicles can exhibit certain properties of life including simple reproduction and metabolism as well as the maintenance of internal chemical environments different from that of their surroundings Abiotically produced vesicles can reproduce on their own and they can increase in size Some vesicles have a selectively permeable bilayer and can perform metabolic reactions using an external source of reagents another important prerequisite for life Self Replicating RNA and the Dawn of Natural Selection The first genetic material was most likely RNA not DNA RNA plays a central role in protein synthesis can also carry out a number of enzyme like catalytic Ribozymes RNA catalysts Some ribozymes can make complementary copies of short pieces of RNA provided that they are supplied with nucleotide building blocks Natural selection on the molecular level has produced ribozymes capable of self replication in the laboratory Single stranded RNA molecules assume a variety of specific 3D shapes mandated by their nucleotide functions sequences The RNA molecule whose sequence is best suited to the surrounding environment and has the greatest ability to replicate itself will leave the most descendant molecules The molecular biology of today may have been preceded by an RNA world in which small RNA molecules that carried genetic information were able to replicate and to store information about the vesicles that carried them A vesicle with self replicating catalytic RNA would differ from its many neighbors If a vesicle could grow split and pass its RNA molecules to its daughters the daughters would be protocells that had some of the properties of their parent The most successful of the early protocells would have increased in number because they could exploit their resources effectively and pass their resources effectively and pass their abilities on to subsequent generations Accurate replication was advantageous as genomes grew larger through gene duplication and other processes and as more properties of the protocells became coded in genetic information Concept 25 1 Summary Conditions on early Earth made the origin of life possible Earth formed 4 6 billion years ago Experiments simulating possible early atmospheres have produced organic molecules from inorganic precursors Amino acids lipids sugars and nitrogenous bases have also been found in meteorites Concept 25 2 The fossil record documents the history of life The fossil record is based primarily on the sequence in which fossils have accumulated in sedimentary rock layers called The Fossil Record strata The known fossil record is biased in favor of species that existed for a long time were abundant and wide spread in certain kinds of environments and had hard shells skeletons or other parts that facilitated their fossilizations How rocks and fossils are dated Chapter 25 The History of Life on Earth 3 While the order of fossils in rock strata tells us the sequence in which the fossils were laid down their relative ages it does not tell us their actual absolute ages Radiometric dating the most common technique of dating based on the decay of radioactive isotopes The rate of decay is expressed by the half life the time required for 50 of the parent isotope to decay The half life is not affected by temperature pressure or other environmental variables When an organisms dies its tops accumulating carbon and the amount of carbon 12 in its tissues does not change over time eons synapsids organisms over time and other methods colonization of land Carbon dating works for fossils up to about 75 000 years old fossils older than that contain too little carbon 14 to be detected with current techniques