EvolutionTHE HETEROTROPH HYPOTHESISEonHadian24-2Aggregates of organic moleculesCONCEPTUAL LIFE SCIENCEEvolutionTHE HETEROTROPH HYPOTHESISAccording to the heterotroph hypothesis, the first organisms to develop were heterotrophs. The autotrophic organisms developed after the heterotrophic ones.Primitive conditions on the EarthThe age of the Earth is thought to be 4.5 x 109 (4.5 billion) years. Evidence for this estimate comes from measurements of the decay of Uranium. This time is divided into four eons as listed in Table XV. In the earliest eon, there was a reducing atmosphere,much heat, lightning (electrical discharge), ultraviolet (UV) light from the Sun and radiation. There was no ozone layer in the earliest days of the Earth. To form an ozone layer requires oxygen. Oxygen is the waste product of photosynthesis. Photosynthesis did not develop until after the formation of the first eukaryotic cells, about 2.5 billion years ago. Therefore, without an ozone layer, UV light came all the way down to the surface.There are no rocks found so far that go back to the formation of the Earth. Geological processes of erosion, transport and deposition have worn away the original rocks. There have been rocks found in Canada and Australia that are dated in the range of 3.9 – 4.1 billion years. Table XV. Eons of TimeEon Duration (Millions of Years) Hadian 4500 – 4000 Archean 4000 – 2500 Proterozoic 2500 - 600 Phanerozoic 600 – Today Synthesis reactionsA reducing atmosphere of ammonia (NH3), methane (CH4), hydrogen (H2) and water (H2O) has been shown to be able to produce such molecules as urea, hydrogen cyanide, organic acids and other molecules under laboratory conditions by random synthesis reactions with each other. There was no oxygen in the early atmosphere but there was some CO2 present. The molecules produced experimentally in Miller’s spark discharge apparatus were similar to biological molecules. For example, amino acids werefound.24-124-2Aggregates of organic moleculesAggregates are collections of organic molecules that may have formed along the shorelines of primitive seas where the molecules were exposed to ultraviolet light. Radiation on rocks causes them to heat up. There is evidence that when organic molecules are heated together dry, they react. Fox took dry amino acids and heated them together. Water was released that condensed on the inside of the test tube at the top. Thiscondensation resulted from reactions between the water molecules. Thus the origin of theterm “condensation reaction.”Reproduction of molecular aggregates and coascervatesCertain larger aggregates (coascervates) might have begun to divide after reachinga certain size. Experiments have shown that CH4, NH3 and H2O can yield bases like those found in DNA.Anaerobic respirationFermentation is found in all cells, not only the anaerobic bacteria but even those that use aerobic respiration. So, anaerobic respiration must have developed first. The first organisms probably got their energy by fermenting organic molecules that were dissolved in the water.Development of autotrophsSome organisms must have developed the means by which to perform photosynthesis. The resulting release of oxygen over time led to the formation of the oxidizing atmosphere we have today.Aerobic respirationThose organisms that could tolerate and use oxygen went on to become aerobic. It is thought that eukaryotic cells developed as a result of one kind of prokaryote living inside another. EndosymbiosisThis explanation is called the endosymbiotic theory. According to this explanation, mitochondria that we have today were originally a form of prokaryote that had a knack for producing energy. Similarly, chloroplasts are thought to have resulted from other prokaryotes that could trap solar energy. Evidence for this theory comes from the fact that mitochondria and chloroplasts have their own DNA molecules, independent of the DNA in the chromosomes of the nucleus of the cell. This DNA is closely related24-3to bacterial DNA, not nuclear DNA. Also, mitochondria and chloroplasts have their own ribosomes. These ribosomes are closely related to bacterial ribosomes and are distinctly different from the cytoplasmic ribosomes found in the eukaryotic cell.THE GEOLOGIC TIME SCALELife began in the proterozoic eon, about 2.5 billion years ago. This is also known as the Pre-Cambrian era. As shown in Table XVI, an era is a subdivision of an eon. Each era is further subdivided into periods. The periods correspond to major divisions in geologic (and biologic) history. Geologists use biology as evidence for different periods. Up until the discovery of radioactivity in 1895, there was uncertainty about the age of theEarth or different layers of rocks. Geologists then and now use fossils to indicate information about the age and history of various rocks. The most recent period, the one we are in now, is known as the Quaternary Period. Glaciations of the Quaternary periodThere were four major glaciations (ice ages) in the Quaternary period. The earliest ice age began 600,000 years ago. Others occurred beginning at 500,000 years and 250,000 years. The last one began at 70,000 years and ended 20,000 years ago.THE FOSSIL RECORDThe oldest rocks are over 3.5 billion years old. Fossils are the remains of living things or are objects that were made by living things. These are left in sand or mud that will eventually be turned into sedimentary rock. Teeth and bones make good fossils as well as tools of primitive humans and worm burrows.In undisturbed rock layers, the oldest fossils (indicating the oldest rocks) should be found in the oldest layers. This is the case in the Grand Canyon that exposes approximately 2.5 billion years of Earth history in one place. Relatedness between fossils is based on comparative anatomy.EARLY MAMMALSThe first mammals came into existence as early as 1.5 x 108 (150,000,000) years ago. The earliest primates developed about 70,000,000 years ago. The earliest monkeys came into being some 40,000,000 years ago. The most primitive of them already had the same dental pattern as modern humans.24-4Table XVI. Geologic time scale.ERA PERIOD EVENTS BEGAN* EPOCHCenozoic Quaternary Age of humans 0.01 HoloceneFour major ice ages 2 PleistoceneTertiary Increase in mammals 7 PlioceneEvolution of higher animals 25 MioceneIncrease 36 Oligocene in