~.. CONCEPTUAL LIFE SCIENCE EvoJution THE HETEROTROPH HYPOTHESIS According to the heterotroph hypothesis, the fust organisms to develop were heterotropbs. The autotrophic organisms developed after the heterotrophic ones. . Primitive conditions OD the Earth The age ofthe Earth is thought to be 4.5 x J(f (4.5 billion) years. Evidence for this estimate comes from measurements ofthe decay ofUranimn. 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 Sm and radiation. ~was DO ozone layer in the earliest days ofthe Earth. To fOlman ozone layer requires oxygen. Oxygen is the waste produet ofphotosynthesis. Photosyn1hesis did not develop mtil after the fmmatioD ofthe 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 DO rocks·found so far that go back to the formation ofthe Earth. .Geological processes oferosion, 1JaDSpOI't and deposition have worn away the original rocks. There have been rocks found in Canada IIDd Australia that IR dated in the nmge of3.9-4.1 billionyears. Table XV. Eons ofTime Eon Duration (MillioDS ofYears) . Hadian 4500-4000 Archean 4000-2500 Proterozoic 2500-600 Phanerozoic 6OO-Todav . Synthesis reactions Areducing atmosphere ofammonia (NH3), methane (CFL), hydrogen (Hv and . water (H~) bas been shown to be able to produce such molecules as urea, hydrogezl cyanide, organic acids and other molecules lDJder laboratory conditions by random synthesis reactions with each other. There was DO oxygen in the early atmo~ but there was some C~ present. The molecules produced experimentally in Miller's'" discharge apparatus were similar to biological molecules. For example, amino acids·'W\ft found. .. 24-1I • . , 24-2 Aggregates oforganicmolecules Aggregates are collectionsoforganic molecules that may have formed along the shorelinesofprimitiveseaswherethemoleculeswere exposedto ultravioletlight. 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 onthe insideofthetest tube at thetop. This condensation resulted from reactions between the water molecules. Thus the origin ofthe term "condensation reaction." Reproduction ofmolecular aggregates and coascervates Certain larger aggregates (coascervates) might have begun to divide after reaching a certain size. Experiments have shown that ~, NH3 and H20 am yield bases like those found in DNA. .Anaerobic respiration Fennentation 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 molecu1es that were dissolved in the water. Development ofautotrophs Some organisms must have developed the means"by which to perform photosynthesis. The resultingreleaseofoxygenovertimeledto the fOJ'DUltjon ofthe . oxidizing atmosphere we have today. . Aerobic respiration .Those organisms that could tolerate and use oxysen went on to become aerobic. Itisthought that eukaryoticcellsdeveloped asa result ofone lciDd ofprokaryote living inside another. Endosymbiosis This explanation is called the endosymbiotic theory. Acconfing 10 this explanation, mitochondria that we have today were originally a form ofprokaryote that had a knack for producing energy. Similarly, chJoroplasts are thought to have resulted . from other probJ'yotes that could trap solarenergy. Evidence for this theory comes fiom the fact that mitochondria aDd chJoroplasts have their OWD DNA molecules, independent ofthe DNA in the chromosomes ofthe nucleusofthe cell. This DNA is~losely mated24-3 to bacterial DNA, not nuclear DNA. Also, mitochondria and chloroplasts have their own ribosomes. These ribosomes are closely related to bacterial n'bosomes and are distinctly different from the cytoplasmic ribosomes found in the eukaryouc ceD. THE GEOLOGIC TIME SCALE Life 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 of8D eon. Each era is further subdivided into periods. The periods conaponcl to major divisiODS in geologic (and biologic) history. Geologists use biology as evidence for differeDtperiods. Upaulthediscoveryofradioactmty in ]895, there was uncertainty about the age ofthe Earth or different layers ofrocks. Geologists then and DOW use fossils to indicate information about the age and history ofvarious rocks. The most recent period, the one we are in DOW, is known as the (luDtemary Period. Glaciations ofthe QuatemarY period There were four major glaciations (ice ages) in the Quaternary period. The earliest ice age began 600,000 years 880. OthCJS occuned beginning at 500,000 years and 250,000 years. The last one began at 70,000 years and ended 20,000 years 880. .. . mE FOSSu.. RECORD The oldest rocks are over 3.5 billion years old. Fossils are the remains ofliving 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 Sood fossils as well as tools ofprimitive hUJDIIDS and worm bmrows. 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 yean ofEarth history in one place. Relatedness betw=1 . fossils is based on comparative anatomy. EARLY MAMMALS The first mammals came iDto existence as early as 1.5 xla' (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 oftbem already had the same dental pattern as modem hlDDlDS.r • 24-4 Table XVI. Geologic time scale. ERA PERIOD EVENTS BEGAN· EPOCH Cenozoic Aee ofbumans 0.0] Ho]occoe FOlD' maior ice I2tS 2 Pleistoc:c:De TertiarY lncrease in mammals 7 P]ioccoe Evolution ofbil!her animals 25 Mioc:eae 1ncreIse 36 Oliaocene in diversitv 58 Eocene ofplants 65 Paleocene Mesozoic Cretaceous Extine:ticm ofdinosaurs 135 . of OoweriJu! Plants rieinof Jurusic riein ofmammals and birds ]80 DomiJJance ofdinosaurs Triassic Be.....nn...