CONCEPTUAL LIFE SCIENCE Evolution THE HETEROTROPH HYPOTHESIS According to the heterotroph hypothesis, the first organisms to develop were heterotrophs. The autotrophic organisms developed after the heterotrophic ones. Primitive conditions on the Earth The 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 Time Eon Hadian Archean Proterozoic Phanerozoic Duration (Millions of Years) 4500 – 4000 4000 – 2500 2500 - 600 600 – Today Synthesis reactions A 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 were found. 24-1 24-2 Aggregates of organic molecules Aggregates 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. This condensation resulted from reactions between the water molecules. Thus the origin of the term “condensation reaction.” Reproduction of molecular aggregates and coascervates Certain larger aggregates (coascervates) might have begun to divide after reaching a certain size. Experiments have shown that CH4, NH3 and H2O can yield bases like those found in DNA. Anaerobic respiration Fermentation 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 autotrophs Some 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 respiration Those 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. Endosymbiosis This 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 related 24-3 to 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 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 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 the Earth 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 period There 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 RECORD The 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 MAMMALS The 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. 24-4 Table XVI. Geologic time scale. ERA Cenozoic PERIOD Quaternary Tertiary Mesozoic Cretaceous Jurassic Triassic Paleozoic Permian Carboniferous Devonian Silurian Ordovician Cambrian Pre-Cambrian EVENTS Age of humans Four major ice ages Increase in mammals Evolution of higher animals Increase in diversity of plants Extinction of dinosaurs Diversity of flowering plants Origin of primates Origin of mammals and birds Dominance of dinosaurs Beginning of dinosaurs Dominance of gymnosperms Rise of flowering plants Reptiles spread and develop Abundant amphibians Reptiles appear Growth of great coal forests Age of fishes First amphibians First large forests on land First land plants First fishes and vertebrates Age of marine invertebrates Beginning of life—at least 5x longer than all geologic time following BEGAN* 0.01 2 7 25 36 58 65 135 EPOCH Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene 180 225 270 350 400 440 500 600 *Dates are in millions of years. Fossil record of early humans Information about the early ancestors of humans is derived from the study of fossil bones and skulls. In the Miocene epoch lived Proconsul. Proconsul may have been the common ancestor of chimpanzees, gorillas and humans. It is 25 million years old and had teeth like those of apes. In the Pliocene epoch lived Ramapithecus. This animal had teeth more like those of the hominids. It may have been the earliest hominid (ape-man). It is 12 million years old. The hominid family includes humans. All hominids other than humans are extinct. 24-5 Early Pleistocene hominids Early Pleistocene hominids lived in the Old Stone Age. Australopithecus afarensis (southern ape of the Afar region in Africa) lived in East Africa at least 3 million years ago. It then evolved into the gracile A. africanus and the robust A. transvaaliensis at about 2 million years ago. Gracile organisms have delicate boning whereas robust organisms have very heavy boning. The robust ones lived for about 1 million years then died out. Humans are gracile. The gracile ancestors evolved into the genus Homo. More recently in the Pleistocene lived Homo habilis. It dates from about 1.8 to 2 million years ago. Homo is the genus that includes humans. All members of the genus Homo, other than Homo sapiens sapiens are extinct. Homo habilis, like Australopithecus was small but had distinct differences in the skull bones and teeth. Middle Pleistocene humans Homo erectus had a more advanced Stone Age culture called Pre-Mousterian. It had the ability to use fire and create simple tools. Some specimens have been given names such as Java Man. Homo erectus developed in Africa about 1.6 million years ago. Homo erectus and Australopithecus lived in the same general areas of Africa and Europe. Australopithecus may have died out because Homo erectus was more advanced and could compete more easily. The earliest prototype Homo sapiens is dated at about 300,000 years ago. During the last ice age it evolved into two sub-species, Homo sapiens neanderthalis and Homo sapiens sapiens. Steinheim Man is of the Neanderthal type. It had Mousterian culture which included ritual burial. The late Pleistocene Homo sapiens neanderthalis lived between 130,000 and 35,000 years ago. In Europe it lived alongside Homo sapiens sapiens. The two sub-species eventually interbred to form the modern human, Homo sapiens sapiens. 24-6 Table XVII. Evolutionary Trends in Primates 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Primitive five-digit hand and foot pattern Free mobility of thumb and big toe Flattened nails instead of sharp claws Increased facial angle Perfection of visual apparatus Reduction in apparatus of smell Increase in size and complexity of the brain Development of uterus and more efficient nourishment of the fetus in pregnancy Erect posture and development of bipedalism Prolongation of post-natal life periods