GOL 106 LAB 6 ANCIENT SEDIMENTARY ENVIRONMENTS Group ________ Member Names (1)_________________________________________________ (2)_________________________________________________ (3)_________________________________________________ Page Question(s) 35 1, 2, 3 36 All 37 All 38 All 39 All 40 All Chapter 6 Sedimentary Rocks (Part I) The Archives of Earth History 1 History from Sedimentary Rocks • Sedimentary rocks are rocks that are formed from the compaction and hardening of sediment • They account for about 5% (by volume) of Earth’s outer 10 miles – preserve evidence of surface depositional processes – and many contain fossils – These give clues to the depositional environment 2 Sedimentary Rocks • Sedimentary rocks – preserve evidence of the physical, chemical and biological processes that formed them • Some sedimentary rocks are resources, or contain resources – – – – petroleum natural gas coal phosphorus 3 Turning Sediment into Rocks • Many changes occur to sediment after it is deposited • Diagenesis – all of the chemical, physical, and biological changes that take place after sediments are deposited • Occurs within the upper few kilometers of Earth’s crust Diagenesis Includes Recrystallization – development of more stable minerals from less stable ones Lithification – unconsolidated sediments are transformed into solid sedimentary rock by – Compaction – Cementation by calcite, silica, and iron oxide 4 Types of Sedimentary Rocks • Sediment originates from mechanical and/or chemical weathering • Rock types are based on the source of the material • Detrital rocks – transported sediment as solid particles • Chemical rocks – sediment that was once in solution • Biogenic rocks – remains of once-living organism 5 Composition of Detrital Rocks • Very common minerals in detrital rocks are: – quartz, feldspars, and clay minerals • Detrital rock composition tells – about source rocks, – not transport and deposition • Quartz sand may have been deposited – in a river system – on a beach or – in sand dunes 6 Grain Size • Detrital grain size gives some indication – of the energy conditions during transport and deposition • High-energy processes such as swift-flowing streams and waves are needed to transport gravel • Conglomerate must have been deposited in areas where these processes prevail • Sand transport also requires high-energy transport • Silt and clay are transported – by weak currents and accumulate – only under low-energy conditions – as in lakes and lagoons 7 Sorting and Rounding • Texture refers to the size, distribution, shape, and arrangement of clasts • Sorting and rounding are two textural features – of detrital sedimentary rocks – that aid in determining depositional processes • Rounding is the degree to which – detrital particles have their sharp corners and edges – warn away by abrasion • Gravel in transport is rounded very quickly – as the particles collide with one another • Smaller particles in suspension – are usually not as rounded 8 Rounding • All of these stones are rounded – and have lost their sharp edges • The stone in the upper left is also spherical 9 Sorting • Sorting refers to the variation – in size of particles making up sediment or sedimentary rocks • If the size range is not very great, – the sediment or rock is well sorted • If they have a wide range of sizes, – they are poorly sorted • Wind has a limited ability to transport sediment – But glaciers can carry any size particles – Glacial deposits are poorly sorted, wind deposits are well10 sorted Rounding and Sorting • A deposit – of well rounded – moderately sorted gravel • Versus a deposit – of angular – poorly sorted gravel 11 Sedimentary Structures • Sedimentary structures are features that formed at the time of deposition or shortly thereafter – They manifest the physical and biological processes that operated in depositional environments • Structures seen in present-day environments or produced in experiments help provide information about depositional environments of rocks with similar structures 12 Bedding • Sedimentary rocks generally have bedding or stratification – Individual layers less than 1 cm thick are laminations • common in mudrocks – Beds are thicker than 1 cm • common in rocks with coarser grains 13 Graded Bedding • Some beds show an upward gradual decrease in grain size, known as graded bedding • Graded bedding is common in turbidity current deposits – which form when sediment-water mixtures flow along the seafloor – As they slow, – the largest particles settle out, 14 – then smaller ones Cross-Bedding • Cross-bedding forms when layers come to rest – at an angle to the surface – upon which they accumulate – as on the downwind side of a sand dune • Cross-beds result from transport – by either water or wind • The beds are inclined or dip downward – in the direction of the prevailing current • They indicate direction of the flow of ancient currents 15 Cross-Bedding • Individual beds are deposited at an angle • Horizontal bedding and crossbedding in Upper Cambrian St. Peter Sandstone in Wisconsin 16 Ripple Marks • Small-scale alternating ridges and troughs – known as ripple marks are common – on bedding planes, especially in sandstone • Current ripple marks – formed from wind or water flow – and have asymmetry – indicating the original flow direction • Wave-formed ripple marks – result from the to-and-fro motion of waves – and tend to be symmetrical 17 Current Ripple Marks • Ripples with an asymmetrical shape • The photo shows current ripples – that formed in a small stream channel – with flow from right to left 18 Wave-Formed Ripples • As the waves wash back and forth, – symmetrical ripples form • The photo shows waveformed ripple marks – in shallow seawater 19 Mud Cracks • When clay-rich sediments dry, they shrink – and crack into polygonal patterns – bounded by fractures called mud cracks • Mud cracks require wetting and drying to form, – as along a lakeshore – or a river flood plain – or where mud is exposed at low tide along a seashore 20 Ancient Mud Cracks • Mud cracks in ancient rocks – in Glacier National Park, Montana • Mud cracks can fill in – with sediment – when they are preserved – as seen here 21 Biogenic Sedimentary Structures • Biogenic sedimentary structures include – tracks – burrows – trails • These are called trace fossils • Extensive burrowing by organisms – is called bioturbation – and may alter sediments so thoroughly – that other structures are disrupted or destroyed 22 Bioturbation • U-shaped burrows • Vertical burrows 23 Bioturbation • Vertical, dark-colored areas in this rock are sediment-filled burrows 24 No Single Structure Is Unique • Sedimentary structures are important – for environmental analyses – but no single structure is unique to a specific environment • Example: – Current ripples are found • in stream channels • in tidal channels • Environmental determinations – are usually successful with – associations of groups of sedimentary structures – taken along with other sedimentary rock properties 25 Sedimentary Structures • Sedimentary structures and fossils allow geologists to resolve the history of an area when rocks have been deformed • Here, the mudcrack “V” opens toward younger strata, – and shape of current ripple marks • Indicate that the youngest layer is lower right 26 Blanket or Sheet Geometry • Some of the most extensive sedimentary rocks – in the geologic record result from – marine transgressions and regressions • The rocks commonly cover – hundreds or thousands of square kilometers – but are perhaps only a few tens to hundreds of meters thick • Their thickness is small compared to their length and width • Thus, they are said to have blanket or sheet geometry 27 Elongate or Shoestring Geometry • Some sand deposits have an elongate or shoestring geometry – especially those deposited in • stream channels • or barrier islands 28 Fossils—The Biologic Content of Sedimentary Rocks • Fossils – are the remains or traces of prehistoric organisms – can be used to establish biostratigraphic units – are important constituents of rocks, sometimes making up the entire rock – are important for determining depositional environments • Some rocks, especially limestones, are composed – largely of shells of marine-dwelling animals – or even the droppings of these organisms 29 Fossils Are Constituents of Sedimentary Rocks • This variety of limestone, – known as coquina, – is made entirely of shell fragments 30 Fossils in Environmental Analyses • Did the organisms in question live where they were buried? • Or where their remains or fossils transported there? • Example: – – – – – Fossil dinosaurs usually indicate deposition in a land environment such as a river floodplain But if their bones are found in rocks with clams, corals and sea lilies, we assume a carcass was washed out to sea 31 Environmental Analyses • What kind of habitat did the organisms originally occupy? • Studies of a fossil’s structure and its living relatives, if any, help environmental analysis • For example: – clams with heavy, thick shells typically live in shallow turbulent water – whereas those with thin shells are found in low-energy environments • Most corals live in warm, clear, shallow marine environments where symbiotic bacteria can carry out photosynthesis 32 Microfossils • Microfossils are particularly useful – because many individuals can be recovered from small rock samples • In oil-drilling operations, small rock chips – called well cuttings are brought to the surface • These cuttings rarely – contain complete fossils of large organisms, but they might have thousands of microfossils that aid in relative dating and environmental analyses 33 Trace Fossils In Place • Trace fossils, too, may be characteristic of particular environments • Trace fossils, of course, are not transported from their original place of origin 34 Depositional Environments • A depositional environment is anywhere sediment accumulates on Earth’s surface – especially a particular area where a distinctive kind of deposit originates from physical, chemical, and biological processes • Three broad environments of deposition are: – Marine Environment – Marginal Marine or (transitional) Environment – Non-Marine or (continental) Environment – Each has several specific environments 35 Depositional Environments Non-marine environments Marginal marine environments Marine environments 36 Non-marine or Continental Environments • Deposition on continents (on land) might take place in – – – – Fluvial systems – rivers and streams Aeolian systems – deserts Glacial systems – areas covered by glaciers Lacustrine system – lakes • Deposits in each of these environments – possess combinations of features that allow us to 37 differentiate among them Fluvial • Fluvial refers to river and stream activity – and to their deposits • Fluvial deposits accumulate in either of two types of systems – Braided stream system • with multiple broad, shallow channels • in which mostly sheets of gravel and cross-bedded sand are deposited • mud is nearly absent 38 Braided Stream Deposits • Braided stream deposits consist of • gravel – cross-bedded sand – but mud is rare or absent 39 QUESTIONS? 40 Chapter 6 Sedimentary Rocks (Part II) The Archives of Earth History 41 Fluvial Systems • The other type of system is a meandering stream – with winding channels – mostly fine-grained sediments on floodplains – cross-bedded sand bodies with shoe-string geometries – point-bar deposits consisting of a sand body overlying an erosion surface that developed on the convex side of a meander loop 42 Meandering Stream Deposits • In meandering stream deposits, – fine-grained floodplain sediment is common – with subordinate sand bodies 43 Aeolian or Desert Environments • Desert environments contain an association of features found in – sand dune deposits, – alluvial fan deposits, and – playa lake deposits • Wind-blown dunes are typically composed – of well-sorted, well-rounded sand – with cross-beds, meters to tens of meters high – land-dwelling plants and animals make up any fossils 44 Alluvial Fans and Playa Lakes • Alluvial fans form best along the margins of desert basins – where streams and debris flows discharge from mountains onto a valley floor – They form a triangular (fan-shaped) deposit of sand and gravel • The more central part of a desert basin – might be the site of a temporary lake, a playa lake, in which laminated mud and evaporites accumulate 45 Associations in Desert Basin • Huge alluvial fans formed at the base of the Panamint Mountains, Death Valley • Sand dunes also are present in Death Valley 46 Playa Lake • This playa lake near Fallon, Nevada – has deposits of rock salt forming 47 Glacial Environments • All sediments deposited in glacial environments are collectively called drift • Till is poorly-sorted, non-stratified drift, deposited directly by glacial ice mostly in ridge-like deposits called moraines • Outwash is sand and gravel deposited by braided streams issuing from melting glaciers • The association of these deposits along with scratched (striated) and polished bedrock is generally sufficient to conclude that glaciers were involved 48 Moraines and Till • Moraines and poorly sorted till 49 Glacial Varves • Glacial lake deposits show alternating dark and light laminations • Each dark-light couplet is called a varve – It represents one year’s accumulation of sediment – light layers accumulate in spring and summer – dark layers in winter • Dropstones – liberated from icebergs – may also be present 50 Marginal or Transitional Environments • Transitional environments include those – with both marine and continental processes • Example: – Deposition where a river or stream (fluvial system) enters the sea yields a body of sediment called a delta with deposits modified by marine processes, especially waves and tides • Transitional environments include – – – – deltas barrier islands lagoons tidal flats 51 Transitional Environments Transitional environments 52 Simple Deltas • The simplest deltas are those in lakes. They consist of – topset beds – foreset beds – bottomset beds – As the delta builds outward, it progrades – and forms a vertical sequence of rocks that becomes coarser-grained from the bottom to top – The bottomset beds may contain marine (or lake) fossils, – whereas the topset beds contain land fossils 53 Marine Deltas • Marine deltas rarely conform precisely – to this simple threefold division because – they are strongly influenced by one or more modifying processes • When fluvial processes prevail – a stream/river-dominated delta results • Strong wave action – produces a wave dominated delta • Tidal influences – result in tide-dominated deltas 54 Stream/River-Dominated Deltas • Stream/riverdominated deltas – have long distributary channels – extending far seaward – Mississippi River delta 55 Wave-Dominated Deltas • Wave-dominated deltas – such as the Nile Delta of Egypt – also have distributary channels – but their seaward margin is modified by wave action 56 Tide-Dominated Deltas • Tide-Dominated Deltas, – such as the Ganges-Brahmaputra delta, west Bengal, India – have tidal sand bodies – along the direction of tidal flow 57 Barrier Islands • On broad continental margins with abundant sand, long barrier islands lie offshore separated from the mainland by a lagoon • Barrier islands are common along the Gulf – and Atlantic Coasts of the United States • Many ancient deposits formed in this environment • Sub-environments of a barrier island complex: – beach sand grading offshore into finer deposits – dune sands contain shell fragments • not found in desert dunes – fine-grained lagoon deposits • with marine fossils and bioturbation 58 Barrier Island Complex • Subenvironments of a barrier island complex 59 Tidal Flats • Tidal flats are present where part of the shoreline is periodically covered by seawater at high tide and then exposed at low tide • Many tidal flats build or prograde seaward – and yield a sequence of rocks grading upward from sand to mud • One of their most distinctive features – is herringbone cross-bedding – or sets of cross-beds that dip in opposite directions 60 Tidal Flats • Tidal-flat deposits showing a prograding shoreline – Notice the distinctive cross-beds – that dip in opposite directions 61 Marine Environments • Marine environments include – – – – continental shelf continental slope continental rise deep-seafloor • Much of the detritus eroded from continents – is eventually deposited in marine environments • but other sediments – are found here as well 62 Marine Environments Marine environments 63 Detrital Marine Environments • The gently sloping area adjacent to a continent – is a continental shelf • It consists of a high-energy inner part that is – periodically stirred up by waves and tidal currents • Its sediment is mostly sand, – shaped into large cross-bedded dunes • Bedding planes are commonly marked – by wave-formed ripple marks • Marine fossils and bioturbation are typical 64 Slope and Rise • The low-energy part of the shelf – has mostly mud with marine fossils, – and interfingers with inner-shelf sand • Much sediment derived from the continents – crosses the continental shelf and is funneled into deeper water through submarine canyons • It eventually comes to rest – on the continental slope and continental rise – as a series of overlapping submarine fans 65 Slope and Rise • Once sediment passes the outer margin – of the shelf, the shelf-slope break, turbidity currents transport it • So sands with graded bedding are common • as well as mud that settled from seawater 66 Detrital Marine Environments • Shelf, slope and rise environments • The main avenues of sediment transport across the shelf are submarine canyons Turbidity currents carry sediment to the submarine fans Sand with graded bedding and mud settled from seawater 67 Deep Sea • Beyond the continental rise, the seafloor is – nearly completely covered by fine-grained deposits • pelagic clay and ooze – with no sediment at all • near mid-ocean ridges – sand and gravel are notably absent • The main sources of sediment are – dust from continents or oceanic islands – volcanic ash – shells of micro-organisms that dwelled in surface waters of the ocean 68 Deep Sea • Types of sediment are – pelagic clay, • which covers most of the deeper parts of the seafloor – calcareous (CaCO3) and siliceous (SiO2) oozes • made up of microscopic shells 69 Deep Sea • Sediments on the deep seafloor consist of – calcareous foraminifera and coccolithophores – siliceous radiolarians and diatoms 70 Carbonate Environments • Carbonate rocks are – limestone, which is composed of calcite – dolostone, which is composed of dolomite • most dolostone is altered limestone • Limestone is similar to detrital rock in some ways – Many limestones are made up of • gravel-sized grains • sand-sized grains • microcrystalline carbonate mud called micrite – but the grains are all calcite and are formed in the environment of deposition, instead of being transported there 71 Limestone Environments • Some limestone form in lakes, – but most limestone is deposited in warm shallow season carbonate shelves and on carbonate platforms rising from oceanic depths • Deposition occurs where little detrital sediment, especially mud, is present • Carbonate barriers form in high-energy areas and may be – reefs – banks of skeletal particles – accumulations of spherical carbonate grains known as ooids • which make up the grains in oolitic limestone 72 Carbonate Shelf • Deposition of limestone is taking place in southern Florida and the Persian Gulf 73 Carbonate Subenvironments • Reef rock tends to be – structureless – composed of skeletons of corals, mollusks, sponges and other organisms • Carbonate banks are made up of – layers with horizontal beds – cross-beds – wave-formed ripple marks • Lagoons tend to have – micrite – with marine fossils – bioturbation 74 Evaporite Environments • Evaporites consist of – rock salt – rock gypsum • They are found in environments such as – playa lakes – saline lakes – but most of the extensive deposits formed in the seas • Evaporites are not nearly as common – as sandstone, mudrocks and limestone, – but can be abundant locally 75 Evaporites • Large evaporite deposits – lie beneath the Mediterranean Seafloor • more than 2 km thick – in western Canada, Michigan, Ohio, New York, – and several Gulf Coast states • How some of these deposits originated is controversial, but geologists agree that high evaporation rates of seawater caused minerals to precipitate from solution • Coastal environments in arid regions such as the present-day Persian Gulf meet the requirements 76 Interpreting Depositional Environments • Jurassic-aged Navajo Sandstone of the South western United States – has all the features of wind-blown sand dunes: • the sandstone is mostly well-sorted, well-rounded quartz • measuring 0.2 to 0.5 mm in diameter • tracks of land-dwelling animals, including dinosaurs, are present • cross-beds up to 30 m high have current ripple marks • like those produced on large dunes by wind today • cross-beds dip generally southwest • indicating a northeast prevailing wind 77 Interpreting Depositional Environments • Lower Silurian strata exposed in New Jersey and Pennsylvania – We can use combined features of sedimentary rocks – and comparisons with present-day deposits • Conclusion: – Sediments were deposited in braided streams – that flowed from east to west 78 Interpreting Depositional Environments • Geologists have also evaluated vertical facies relationships, rock types, sedimentary structures, fossils in Ordovician rocks in Arkansas and conclude they formed as transgressive shelf carbonate deposits • The trend was trangressive although several regressions occurred 79 Paleogeography • Paleogeography deals with – Earth’s geography of the past • Using interpretations of depositional environment, we can attempt to reconstruct what Earth’s geography was like • For example, – the Navajo Sandstone shows that a vast desert was present in what is now the southwest during the Jurassic Period • and from Late Precambrian to Middle Cambrian – the shoreline migrated inland from east and west during a marine transgression in North America 80 Paleogeography • Detailed studies of various rocks – in several western states allow us to determine with some accuracy how the area appeared during the Late Cretaceous • A broad coastal plain – sloped gently eastward – from a mountainous region 81 – to the sea Paleogeography • Later, vast lakes, – river floodplains, alluvial fans – covered much of this area – and the sea had withdrawn from the continent 82 QUESTIONS? 83 Chapter 7 Evolution (Part I) The Theory and Its Supporting Evidence 1 Darwin and the Galápagos • During Charles Darwin’s five-year voyage – (1831-1836) on the HMS Beagle, he visited the Galápagos Islands where he made important observations that changed his ideas about the then popular concept called the fixity of species • an idea holding that all present-day species had been created in their present form and had changed little or not at all • Darwin fully accepted – the Biblical account of creation before the voyage 2 Route of HMS Beagle • Map showing the route (red line) followed – by Charles Darwin when he was aboard HMS Beagle from 1831 to 1836 • The Galápagos Islands – are in the Pacific Ocean west of Ecuador 3 The Galápagos Islands • The Galápagos Islands – are specks of land – composed of basalt – in the eastern Pacific 4 Darwin Developed the Theory • During the voyage Darwin observed – that fossil mammals in South America are similar yet different from present-day llamas, sloths, and armadillos – that the finches and giant tortoises living on the Galápagos Islands vary from island to island – and still resemble ones from South America, even though they differ in subtle ways • These observations convinced Darwin – that organisms descended with modification from ancestors that lived during the past – the central idea of the theory of evolution 5 Galápagos Finches • Darwin’s finches from the Galápagos Islands – arranged to show evolutionary relationships Insect eaters Insect eaters Berry eater Seed Cactus eaters eaters – Notice that beak shape – varies depending on diet 6 Why Study Evolution? • Evolution – involving inheritable changes in organisms through time • is fundamental to biology and paleontology – Paleontology is the study of life history as revealed by fossils • Evolution is a unifying theory • like plate tectonic theory – that explains an otherwise encyclopedia collection of facts • Evolution provides a framework for discussion of life history 7 Evolution: Historical Background • Evolution, the idea that today’s organisms have descended with modification from ancestors that lived during the past is usually attributed solely to Charles Darwin – but it was seriously considered long before he was born, even by some ancient Greeks and by philosophers and theologians • during the Middle Ages • Nevertheless, the prevailing belief in the 1700s was that Genesis and the works of Aristotle explained the origin of life and contrary views were heresy 8 Evolution: Historical Background • During the 18th century, naturalists were discovering evidence that could not be reconciled with literal reading of the Bible • In this changing intellectual atmosphere, scientists gradually accepted a number of ideas: • • • • the principle of uniformitarianism, Earth’s great age, that many types of plants and animals had become extinct, and that change from one species to another occurred • What was lacking, though, – was a theoretical framework to explain evolution 9 Lamarck • Jean-Baptiste de Lamarck (1744-1829) is best remembered for his theory of inheritance of acquired characteristics, though he greatly contributed to our understanding of the natural world • According to this theory, new traits arise in organisms because of their needs and are somehow passed on to their descendants • Lamarck’s theory seemed logical at the time 10 Lamarck’s Theory • Lamark’s theory was not totally refuted – until decades later – with the discovery that genes • units of heredity – cannot be altered by any effort by an organism during its lifetime 11 Darwin • In 1859, Charles Robert Darwin (1809-1882) – published On the Origin of Species • in which he detailed – his Natural Selection ideas – formulated 20 years earlier – and proposed a mechanism for evolution 12 Natural Selection • Plant and animal breeders practice artificial selection by selecting those traits they deem desirable and then breed plants and animals with those traitsthereby bringing about a great amount of change • Observing artificial selection gave Darwin the idea that a process of selection among variant types in nature could also bring about change • Therefore, a natural process was selecting only a few individuals for survival 13 Artificial Selection • Through artificial selection, humans have given rise to dozens of varieties of – domestic dogs, pigeons, sheep, cereal crops, and vegetables • Wild mustard was selectively bred – to yield broccoli, cauliflower, kale, and cabbage 14 Natural Selection—Main Points • Organisms in all populations possess heritable variations such as – size, speed, agility, visual acuity, digestive enzymes, color, and so forth • Some variations are more favorable than others – some have a competitive edge in acquiring resources and/or avoiding predators • Those with favorable variations – are more likely to survive – and pass on their favorable variations 15 “Survival of the Fittest” • In colloquial usage, natural selection is sometimes expressed as “survival of the fittest” • This is misleading because – natural selection is not simply a matter of survival but involves inheritable variations leading to reproductive success 16 Not only Biggest, Strongest, Fastest • One misconception about natural selection – is that among animals, only the biggest, strongest, and fastest are likely to survive – These characteristics might provide an advantage • but natural selection may favor – – – – – the smallest if resources are limited the most easily concealed those that adapt most readily to a new food source those having the ability to detoxify some substance and so on… 17 Limits of Natural Selection • Natural selection works on existing variation in a population • It could not account for the origin of variations • Critics reasoned that should a variant trait arise, – it would blend with other traits and would be lost • The answer to these criticisms – existed even then in the work of Gregor Mendel, but remained obscure until 1900 18 Mendel and the Birth of Genetics • During the 1860s, Gregor Mendel, an Austrian monk, performed a series of controlled experiments with true-breeding strains of garden peas strains that when self-fertilized always display the same trait, such as flower color • Traits are controlled by a pair of factors, now called genes • Genes occur in alternate forms, called alleles – One allele may be dominant over another – Offspring receive one allele of each pair from each parent 19 Mendel’s Experiments • The parental generation consisted of – true-breeding strains, – RR = red flowers, rr = white flowers • Cross-fertilization yielded a second generation – all with the Rr combination of alleles, – in which the R (red) is dominant over r (white) 20 Mendel’s Experiments • The second generation, when self-fertilized produced a third generation with a ratio of three red-flowered plants to one white-flowered plant 21 Importance of Mendel’s Work • The factors (genes) controlling traits – do not blend during inheritance • Traits not expressed in each generation – may not be lost (it’s simply recessive) • Therefore, some variation in populations – results from alternate expressions of genes (alleles) • Variation can be maintained 22 Genes and Chromosomes • Complex, double-stranded helical molecules of deoxyribonucleic acid (DNA) • called chromosomes – are found in cells of organisms • Specific segments of DNA are the basic units of heredity (genes) • The number of chromosomes – varies from one species to another – fruit flies 8; humans 46; horses 64 23 Sexually Reproducing Organisms • In sexually reproducing organisms, – the production of sex cells • pollen and ovules in plants • sperm and eggs in animals – results when cells undergo a type of cell division known as meiosis • This process yields cells with only one chromosome of each pair – so all sex cells (gametes) have only 1/2 the chromosome number of the parent cell 24 Meiosis • During meiosis, – sex cells that contain one member of each chromosome pair form • Formation of sperm is shown here • Eggs form the same way, – but only one of the four final eggs is functional 25 Fertilization • The full number of chromosomes is restored when a sperm fertilizes an egg – or when pollen fertilizes an ovule • The zygote then – has a full set of chromosomes typical for that species • As Mendel deduced, – 1/2 the genetic makeup of fertilized egg comes from each parent • The fertilized egg – grows by mitosis 26 Mitosis • Mitosis is cell division – that results in the complete duplication of a cell • In this example, – a cell with four chromosomes (two pairs) produces two cells each with four chromosomes • Mitosis takes place in all cells except sex cells • Once an egg has been fertilized, the developing embryo grows by mitosis 27 Modern View of Evolution • During the 1930s and 1940s, paleontologists, population biologists, geneticists, and others developed ideas that merged to form a modern synthesis or neo-Darwinian view of evolution • They incorporated chromosome theory of inheritance into evolutionary thinking • They saw changes in genes (mutations) as one source of variation • They completely rejected Lamarck’s idea of 28 inheritance of acquired characteristics Modern View of Evolution • They reaffirmed the importance of natural selection • According to modern evolutionary theory, populations rather than individuals evolve. – Individuals with favorable traits • are more likely to survive and reproduce • if their variations are favorable – As a result, descendant populations possess variations in greater frequency 29 What Brings about Variation? • Evolution by natural selection – works on variation in populations most of which is accounted for by the reshuffling of genes from generation to generation during sexual reproduction • The potential for variation is enormous – with thousands of genes, each with several alleles, – and with offspring receiving 1/2 of their genes from each parent • New variations arise by mutations – change in the 30 chromosomes or genes Mutations • Mutations result in a change – in hereditary information • Mutations that take place in sex cells – are inheritable, – whether they are chromosomal mutations • affecting a large segment of a chromosome – or point mutations • individual changes in particular genes • Mutations are random with respect to fitness – they may be beneficial, neutral, or harmful 31 Mutations • If a species is well adapted to its environment, – most mutations would not be particularly useful and perhaps would be harmful • But what was a harmful mutation – can become a useful one if the environment changes 32 Neutral Mutations • Information in cells is carried on chromosomes – which direct the formation of proteins – by selecting the appropriate amino acids – and arranging them into a specific sequence • Neutral mutations may occur – if the information carried on the chromosome – does not change the amino acid or protein that is produced 33 What Causes Mutations? • Some mutations are induced by mutagens – agents that bring about higher mutations rates such as • • • • some chemicals ultraviolet radiation X-rays extreme temperature changes • Some mutations are spontaneous – occurring without any known mutagen 34 Species • Species is a biological term for a population – of similar individuals that naturally interbreed and produce fertile offspring • Species are reproductively isolated from one another • Goats and sheep do not interbreed in nature, – so they are separate species • Yet in captivity – they can produce fertile offspring 35 Speciation • Speciation is the phenomenon of a new species arising from an ancestral species • It involves change in the genetic makeup – of a population, – which also may bring about changes in form and structure • During allopatric speciation, – species arise when a small part of a population – becomes isolated from its parent population 36 Allopatric Speciation • A few individuals of a species on the mainland – reach isolated island 1 – Speciation follows genetic divergence in a new habitat. 37 Allopatric Speciation • Later in time, a few individuals of the new species colonize island 2 – In this new habitat, speciation follows genetic divergence. 38 Allopatric Speciation • Speciation may also follow colonization of islands 3 and 4 • Invasion of island 1 by genetically different descendants of the ancestral species! 39 Rate of Speciation • Although widespread agreement exists on allopatric speciation scientists disagree on how rapidly a new species might evolve – Phyletic gradualism • the gradual accumulation of minor changes eventually brings about the origin of new species – Punctuated equilibrium • holds that little or no change takes place in a species during most of its existence • Then evolution occurs rapidly giving rise to a new species – in as little as a few thousands of years 40 Styles of Evolution • Divergent evolution occurs when an ancestral species gives rise to diverse descendants that differ markedly from their ancestors • Convergent evolution involves the development of similar characteristics in distantly related organisms • Parallel evolution involves the development of similar characteristics in closely related organisms 41 Microevolution and Macroevolution • Microevolution is any change in the genetic make-up of a species, and involves changes within a species • Macroevolution involves changes such as the origin of a new species or changes at even higher levels – For example, the origin of birds from reptiles • The cumulative effects of microevolution are responsible for macroevolution 42 Cladistics and Cladograms • Traditionally, scientists have depicted evolutionary relationships with phylogenetic trees • in which the horizontal axis represents anatomical differences • and the vertical axis denotes time • In contrast, a cladogram shows the relationships among members of a clade • a group of organisms including its most recent common ancestor • Cladistics focus on derived characteristics sometimes called evolutionary novelties – as opposed to primitive characteristics 43 Phylogenetic Tree • A phylogenetic tree showing the relationships among various organisms 44 Cladogram • A cladogram showing inferred relationships • Some of the characteristics used – to construct this cladogram are indicated 45 QUESTIONS? 46 Chapter 7 Evolution (Part II) The Theory and Its Supporting Evidence 47 Evolutionary Novelties • All land-dwelling vertebrate animals possess bone and paired limbs so these characteristics are primitive and of little use in establishing relationships among land vertebrates • However, hair and three middle ear bones are derived characteristics because only one subclade, the mammals, has them 48 Evolutionary Novelties • If considering only mammals, hair and middle ear bones are primitive characteristics – but live birth is a derived characteristic that serves to distinguish most mammals from the egg-laying mammals 49 Evolutionary Trends • Evolutionary changes do not involve – all aspects of an organism simultaneously • A key feature we associate with a descendant group might appear before other features typical of that group • For example, the oldest known bird – had feathers and the typical fused clavicles of birds, – but it also retained many reptile characteristics • Mosaic evolution is the concept that – organisms possess recently evolved characteristics as well as some features of their ancestral group 50 Phylogeny • Phylogeny is the evolutionary history of a group of organisms • If sufficient fossil material is available, – paleontologists determine the phylogeny and evolutionary trends for groups of organisms • For example, one trend in ammonoids • extinct relatives of squid and octopus – was the evolution of an increasingly complex shell 51 Evolutionary Trends • Abundant fossils show the evolutionary trends of – the Eocene mammals, Titanotheres • These extinct relative of horses and rhinoceroses – evolved from small ancestors – to giants standing 2.4 m at the shoulder – developed large horns – and the shape of their skull changed – Only 4 of the 16 known genera are shown 52 Evolutionary Trends • Size increase is one of the most common evolutionary trends • However, trends are complex – they might reverse – more than one can take place at the same time at different rates • Trends in horses included – generally larger size • but size decreased in some now-extinct horses – changes in teeth and skull – lengthening legs – reduction in number of toes • These trends occurred at different rates 53 Adaptations • Evolutionary trends are a series of adaptations to changing environment or in response to exploitation of new habitats • Some organisms – show little evolutionary change for long periods • Lingula is a brachiopod – whose shell has not changed significantly since the Ordovician 54 “Living Fossils” • Several organisms have shown – little or no change for long periods • If these still exist as living organisms today – they are sometimes called living fossils • For example: – horseshoe crabs – Latrimaria (fish) – Ginkgo trees • Some of these are generalized and can live under a wide variety of environments 55 A Living Fossil • Latrimaria – belongs to a group of fish – once thought to have gone extinct at the end of the Mesozoic Era A specimen was caught off the coast of East Africa in 1938 56 A Second Living Fossil • Ginkgos – have changed very little for millions of years 57 Randomness in Natural Selection? • But isn’t evolution by natural selection a random process? • If so, how is it possible – for a trend to continue long enough to account just by chance for such complex structures as – eyes, wings, and hands? 58 Two Steps in Natural Selection • Evolution by natural selection – is a two-step process – Only the first step involves chance • Variation must be present – or arise in a population • Whether a mutation is favorable – is a matter of chance • The natural selection of favorable variations – is not by chance 59 Extinctions • Perhaps as many as 99% of all species – that ever existed are now extinct • Organisms do not always evolve toward some kind of higher order of perfection or greater complexity • Vertebrates are more complex – but not necessarily superior in some survival sense. – Bacteria have persisted for at least 3.5 billion years! • Natural selection yields organisms adapted – to a specific set of circumstances – at a particular time 60 Background and Mass Extinction • The continual extinction of species is referred to as background extinction • It is clearly different from mass extinction – during which accelerated extinction rates sharply reduce Earth’s biotic diversity • Extinction is a continual occurrence – but so is the evolution of new species that usually quickly exploit the opportunities another species’ extinction creates • Mammals began a remarkable diversification – when they began occupying niches the extinction of dinosaurs and their relatives left vacant 61 Evidence in Support of Evolution • Darwin cited supporting evidence – for evolutionary theory such as • • • • • classification embryology comparative anatomy geographic distribution fossil record, to a limited extent • He had little knowledge – of the mechanism of inheritance, – and biochemistry and molecular biology were unknown at his time 62 Evidence in Support of Evolution • Since Darwin’s time, studies from additional fields – in biochemistry – molecular biology – more complete and better understood fossil record • have convinced scientists that the theory is as well supported by evidence as any other major theory 63 Is the Theory of Evolution Scientific? • An idea can only be a truly scientific theory if testable predictive statements can be made from it • No theory in science is ever proven in the final sense, although substantial evidence may support it • All theories are always open – to question, revision and occasionally – to replacement by a more comprehensive theory 64 Theories Must Be Predictive • Not all predictions are about future events, and – evolutionary theory cannot make predictions about the far distant future • Nevertheless, we can make a number of predictions – about the present-day biological world and about the fossil record that should be consistent with evolutionary theory if it is correct 65 Some Predictions from Evolution • If evolution has taken place, – closely related species such as wolves and coyotes should be similar in anatomy and biochemistry, genetics, and embryonic development 66 Testable • Suppose that contrary to evolutionary prediction – wolves and coyotes were not similar in terms of their biochemistry, genetics and embryonic development, then – our prediction would fail and we would at least have to modify the theory • If other predictions also failed, – for example, if mammals appeared in the fossil record before fishes then we would have to abandon the theory and find a better explanation for our 67 observations Classification • Classification uses a nested pattern of similarities • Carolus Linneaus (1707-1778) proposed – a classification scheme in which organisms receive a two-part name consisting of genus and species – for example, the coyote is Canis latrans • Linnaeus’s classification is an ordered list – of categories that becomes more inclusive as one proceeds up the list 68 Linnaean Classification • the coyote, Canis latrans • Animalia – Chordata Most inclusive • Kingdom – Phylum • Subphylum – Class » Order • Family – Genus • Species • Vertebrata – Mammalia » Carnivora • Canidae – Canis • latrans Least inclusive 69 Classification —shared Characteristics • Subphylum vertebrata – including fishes, amphibians, reptiles, birds and mammals, – have a segmented vertebral column • Only warmblooded animals with hair/fur and mammary glands are mammals 70 Coyote, Canis latrans • 18 orders of mammals exist including order Carnivora • The Family Canidae are doglike carnivores • and the genus Canis includes only closely related species • Coyote, Canis latrans, stands alone as a species 71 Coyote and Wolf • Coyote (Canis latrans) and wolf (Canis lupus) – share numerous characteristics as members of the same genus • They share some but fewer characteristics – with the red fox (Volpes fulva) in the family Canidae • All canids share some characteristics with cats, – Bears, and weasels in the order Carnivora – which is one of 18 living orders of the class Mammalia • Shared characteristics are evidence for evolutionary relationships 72 Biological Evidence Supporting Evolution • If all existing organisms descended with modification from ancestors that lived during the past, • all life forms should have fundamental similarities: – all living things consist mainly of carbon, nitrogen hydrogen and oxygen – their chromosomes consist of DNA – all cells synthesize proteins • in essentially the same way 73 Evolutionary Relationships • Biochemistry provides evidence for evolutionary relationships • Blood proteins are similar among all mammals – Humans’ blood chemistry is related • • • • most closely to the great apes then to Old World monkeys then New World monkeys then lower primates such as lemurs • Biochemical tests support the idea – that birds descended from reptiles • a conclusion supported by evidence in the fossil record 74 Structures with Similarities • Homologous structures – are basically similar structures that have been modified for different functions – They indicate derivation from a common ancestor. 75 Homologous Structures • Forelimbs of humans, whales, dogs, and birds – are superficially dissimilar, – yet all are made up of the same bones, – have similar arrangement – of muscles, nerves and blood vessels, – are similarly arranged with respect to other structures, – have similar pattern of embryonic development 76 Structures with Similarities • Analogous structures are structures with similarities unrelated to evolutionary relationships that serve the same function but are quite dissimilar in both structure and development • Wings of insects and birds – serve the same function but differ considerably – in structure and development 77 Vestigial Structures • Vestigial structures are remnants – of structures in organisms that were functional – in their ancestors • Why do dogs have tiny, – functionless toes on their feet (dewclaws)? • Ancestral dogs had five toes – on each foot, – all of which contacted the ground • As they evolved – they became toe-walkers with only four toes on the ground – and the big toes and thumbs were lost or reduced – to their present state 78 Remnants of Rear Limbs in Whales • The Eocene-aged whale, Basilosaurus, – had tiny vestigial back limbs – but it did not use limbs to support its body weight. 79 Evolution in Living Organisms • Small-scale evolution can be observed today. • For example – adaptations of some plants to contaminated soils – insects and rodents developing resistance to new insecticides and pesticides – development of antibiotic-resistant strains of bacteria • Variations in these populations – allowed some variant types to live and reproduce, – bringing about a genetic change 80 What do We Learn from Fossils? • The fossil record consists of first appearances of various organisms through time – One-celled organisms appeared before multi-celled ones – plants appeared before animals – invertebrates before vertebrates • Fish appeared first followed – in succession by amphibians, reptiles, mammals, and birds 81 Advent of Various Vertebrates • Times when major groups of vertebrates appeared in the fossil record • Thickness of spindles shows relative abundance 82 Fossils Are Common • Fossils are much more common than many people realize • However the origin and initial diversification – of a group is generally the most poorly represented • But fossils showing the diversification of horses, rhinoceroses, and tapirs from a common ancestor are known • as are ones showing the origin – of birds from reptiles • and the evolution – of whales from a land-dwelling ancestor 83 The Evidence: Summary • Scientists agree that the theory of evolution is as well-supported by evidence as any other theory • Transitional fossils provide compelling evidence – but fossils aren’t the only evidence • Much evidence comes from – – – – – – comparative anatomy biogeography molecular biology genetics embryology and biochemistry 84 Summary • The central claim of evolution is that all organisms – have descended from ancestors that lived during the past , with modification • Jean Baptiste de Lamarck proposed – the first mechanism to account for evolution – Inheritance of Acquired Characteristics • Darwin’s observation of variation in populations and artificial selection – and his reading of Malthus’ essay on population – helped him formulate his idea of natural selection 85 Summary • In 1859 Charles Darwin and Alfred Wallace – published their ideas of natural selection – which hold that in populations of organisms, some have favorable traits that make it more likely that they will survive and reproduce 86 Summary • Gregor Mendel’s breeding experiments – with garden peas provided some of the answers regarding how variation is maintained in populations – Mendel’s work is the basis for present-day genetics • Genes are the hereditary units in all organisms – Only the genes in sex cells are inheritable 87 Summary • Sexual reproduction and mutations – account for most variation in populations • Evolution by natural selection has two steps – First, variation must exist or arise and be maintained in interbreeding populations – Second, favorable variants must be selected for survival 88 Summary • Most species evolve by allopatric speciation – which involves isolation of a small population from its parent population, that is then subjected to different selection pressures • Divergent evolution involves – A common ancestor stock giving rise to diverse species • The development of similar adaptive types in different groups of organisms results from parallel and convergent evolution 89 Summary • Microevolution involves changes within a species, – while macroevolution encompasses all changes above the species level. • Scientists traditionally used phylogenetic trees to depict evolutionary relationships – but now they more commonly use cladistic analyses and cladograms to show these relationships 90 Summary • Background extinctions occur continually, – but several mass extinctions have also taken place, during which Earth’s biologic diversity has decreased markedly • The theory of evolution is truly scientific 91 Summary • Much of the evidence supporting the theory of evolution comes from – classification, comparative anatomy, embryology, genetics, biochemistry, molecular biology, and present-day examples of microevolution • The fossil record also provides evidence for evolution – in that it shows a sequence of different groups appearing through time, – and some fossils show features we would expect in the ancestors of birds or mammals, horses, whales, and so on 92 QUESTIONS? 93 Precambrian Earth and Life History Part I (The Archean Eon) 1 Archean Rocks • The Beartooth Mountains on the Wyoming and Montana border consists of Archaean-age gneisses – some of the oldest rocks in the US. 2 Precambrian • The Precambrian lasted for more than 4 billion years! – This large time span is difficult for humans to comprehend • Suppose that a 24-hour clock represented all 4.6 billion years of geologic time then the Precambrian would be slightly more than 21 hours long, constituting about 88% of all geologic time 3 Precambrian 4 Precambrian • The term Precambrian is informal but widely used when referring to both time and rocks • The Precambrian includes – time from Earth’s origin 4.6 billion years ago to the beginning of the Phanerozoic Eon, 542 million years ago • It encompasses all rocks below the Cambrian system • No rocks are known for the first 600 million years of geologic time – The oldest known rocks on Earth are 4.0 billion years old 5 Rocks Difficult to Interpret • The earliest record of geologic time preserved in rocks is difficult to interpret because many Precambrian rocks have been • • • • • altered by metamorphism complexly deformed buried deep beneath younger rocks fossils are rare, and the few fossils present are not useful in biostratigraphy • Subdivisions of the Precambrian have been difficult to establish • Two eons for the Precambrian are the Archean and Proterozoic which are based on absolute ages 6 Eons of the Precambrian • Eoarchean refers to all time from Earth’s origin to the Paleoarchean, 3.6 billion years ago • Earth’s oldest body of rocks, the Acasta Gneiss in Canada is about 4.0 billion years old • We have no geologic record for much of the Archaen • Precambrian eons have no stratotypes – unlike the Cambrian Period, for example 7 What Happened During the Eoarchean? • Although no rocks of Eoarchean age are present on Earth, – except for meteorites, • We do know some events that took place then – Earth accreted from planetesimals and differentiated into core and mantle • and at least some crust was present – – – – Earth was bombarded by meteorites Volcanic activity was widespread An atmosphere formed, quite different from today’s Oceans began to accumulate 8 Hot, Barren, Waterless Early Earth • about 4.6 billion years ago • Shortly after accretion, Earth was – – – – http://www.youtube.com/watch?v=QDqskltCixA a rapidly rotating, hot, barren, waterless planet bombarded by meteorites and comets with no continents, intense cosmic radiation and widespread volcanism 9 Oldest Rocks • Continental crust was present by 4.0 billion years ago – Sedimentary rocks in Australia contain detrital zircons (ZrSiO4) dated at 4.4 billion years old – so source rocks at least that old existed • The Eoarchean Earth probably rotated in as little as 10 hours – and the Earth was closer to the Moon • By 4.4 billion years ago, the Earth cooled sufficiently for surface waters to accumulate 10 Eoarchean Crust • Early crust formed as upwelling mantle currents of mafic magma, and numerous subduction zones developed to form the first island arcs • Eoarchean continental crust may have formed – by collisions between island arcs – as silica-rich materials were metamorphosed. – Larger groups of merged island arcs • protocontinents – grew faster by accretion along their margins 11 Origin of Continental Crust • Andesitic island arcs – form by subduction – and partial melting of oceanic crust • The island arc collides with another 12 Continental Foundations • Continents consist of rocks with composition similar to that of granite • Continental crust is thicker and less dense than oceanic crust which is made up of basalt and gabbro • Precambrian shields consist of vast areas of exposed ancient rocks and are found on all continents • Outward from the shields are broad platforms of buried Precambrian rocks that underlie much of each continent 13 Cratons • A shield and its platform make up a craton, – a continent’s ancient nucleus • Along the margins of cratons, more continental crust was added as the continents took their present sizes and shapes • Both Archean and Proterozoic rocks are present in cratons and show evidence of episodes of deformation accompanied by igneous activity, metamorphism, and mountain building • Cratons have experienced little deformation since the Precambrian 14 Distribution of Precambrian Rocks • Areas of exposed – Precambrian rocks – constitute the shields • Platforms consist of – buried Precambrian rocks – Shields and adjoining platforms make up cratons 15 Canadian Shield • The exposed part of the craton in North America is the Canadian shield – which occupies most of northeastern Canada – a large part of Greenland – parts of the Lake Superior region • in Minnesota, Wisconsin, and Michigan – and the Adirondack Mountains of New York • Its topography is subdued, – with numerous lakes and exposed Archean and Proterozoic rocks thinly covered in places by Pleistocene glacial deposits 16 Evolution of North America • North America evolved by the amalgamation of Archean cratons that served as a nucleus around which younger continental crust was added. 17 Archean Rocks • Only 22% of Earth’s exposed Precambrian crust is Archean • The most common Archean rock associations are granitegneiss complexes • Other rocks range from peridotite to various sedimentary rocks – all of which have been metamorphosed • Greenstone belts are subordinate in quantity, – account for only 10% of Archean rocks – but are important in unraveling Archean tectonic events • Outcrop of Archean gneiss cut by a granite dike from a granitegneiss complex in Ontario, Canada 18 Archean Rocks • Shell Creek in the Bighorn Mountains of Wyoming has cut a gorge into this 2.9 billion year old granite 19 Archean Plate Tectonics • Plate tectonic activity has operated since the Paleoproterozoic or earlier • Most geologists are convinced that some kind of plate tectonic activity took place during the Archean as well – but it differed in detail from today • Plates must have moved faster – with more residual heat from Earth’s origin – and more radiogenic heat, – and magma was generated more rapidly 20 Archean Plate Tectonics • As a result of the rapid movement of plates, – continents grew more rapidly along their margins, a process called continental accretion, as plates collided with island arcs and other plates • Also, ultramafic extrusive igneous rocks, – komatites, were more common 21 The Origin of Cratons • Certainly several small cratons existed during the Archean and grew by accretion along their margins • They amalgamated into a larger unit – during the Proterozoic • By the end of the Archean, – 30-40% of the present volume of continental crust existed • Archean crust probably evolved similarly to the evolution of the southern Superior craton of Canada 22 Atmosphere and Hydrosphere • Earth’s early atmosphere and hydrosphere were quite different than they are now • They also played an important role in the development of the biosphere • Today’s atmosphere is mostly – nitrogen (N2) – abundant free oxygen (O2), • or oxygen not combined with other elements such as in carbon dioxide (CO2) – water vapor (H2O) – small amounts of other gases, like ozone (O3) • which is common enough in the upper atmosphere to block most of the Sun’s ultraviolet radiation 23 Present-day Atmosphere Composition • Nonvariable gases Nitrogen N2 78.08% Oxygen O2 20.95 Argon Ar 0.93 Neon Ne 0.002 Others 0.001 • Variable gases Water vapor H2O Carbon dioxide CO2 Ozone O3 Other gases 0.1 to 4.0 0.038 0.000006 Trace • Particulates normally trace in percentage by volume 24 Earth’s Very Early Atmosphere • Earth’s very early atmosphere was probably composed of – hydrogen and helium, • the most abundant gases in the universe • If so, it would have quickly been lost into space – because Earth’s gravity is insufficient to retain them – because Earth had no magnetic field until its core formed (magnetosphere) • Without a magnetic field, – the solar wind would have swept away any atmospheric gases 25 Outgassing • Once a magnetosphere was present – Atmosphere began accumulating as a result of outgassing, released during volcanism • Water vapor is the most common volcanic gas today – but volcanoes also emit carbon dioxide, sulfur dioxide, carbon monoxide, sulfur, hydrogen, chlorine, and nitrogen 26 Archean Atmosphere • Archean volcanoes probably emitted the same gases, and thus an atmosphere developed – but one lacking free oxygen and an ozone layer • It was rich in carbon dioxide, – and gases reacting in this early atmosphere probably formed • ammonia (NH3) • methane (CH4) • This early atmosphere persisted throughout the 27 Archean Evidence for an Oxygen-Free Atmosphere • The atmosphere was chemically reducing – rather than an oxidizing one • Some of the evidence for this conclusion comes from detrital deposits containing minerals that oxidize rapidly in the presence of oxygen • pyrite (FeS2) • uraninite (UO2) • But oxidized iron becomes increasingly common in Proterozoic rocks – indicating that at least some free oxygen was present then 28 Introduction of Free Oxygen • Two processes account for introducing free oxygen into the atmosphere, • one or both of which began during the Eoarchean. 1. Photochemical dissociation involving ultraviolet radiation in the upper atmosphere • The radiation disrupts water molecules and releases their oxygen and hydrogen • This could account for 2% of present-day oxygen • but with 2% oxygen, ozone forms, creating a barrier against ultraviolet radiation 2. More important were the activities of organisms that practiced photosynthesis 29 Photosynthesis • Photosynthesis is a metabolic process – in which carbon dioxide and water are used in making organic molecules – and oxygen is released as a waste product 6CO2 + 6H2O ==> C6H12O6 + 6O2 • Even with photochemical dissociation and photosynthesis, – probably no more than 1% of the free oxygen level of today was present by the end of the Archean 30 Oxygen Forming Processes • Photochemical dissociation and photosynthesis added free oxygen to the atmosphere – Once free oxygen was present, an ozone layer formed – and blocked incoming ultraviolet radiation 31 Earth’s Surface Waters • Outgassing was responsible for the early atmosphere and also for some of Earth’s surface water • the hydrosphere, most of which is in the oceans > 97% • Another source of our surface water was meteorites and icy comets • Numerous erupting volcanoes, and an early episode of intense meteorite and comet bombardment accounted for rapid rate of surface water accumulation 32 Ocean Water • Volcanoes still erupt and release water vapor – Is the volume of ocean water still increasing? – Perhaps it is, but if so, the rate has decreased considerably because the amount of heat needed to generate magma has diminished 33 Decreasing Heat • Ratio of radiogenic heat production in the past to the present – The width of the colored band indicates variations in ratios from different models • Heat production 4 billion years ago was 3 to 6 times as great as it is now • With less heat outgassing decreased 34 First Organisms • Today, Earth’s biosphere consists of millions of species of Archea, Bacteria, Fungi, Protists, Plants, and Animals, – whereas only bacteria and archea are found in Archean rocks • We have fossils from Archean rocks – 3.5 billion years old • Chemical evidence in rocks in Greenland that are 3.8 billion years old convince some investigators that organisms were present then 35 What Is Life? • Minimally, a living organism must reproduce – and practice some kind of metabolism • Reproduction ensures the long-term survival of a group of organisms • whereas metabolism maintains the organism • The distinction between living and nonliving things is not always easy • Are viruses living? – When in a host cell they behave like living organisms – but outside, they neither reproduce nor metabolize 36 What Is Life? • Comparatively simple organic (carbon based) molecules known as microspheres – form spontaneously – can even grow and divide in a somewhat organism-like fashion – but their processes are more like random chemical reactions, so they are not living 37 How Did Life First Originate? • To originate by natural processes, from non-living matter (abiogenesis), life must have passed through a prebiotic stages – in which it showed signs of living – but was not truly living • The origin of life has 2 requirements – a source of appropriate elements for organic molecules – energy sources to promote chemical reactions 38 Elements of Life • All organisms are composed mostly of – – – – carbon (C) hydrogen (H) nitrogen (N) oxygen (O) • all of which were present in Earth’s early atmosphere as – – – – – carbon dioxide (CO2) water vapor (H2O) nitrogen (N2) and possibly methane (CH4) and ammonia (NH3) 39 Basic Building Blocks of Life • Energy from • Lightning, volcanism, • and ultraviolet radiation – probably promoted chemical reactions during which C, H, N, and O combined – to form monomers • such as amino acids • Monomers are the basic building blocks of more complex organic molecules 40 Experiment on the Origin of Life • Is it plausible that monomers originated in the manner postulated? – Experimental evidence indicates that it is • During the late 1950s – Stanley Miller synthesized several amino acids – by circulating gases approximating the early atmosphere – in a closed glass vessel 41 Experiment on the Origin of Life • This mixture was subjected to an electric spark – to simulate lightning • In a few days – it became cloudy • Analysis showed that – several amino acids typical of organisms had formed • Since then, – scientists have synthesized all 20 amino acids found in organisms 42 Polymerization • The molecules of organisms are polymers – such as proteins – and nucleic acids • RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) consisting of monomers linked together in a specific sequence • How did polymerization take place? • Water usually causes depolymerization, – however, researchers synthesized molecules known as proteinoids or thermal proteins – some of which consist of more than 200 linked amino acids – when heating dehydrated concentrated amino acids 43 Proteinoids • These concentrated amino acids – spontaneously polymerized to form proteinoids • Perhaps similar conditions for polymerization existed on early Earth, – but the proteinoids needed to be protected by an outer membrane or they would break down • Experiments show that proteinoids spontaneously aggregate into microspheres – which are bounded by cell-like membranes and grow and divide much as bacteria do 44 Proteinoid Microspheres • Proteinoid microspheres produced in experiments • Proteinoids grow and divide much as 45 bacteria do Protobionts • These proteinoid molecules can be referred to as protobionts – These are intermediates between inorganic chemical compounds and living organisms 46 Monomer and Proteinoid Soup • The origin-of-life experiments are interesting, – but what is their relationship to early Earth? • Monomers likely formed continuously and in billions – They accumulated in the early oceans into a “hot, dilute soup” – The amino acids in the “soup” might have washed up onto a beach or perhaps cinder cones – where they were concentrated by evaporation and polymerized by heat • The polymers then washed back into the ocean – where they reacted further 47 QUESTIONS? 48 Precambrian Earth and Life History Part II (The Archean Eon) 49 Next Critical Step • Not much is known about the next critical step in the origin of life • the development of a reproductive mechanism • The microspheres divide and may represent a protoliving system – but in today’s cells, nucleic acids, • either RNA or DNA – are necessary for reproduction • The problem is that nucleic acids cannot replicate without protein enzymes, – and the appropriate enzymes cannot be made without nucleic acids, 50 – or so it seemed until fairly recently RNA World? • Now we know that small RNA molecules – can replicate without the aid of protein enzymes • Thus, the first replicating systems – may have been RNA molecules • Some researchers propose – an early “RNA world” in which these molecules were intermediate between • inorganic chemical compounds • and the DNA-based molecules of organisms • How RNA was naturally synthesized – remains an unsolved problem 51 Much Remains to Be Learned • Scientists agree on some basic requirements for the origin of life, – but the exact steps involved and significance of results are debated • Many researchers believe that – the earliest organic molecules were synthesized from atmospheric gases – but some scientist suggest that life arose instead near hydrothermal vents on the seafloor 52 Submarine Hydrothermal Vents • Seawater seeps into the crust near spreading ridges, becomes heated, rises and discharges • Black smokers – Discharge water saturated with dissolved minerals – Life may have formed near these in the past 53 Submarine Hydrothermal Vents • Several minerals containing zinc, copper, and iron precipitate around them • Communities of organisms – previously unknown to science, are supported here. – Necessary elements, sulfur, and phosphorus are present in seawater – Polymerization can take place on surface of clay minerals – Protocells were deposited on the ocean floor 54 Oldest Known Organisms • The first organisms were archaea and bacteria – both of which consist of prokaryotic cells, – cells that lack an internal, membrane-bounded nucleus and other structures • Prior to the 1950s, scientists assumed that life – must have had a long early history – but the fossil record offered little to support this idea • The Precambrian, once called Azoic – (“without life”), seemed devoid of life 55 Oldest Know Organisms • Charles Walcott (early 1900s) described structures from the Paleoproterozoic Gunflint Iron Formation of Ontario, Canada – that he proposed represented reefs constructed by algae • Now called stromatolites, – not until 1954 were they shown to be products of organic activity 56 Present-day stromatolites (Shark Bay, Australia) Stromatolites • Different types of stromatolites include – irregular mats, columns, and columns linked by mats 57 Stromatolites • Present-day stromatolites form and grow as sediment grains are trapped on sticky mats of photosynthesizing cyanobacteria – although now they are restricted to environments where snails cannot live • The oldest known undisputed stromatolites are found in rocks in South Africa – that are 3.0 billion years old • But probable ones are also known from the Warrawoona Group in Australia – which is 3.3 to 3.5 billion years old 58 Other Evidence of Early Life • Chemical evidence in rocks 3.85 billion years old in Greenland indicate life was perhaps present then • The oldest known cyanobacteria were photosynthesizing organisms – but photosynthesis is a complex metabolic process • A simpler type of metabolism must have preceded it • No fossils are known of these earliest organisms 59 Earliest Organisms • The earliest organisms must have resembled – tiny anaerobic bacteria – meaning they required no oxygen • They must have totally depended on an external source of nutrients – that is, they were heterotrophic, as opposed to autotrophic organisms • that make their own nutrients, as in photosynthesis • They all had prokaryotic cells 60 Earliest Organisms • The earliest organisms, then, were anaerobic, heterotrophic prokaryotes • Their nutrient source was most likely – adenosine triphosphate (ATP) from their environment which was used to drive the energyrequiring reactions in cells • ATP can easily be synthesized from simple gases and phosphate – so it was available in the early Earth environment 61 Fermentation • Obtaining ATP from the surroundings could not have persisted for long – because more and more cells competed for the same resources • The first organisms to develop a more sophisticated metabolism – probably used fermentation to meet their energy needs • Fermentation is an anaerobic process in which molecules such as sugars are split, releasing carbon dioxide, alcohol, and energy 62 Photosynthesis • A very important biological event occurring in the Archean was the development of the autotrophic process of photosynthesis • This may have happened as much as 3.5 billion years ago • These prokaryotic cells were still anaerobic, – but as autotrophs they were no longer dependent on preformed organic molecules as a source of nutrients 63 Fossil Prokaryotes • Photomicrographs from western Australia’s – 3.3- to 3.5-billion-year-old Warrawoona Group, – with schematic restoration shown at the right of each 64 Archean Mineral Resources • A variety of mineral deposits are of Archean-age – but gold is the most commonly associated, although it is also found in Proterozoic and Phanerozoic rocks • This soft yellow metal is prized for jewelry, – but it is or has been used as a monetary standard, in glass making, electric circuitry, and chemical industry • About half the world’s gold since 1886 has come from Archean and Proterozoic rocks in South Africa • Gold mines also exist in Archean rocks of the Superior craton in Canada 65 Archean Sulfide Deposits • Archean sulfide deposits of • zinc, • copper • and nickel – occur in Australia, Zimbabwe, and in the Abitibi greenstone belt in Ontario, Canada • Some, at least, formed as mineral deposits – next to hydrothermal vents on the seafloor, much as they do now around black smokers 66 Chrome • About 1/4 of Earth’s chrome reserves are in Archean rocks, especially in Zimbabwe • These ore deposits are found in – the volcanic units of greenstone belts – where they appear to have formed when crystals settled and became concentrated in the lower parts of plutons – such as mafic and ultramafic sills • Chrome is needed in the steel industry • The United States has very few chrome deposits – so must import most of what it uses 67 Chrome and Platinum • One chrome deposit in the United States is in the Stillwater Complex in Montana • Low-grade ores were mined there during war times, – but they were simply stockpiled and never refined for chrome • These rocks also contain platinum, – a precious metal, that is used • in the automotive industry in catalytic converters • in the chemical industry • for cancer chemotherapy 68 Iron • Banded Iron formations are sedimentary rocks – consisting of alternating layers of silica (chert) and iron minerals • About 6% of the world’s banded iron formations were deposited during the Archean Eon • Although Archean iron ores are mined in some areas – they are neither as thick nor as extensive as those of the Proterozoic Eon, which constitute the world’s major source of iron 69 Pegmatites • Pegmatites are very coarsely crystalline igneous rocks, commonly associated with granite plutons • Some Archean pegmatites, – such in the Herb Lake district in Manitoba, Canada, – and Rhodesian Province in Africa, – contain valuable minerals • In addition to minerals of gem quality, – Archean pegmatites contain minerals mined for lithium, beryllium, rubidium, and cesium 70 QUESTIONS? 71 ***NU Ripple marks in intertidal zone at Puerto Penasco, Sonora, Mexico. Tides in this area have the greatest range of any location in the Gulf of California (exceeding 5 m). Courtesy of De Hermosillo versidad de Sonor mento de Geologia, 1. FIC om license do to es winnin sro Cime. Door 36 Ancient Sedimentary Environments

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