Chapter 9 Precambrian Earth and Life HistoryThe Proterozoic

Chapter 9 Precambrian Earth and Life History—The Proterozoic Eon

Proterozoic Rocks, Glacier NP • Proterozoic sedimentary rocks – in Glacier National Park, Montana • The angular peaks, ridges and broad valleys – were carved by Pleistocene and Recent glaciers

The Length of the Proterozoic • the Proterozoic Eon alone, – at 1. 955 billion years long, – accounts for 42. 5% of all geologic time – yet we review this long episode of Earth and life history in a single section

The Phanerozoic • Yet the Phanerozoic, – consisting of • Paleozoic, • Mesozoic, • Cenozoic eras, – lasted a comparatively brief 545 million years – is the subject of the rest of the course

Disparity in Time • Perhaps this disparity – between the coverage of the Proterozoic and the Phanerozoic – seems disproportionate, • but we know far more – about Phanerozoic events – than we do for either of the Precambrian eons

Archean-Proterozoic Boundary • Geologist have rather arbitrarily placed – the Archean-Proterozoic boundary – at 2. 5 billion years ago – because it marks the approximate time – of changes in the style of crustal evolution • However, we must emphasize "approximate, " – because Archean-type crustal evolution – was largely completed in South Africa – nearly 3. 0 billion years ago, – whereas in North America the change took place – from 2. 95 to 2. 45 billion years ago

Style of Crustal Evolution • Archean crust-forming processes generated – granite-gneiss complexes – and greenstone belts – that were shaped into cratons • Although these same rock associations – continued to form during the Proterozoic, – they did so at a considerably reduced rate

Contrasting Metamorphism • In addition, Archean and Proterozoic rocks – contrast in metamorphism • Many Archean rocks have been metamorphosed, – although their degree of metamorphism – varies and some are completely unaltered • However, vast exposures of Proterozoic rocks – show little or no effects of metamorphism, – and in many areas they are separated – from Archean rocks by a profound unconformity

Other Differences – In addition to changes in the style of crustal evolution, • the Proterozoic is characterized – by widespread rock assemblages • that are rare or absent in the Archean, – by a plate tectonic style essentially the same as that of the present – by important evolution of the atmosphere and biosphere – by the origin of some important mineral resources

Proterozoic Evolution of Oxygen-Dependent Organisms • It was during the Proterozoic – that oxygen-dependent organisms – made their appearance • and the first cells evolved – that make up most organisms today

Evolution of Proterozoic Continents • Archean cratons assembled during collisions – of island arcs and minicontinents, – providing the nuclei around which – Proterozoic crust accreted, – thereby forming much larger landmasses • Proterozoic accretion at craton margins – probably took place more rapidly than today – because Earth possessed more radiogenic heat, – but the process continues even now

Proterozoic Greenstone Belts • Most greenstone belts formed – during the Archean – between 2. 7 and 2. 5 billion years ago • They also continued to form – during the Proterozoic and at least one is known – from Cambrian-aged rocks in Australia • They were not as common after the Archean, – and differed in one important detail • the near absence of ultramafic rocks • which no doubt resulted from • Earth's decreasing amount of radiogenic heat

Focus on Laurentia • Our focus here is on the geologic evolution of Laurentia, – a large landmass that consisted of what is now • North America, • Greenland, • parts of northwestern Scotland, • and perhaps some of the Baltic shield of Scandinavia

Early Proterozoic History of Laurentia • Laurentia originated and underwent important growth – between 2. 0 and 1. 8 billion years ago • During this time, collisions – among various plates formed several orogens, – which are linear or arcuate deformation belts – in which many of the rocks have been • metamorphosed • and intruded by magma • thus forming plutons, especially batholiths

Proterozoic Evolution of Laurentia • Archean cratons were sutured – along deformation belts called orogens, – thereby forming a larger landmass • By 1. 8 billion years ago, – much of what is now Greenland, central Canada, – and the north-central United States existed • Laurentia grew along its southern margin – by accretion

Craton-Forming Processes • Examples of these craton-forming processes – are recorded in rocks – in the Thelon orogen in northwestern Canada • where the Slave and Rae cratons collided,

Craton-Forming Processes • the Trans Hudson orogen • in Canada and the United States, – where the Superior, Hearne, and Wyoming cratons – were sutured • The southern margin of Laurentia – is the site of the Penokian orogen

Wilson Cycle • Rocks of the Wopmay orogen – in northwestern Canada are important – because they record the opening and closing – of an ocean basin – or what is called a Wilson cycle • A complete Wilson cycle, • named for the Canadian geologist J. Tuzo Wilson, – involves • • fragmentation of a continent, opening followed by closing of an ocean basin, and finally reassembly of the continent

Wopmay Orogen • Some of the rocks in Wopmay orogen – are sandstonecarbonate-shale assemblages, – a suite of rocks typical of passive continental margins – that first become widespread during the Proterozoic

Early Proterozoic Rocks in Great Lakes Region • Early Proterozoic sandstone-carbonate-shale assemblages are widespread near the Great Lakes

Outcrop of Sturgeon Quartzite • The sandstones have a variety of sedimentary structures – such as – ripple marks – and crossbeds – Northern Michigan

Outcrop of Kona Dolomite • Some of the carbonate rocks, now mostly dolostone, – such as the Kona Dolomite, – contain abundant bulbous structures known as stromatolites – Northern Michigan

Penkean Orogen • These rocks of northern Michigan – have been only moderately deformed – and are now part of the Penokean orogen

Accretion along Laurentia’s Southern Margin • Following the initial episode – of amalgamation of Archean cratons • 2. 0 to 1. 8 billion years ago – accretion took place along Laurentia's southern margin • From 1. 8 to 1. 6 billion years ago, – continental accretion continued • in what is now the southwestern and central United States – as successively younger belts were sutured to Laurentia, – forming the Yavapai and Mazatzal-Pecos orogens

Southern Margin Accretion • Laurentia grew along its southern margin – by accretion of the Central Plains, Yavapai, and Mazatzal orogens • Also notice that the Midcontinental Rift – had formed in the Great Lakes region by this time

BIF, Red Beds, Glaciers • This was also the time during which – most of Earth’s banded iron formations (BIF) – were deposited • The first continental red beds – sandstone and shale with oxidized iron – were deposited about 1. 8 billion years ago • We will have more to say about BIF – and red beds in the section on “The Evolving Atmosphere” • In addition, some Early Proterozoic rocks – and associated features provide excellent evidence – for widespread glaciation

Early and Middle Proterozoic Igneous Activity • During the interval – from 1. 8 to 1. 1 billion years ago, – extensive igneous activity took place – that seems to be unrelated to orogenic activity • Although quite widespread, – this activity did not add to Laurentia’s size – because magma was either intruded into – or erupted onto already existing continental crust

Igneous Activity • These igneous rocks are exposed – in eastern Canada, extend across Greenland, – and are also found in the Baltic shield of Scandinavia

Igneous Activity • However, the igneous rocks are deeply buried – by younger rocks in most areas • The origin of these – granitic and anorthosite plutons, • Anorthosite is a plutonic rock composed • almost entirely of plagioclase feldspars – calderas and their fill, – and vast sheets of rhyolite and ash flows – are the subject of debate • According to one hypothesis – large-scale upwelling of magma – beneath a Proterozoic supercontinent – produced the rocks

Middle Proterozoic Orogeny and Rifting • The only Middle Proterozoic event in Laurentia – was the Grenville orogeny – in the eastern part of the continent – 1. 3 to 1. 0 billion years old • Grenville rocks are well exposed – in the present-day northern Appalachian Mountains – as well as in eastern Canada, Greenland, and Scandinavia

Grenville Orogeny • A final episode of Proterozoic accretion – occurred during the Grenville orogeny

Grenville Orogeny • Many geologists think the Grenville orogen – resulted from closure of an ocean basin, • the final stage in a Wilson cycle • Others disagree and think – intracontinental deformation or major shearing – was responsible for deformation • Whatever the cause of the Grenville orogeny, – it was the final stage – in the Proterozoic continental accretion of Laurentia

75% of North America • By this final stage, about 75% – of present-day North America existed • The remaining 25% – accreted along its margins, – particularly its eastern and western margins, – during the Phanerozoic Eon

Midcontinent Rift • Grenville deformation in Laurentia – was accompanied by the origin – of the Midcontinent rift, • a long narrow continental trough bounded by faults, • extending from the Lake Superior basin southwest into Kansas, • and a southeasterly branch extends through Michigan into Ohio • It cuts through Archean and Early Proterozoic rocks – and terminates in the east against rocks – of the Grenville orogen

Location of the Midcontinent Rift • Rocks filling the rift – are exposed around Lake Superior – but are deeply buried elsewhere

Midcontinental Rift • Most of the rift is buried beneath younger rocks – except in the Lake Superior region – where various igneous and sedimentary rocks – are well exposed • The central part of the rift contains – numerous overlapping basalt lava flows – forming a volcanic pile several kilometers thick • In fact, the volume of volcanic rocks, – between 300, 000 and 1, 000 km 3, – is comparable in volume although not areal extent – to the great outpourings of lava during the Cenozoic

Midcontinental Rift • Along the rift's margins – coarse-grained sediments were deposited – in large alluvial fans – that grade into sandstone and shale – with increasing distance – from the sediment source • In the vertical section – Freda Sandstone overlies – Cooper Harbor conglomerate, – which overlies Portage Lake Volcanics

Cooper Harbor Conglomerate Michigan

Portage Lake Volcanics Michigan

Middle and Late Proterozoic Sedimentation • Remember the Grenville orogeny – took place 1. 2 billion – 900 million years ago, – the final episode of continental accretion – in Laurentia until the Ordovician Period • Nevertheless, important geologic events – were taking place, – such as sediment deposition in what is now – the eastern United States and Canada, – in the Death Valley region of California and Nevada, – and in three huge basins in the west

Sedimentary Basins in the West • Map showing the locations of sedimentary Basins – in the western United States and Canada • Belt Basin • Uinta Basin • Apache Basin

Sedimentary Rocks • Middle to Late Proterozoic sedimentary rocks – are exceptionally well exposed – in the northern Rocky Mountains – of Montana and Alberta, Canada • Indeed, their colors, deformation features, – and erosion by Pleistocene and recent glaciers – have yielded some fantastic scenery • Like the rocks in the Great Lakes region – and the Grand Canyon, – they are mostly sandstones, shales, – and stromatolite-bearing carbonates

Proterozoic Mudrock • Outcrop of red mudrock in Glacier National Park, Montana

Proterozoic Limestone • Outcrop of limestone with stromatolites in Glacier National Park, Montana

Proterozoic Sandstone • Proterozoic rocks – of the Grand Canyon Super-group lie – unconformably upon Archean rocks – and in turn are overlain unconformably – by Phanerozoic-age rocks • The rocks, consisting mostly – of sandstone, shale, and dolostone, – were deposited in shallow-water marine – and fluvial environments • The presence of stromatolites and carbonaceous – impression of algae in some of these rocks – indicate probable marine deposition

Grand Canyon Super-group • Proterozoic Sandstone of the Grand Canyon Super-group in the Grand Canyon Arizona

Style of Plate Tectonics • The present style of plate tectonics – involving opening and then closing ocean basins – had almost certainly been established by the Early Proterozoic • In fact, the oldest known complete ophiolite – providing evidence for an ancient convergent plate boundary – is the Jormua mafic-ultramafic complex in Finland • It is about 1. 96 billion years old, – but nevertheless compares closely in detail – with younger well-documented ophiolites

Jormua Complex, Finland • Reconstruction – of the highly deformed – Jormua maficultramafic complex – in Finland • This sequence of rock – is the oldest known complete ophiolite – at 1. 96 billion years old

Jormua Complex, Finland • Metamorphosed basaltic pillow lava 12 cm

Jormua Complex, Finland • Metamorphosed gabbro between mafic dikes 65 cm

Proterozoic Supercontinents • You already know that a continent – is one of Earth's landmasses – consisting of granitic crust – with most of its surface above sea level • A supercontinent consists of all – or at least much of the present-day continents, – so other than size it is the same as a continent • The supercontinent Pangaea, – which existed at the end of the Paleozoic Era, – is familiar, – but few people are aware of earlier supercontinents

Early Supercontinents • Supercontinents may have existed – as early as the Late Archean, – but if so we have little evidence of them • The first that geologists recognize – with some certainty, known as Rodinia – assembled between 1. 3 and 1. 0 billion years ago – and then began fragmenting 750 million years ago

Early Supercontinent • Possible configuration – of the Late Proterozoic supercontinent Rodinia – before it began fragmenting about 750 million years ago

Pannotia • Rodinia's separate pieces reassembled – and formed another supercontinent – this one known as Pannotia – about 650 million years ago – judging by the Pan-African orogeny • the large-scale deformation that took place • in what are now the Southern Hemisphere continents • Fragmentation was underway again, – by the latest Proterozoic, about 550 million years ago, – giving rise to the continental configuration – that existed at the onset of the Phanerozoic Eon

Ancient Glaciers • Very few times of widespread glacial activity – have occurred during Earth history • The most recent one during the Pleistocene – 1. 6 million to 10, 000 years ago – is certainly the best known, – but we also have evidence for Pennsylvanian glaciers – and two major episodes of Proterozoic glaciation

Recognizing Glaciation • How can we be sure that there were Proterozoic glaciers? – After all, their most common deposit – called tillite is simply a type of conglomerate – that may look much like conglomerate – that originated by other processes • Tillite or tillite-like deposits are known – from at least 300 Precambrian localities, – and some of these are undoubtedly not glacial deposits

Glacial Evidence • But the extensive geographic distribution – of other conglomerates – and their associated glacial features – is distinctive, – such as striated and polished bedrock

Proterozoic Glacial Evidence • Bagganjarga tillite in Norway – overlies striated bedrock surface – on sandstone of the Veidnesbotn Formation

Geologists Convinced • Geologists are now convinced • based on this kind of evidence – that widespread glaciation – took place during the Early Proterozoic • The occurrence of tillites of about the same age – in Michigan, Wyoming, and Quebec – indicates that North America may have had – an Early Proterozoic ice sheet centered southwest of Hudson Bay

Early Proterozoic Glaciers • Deposits in North America – indicate that Laurentia – had an extensive ice sheet – centered southwest of Hudson Bay

One or More Glaciations? • Tillites of about this age are also found – in Australia and South Africa, – but dating is not precise enough to determine – if there was a single widespread glacial episode – or a number of glacial events at different times in different areas • One tillite in the Bruce Formation in Ontario, Canada – may date from 2. 7 billion years ago, – thus making it Late Archean

Glaciers of the Late Proterozoic • Tillites and other glacial features – dating from between 900 and 600 million years ago – are found on all continents except Antarctica • Glaciation was not continuous during this entire time – but was episodic with four major glacial episodes so far recognized

Late Proterozoic Glaciers • The approximate distribution of Late Proterozoic glaciers

Most Extensive Glaciation in Earth History • The map shows only approximate distribution – of Late Proterozoic glaciers – The actual extent of glaciers is unknown • Not all the glaciers were present at the same time • Despite these uncertainties, – this Late Proterozoic glaciation – was the most extensive in Earth history • In fact, Late Proterozoic glaciers – seem to have been present even – in near-equatorial areas

The Evolving Atmosphere • Geologists agree that the Archean atmosphere – contained little or no free oxygen so the atmosphere – was not strongly oxidizing as it is now • Even though processes were underway – that added free oxygen to the atmosphere, – the amount present – at the beginning of the Proterozoic – was probably no more than 1% of that present now • In fact, it might not have exceeded – 10% of present levels even – at the end of the Proterozoic

Cyanobacteria and Stromatolites • Remember from our previous discussions – that cyanobacteria, • also known as blue-green algae, – were present during the Archean, – but stromatolites • the structures they formed, – did not become common until about 2. 3 billion years ago, • that is, during the Early Proterozoic • These photosynthesizing organisms – and to a lesser degree photochemical dissociation • added free oxygen to the evolving atmosphere

Oxygen Versus Carbon Dioxide • Earth's early atmosphere – had abundant carbon dioxide • More oxygen became available – whereas the amount of carbon dioxide decreased • Only a small amount of CO 2 – still exists in the atmosphere today • It is one of the greenhouse gases – partly responsible for global warming • What evidence indicates – that the atmosphere became oxidizing? • Where is all that additional the carbon dioxide now?

Evidence from Rocks • Much carbon dioxide is now tied up – in various minerals and rocks • especially the carbonate rocks – limestone and dolostone, – and in the biosphere • For evidence that the Proterozoic atmosphere was evolving – from a chemically reducing one – to an oxidizing one • we must discuss types – of Proterozoic sedimentary rocks, in particular – banded iron formations – and red beds

Banded Iron Formations (BIF) • Banded iron formations (BIFs), – consist of alternating layers of • iron-rich minerals • and chert – Some are found in Archean rocks, – but about 92% of all BIFs • formed during the interval • from 2. 5 to 2. 0 billion years ago

Early Proterozoic Banded Iron Formation • • At this outcrop in Ishpeming, Michigan the rocks are alternating layers of red chert and silvercolored iron minerals

Typical BIF • A more typical outcrop of BIF near Nagaunee, Michigan

BIFs and the Atmosphere • How are these rocks related to the atmosphere? • Their iron is in iron oxides, especially – hematite (Fe 2 O 3) – and magnetite (Fe 3 O 4) • Iron combines with oxygen in an oxidizing atmosphere – to from rustlike oxides – that are not readily soluble in water • If oxygen is absent in the atmosphere, though, – iron easily dissolves – so that large quantities accumulate in the world's oceans, – which it undoubtedly did during the Archean

Formation of BIFs • The Archean atmosphere was deficient in free oxygen • so that little oxygen was dissolved in seawater • However, as photosynthesizing organisms – increased in abundance, • as indicated by stromatolites, – free oxygen, • released as a metabolic waste product into the oceans, – caused the precipitation of iron oxides along with silica – and thus created BIFs

Formation of BIFs • One model accounting for the details – of BIF precipitation involves – a Precambrian ocean with an upper oxygenated layer – overlying a large volume of oxygen-deficient water – that contained reduced iron and silica • Upwelling, – that is transfer of water from depth to the surface, – brought iron- and silica-rich waters – onto the shallow continental shelves – and resulting in widespread precipitation of BIFs

Formation of BIFs • Depositional model for the origin of banded iron formation

Source of Iron and Silica • A likely source of the iron and silica – was submarine volcanism, – similar to that now talking place – at or near spreading ridges • Huge quantities of dissolved minerals are – also discharged at submarine hydrothermal vents • In any case, the iron and silica – combined with oxygen – thus resulting in the precipitation – of huge amounts of banded iron formation • Precipitation continued until – the iron in seawater was largely used up

Continental Red Beds • Obviously continental red beds refers – to red rocks on the continents, – but more specifically it means red sandstone or shale – colored by iron oxides, – especially hematite (Fe 2 O 3) Red mudrock in Glacier National Park, Montana

Red Beds • Red beds first appear – in the geologic records about 1. 8 billion years ago, – increase in abundance throughout the rest of the Proterozoic, – and are quite common in rocks of Phanerozoic age • The onset of red bed deposition – coincides with the introduction of free oxygen – into the Proterozoic atmosphere • However, the atmosphere at that time – may have had only 1% – or perhaps 2% of present levels

Red Beds • Is this percentage sufficient to account – for oxidized iron in sediment? • Probably not, – but no ozone (O 3) layer existed in the upper atmosphere – before free oxygen (O 2) was present • As photosynthesizing organisms released – free oxygen into the atmosphere, – ultraviolet radiation converted some of it – to elemental oxygen (O) and ozone (O 3), – both of which oxidize minerals more effectively than O 2

Red Beds • Once an ozone layer became established, – most ultraviolet radiation failed – to penetrate to the surface, – and O 2 became the primary agent – for oxidizing minerals

Important Events in Life History • Archean fossils are not very common, – and all of those known are varieties – of bacteria and cyanobacteria (blue-green algae), – although they undoubtedly existed in profusion • Likewise, the Early Proterozoic fossil record – has mostly bacteria and cyanobacteria • Apparently little diversification – had taken place; – all organisms were single-celled prokaryotes, – until about 2. 1 billion years ago – when more complex eukaryotic cells evolved

Gunflint Microfossils • Even in well-known Early Proterozoic fossils assemblages, – such as the Gunflint Iron Formation of Canada, – only fossils of bacteria are recognized Photomicrograph of spheroidal and filamentous microfossils from the Gunflint Chert of Ontario Canada

Prokaryote and Eukaryotes • An organism made up of prokaryotic cells is called a prokaryote – whereas those composed of eukaryotic cells are eukaryotes • In fact, the distinction between prokaryotes and eukaryotes – is the basis for the most profound distinction between all living things

Lack of Organic Diversity • Actually, the lack of organic diversity – during this early time in life history – is not too surprising – because prokaryotic cells reproduce asexually • Most variation in – sexually reproducing populations comes from – the shuffling of genes, – and their alleles, – from generation to generation • Mutations introduce new variation into a population, – but their effects are limited in prokaryotes

Genetic Variation in Bacteria • A beneficial mutation would spread rapidly – in sexually reproducing organism, – but have a limited impact in bacteria – because they do not share their genes with other bacteria • Bacteria usually reproduce by binary fission – and give rise to two cells – having the same genetic makeup • Under some conditions, – they engage in conjugation during – which some genetic material is transferred

Sexual Reproduction Increased the Pace of Evolution • Prior to the appearance of cells capable of sexual reproduction, – evolution was a comparatively slow process, – thus accounting for the low organic diversity • This situation did not persist • Sexually reproducing cells probably – evolved by Early Proterozoic time, – and thereafter the tempo of evolution – increased markedly

Eukaryotic Cells Evolve • The appearance of eukaryotic cells – marks a milestone in evolution – comparable to the development • of complex metabolic mechanisms • such as photosynthesis during the Archean • Where did these cells come from? • How do they differ from their predecessors, – the prokaryotic cells? • All prokaryotes are single-celled, – but most eukaryotes are multicelled, – the notable exception being the protistans

Eukaryotes • Most eukaryotes reproduce sexually, – in marked contrast to prokaryotes, • and nearly all are aerobic, – that is, they depend on free oxygen – to carry out their metabolic processes • Accordingly, they could not have evolved – before at least some free oxygen was present in the atmosphere

Prokaryotic Cell • Prokaryotic cells – do not have a cell nucleus – do not have organelles – are smaller and not nearly as complex as eukaryotic cells

Eukaryotic Cell • Eukaryotic cells have – a cell nucleus containing – the genetic material – and organelles – such as mitochondria – and plastids, – as well as chloroplasts in plant cells

Eukaryotic Fossil Cells • The Negaunee Iron Formation in Michigan – which is 2. 1 billion years old – has yielded fossils now generally accepted – as the oldest known eukaryotic cells • Even though the Bitter Springs Formation – of Australia is much younger • 1 billion years old – it has some remarkable fossils of single-celled eukaryotes – that show evidence of meiosis and mitosis, – processes carried out only by eukaryotic cells

Evidence for Eukaryotes • Prokaryotic cells are mostly rather simple – spherical or platelike structures • Eukaryotic cells – are larger, commonly much larger – much more complex – have a well-defined, membrane-bounded cell nucleus, which is lacking in prokaryotes – have several internal structures – called organelles such as plastids and mitochondria – their organizational complexity – is much greater than it is for prokaryotes

Acritarchs • Other organisms that were – almost certainly eukaryotes are the acritarchs – that first appeared about 1. 4 billion years ago – they were very common by Late Proterozoic time – and were probably cysts of planktonic (floating) algae

Acritarchs • These common Late Proterozoic microfossils – are probably from eukaryotic organisms • Acritarchs are very likely the cysts of algae

Late Proterozoic Microfossil • Numerous microfossils of organisms – with vase-shaped skeletons – have been found – in Late Proterozoic rocks – in the Grand Canyon • These too have tentatively been identified as – cysts of some kind of algae

Endosymbiosis and the Origin of Eukaryotic Cells • Eukaryotic cells probably formed – from several prokaryotic cells – that entered into a symbiotic relationship – Symbiosis, • involving a prolonged association of two or more dissimilar organisms, – is quite common today • In many cases both symbionts benefit from the association – as occurs in lichens, • once thought to be plants • but actually symbiotic fungi and algae

Endosymbiosis • In a symbiotic relationship, – each symbiont must be capable – of metabolism and reproduction, – but in some cases one symbiont – cannot live independently • This may have been the case – with Proterozoic symbiotic prokaryotes – that became increasingly interdependent – until the unit could exist only as a whole • In this relationship – one symbiont lived within the other, – which is a special type of symbiosis – called endosymbiosis

Evidence for Endosymbiosis • Supporting evidence for endosymbiosis – comes from studies of living eukaryotic cells – containing internal structures called organelles, • such as mitochondria and plastics, – which contain their own genetic material • In addition, prokaryotic cells – synthesize proteins as a single system, • whereas eukaryotic cells – are a combination of protein-synthesizing systems

Organelles Capable of Protein Synthesis • That is, some of the organelles – within eukaryotic cells are capable of protein synthesis • These organelles • with their own genetic material • and protein-synthesizing capabilities – are thought to have been free-living bacteria • that entered into a symbiotic relationship, • eventually giving rise to eukaryotic cells

Multicelled Organisms • Obviously multicelled organisms – are made up of many cells, – perhaps billions, – as opposed to a single cell as in prokaryotes • In addition, multicelled organisms – have cells specialized to perform specific functions – such as respiration, – food gathering, – and reproduction

Dawn of Multicelled Organisms • We know from the fossil record – that multicelled organisms – were present during the Proterozoic, – but we do not know exactly when they appeared • What seem to be some kind of multicelled algae appear – in the 2. 1 -billion-year-old fossils • from the Negaunee Iron Formation in Michigan – as carbonaceous filaments • from 1. 8 billion-year-old rocks in China – as somewhat younger carbonaceous impressions – of filaments and spherical forms

Multicelled Algae? • Carbonaceous impressions – in Proterozoic rocks – in the Little Belt Mountains, Montana • These may be impressions of multicelled algae

Studies of Present-Day Organisms • How did this important transition taken place? • Perhaps a single-celled organism divided – but the daughter cells formed – an association as a colony • Each cell would have been capable – of an independent existence, – and some cells might have become somewhat specialized • as are the cells of colonial organisms today • Increased specialization of cells – may have given rise to – comparatively simple multicelled organisms – such as algae and sponges

The Multicelled Advantage? • Is there any particular advantage to being multicelled? • For something on the order of 1. 5 billion years – all organisms were single-celled – and life seems to have thrived • In fact, single-celled organisms – are quite good at what they do – but what they do is very limited

The Multicelled Advantage? • For example, single celled organisms – can not grow very large, because as size increases, – proportionately less of a cell is exposed – to the external environment in relation to its volume – and the proportion of surface area decreases • Transferring materials from the exterior – to the interior becomes less efficient

The Multicelled Advantage? • Also, multicelled organisms live longer, – since cells can be replaced and more offspring can be produced • Cells have increased functional efficiency – when they are specialized into organs with specific capabilities

Late Proterozoic Animals • Biologists set forth criteria such as – method of reproduction – and type of metabolism – to allow us to easily distinguish – between animals and plants • Or so it would seem, – but some present-day organisms – blur this distinction and the same is true – for some Proterozoic fossils • Nevertheless, the first – relatively controversy-free fossils of animals – come from the Ediacaran fauna of Australia – and similar faunas of similar age elsewhere

The Ediacaran Fauna • In 1947, an Australian geologist, R. C. Sprigg, – discovered impressions of soft-bodied animals – in the Pound Quartzite in the Ediacara Hills of South Australia • Additional discoveries by others turned up what appeared to be – impressions of algae and several animals – many bearing no resemblance to any existing now • Before these discoveries, geologists – were perplexed by the apparent absence – of fossil-bearing rocks predating the Phanerozoic

Ediacaran Fauna • The Ediacaran fauna of Australia Tribrachidium heraldicum, a possible primitive echinoderm Spriggina floundersi, a possible ancestor of trilobites

Ediacaran Fauna Pavancorina minchami • Restoration of the Ediacaran Environment

Ediacaran Fauna • Geologists had assumed that – the fossils so common in Cambrian rocks – must have had a long previous history – but had little evidence to support this conclusion • The discovery of Ediacaran fossils and subsequent discoveries – have not answered all questions about pre. Phanerozoic animals, – but they have certainly increased our knowledge – about this chapter in the history of life

Represented Phyla • Three present-day phyla may be represented – in the Ediacaran fauna: • jellyfish and sea pens (phylum Cnidaria), • segmented worms (phylum Annelida), • and primitive members of the phylum Arthropoda (the phylum with insects, spiders crabs, and others) • One Ediacaran fossil, Spriggina, – has been cited as a possible ancestor of trilobites • Another might be a primitive member – of the phylum Echinodermata

Distinct Evolutionary Group • However, some scientists think – these Ediacaran animals represent – an early evolutionary group quite distinct from – the ancestry of today’s invertebrate animals • Ediacara-type faunas are known – from all continents except Antarctica, – are collectively referred to as the Ediacaran fauna – were widespread between 545 and 670 million years ago – but their fossils are rare • Their scarcity should not be surprising, though, – because all lacked durable skeletons

Other Proterozoic Animal Fossils • Although scarce, a few animal fossils – older than those of the Ediacaran fauna are known • A jellyfish-like impression is present – in rocks 2000 m below the Ediacara Hills Pound Quartzite, • Burrows, in many areas, – presumably made by worms, – occur in rocks at least 700 million years old • Wormlike and algae fossils come – from 700 to 900 million-year-old rocks in China – but the identity and age of these "fossils" has been questioned

Wormlike Fossils from China • Wormlike fossils from Late Proterozoic rocks in China

Soft Bodies • All known Proterozoic animals were softbodied, – but there is some evidence that the earliest stages in the origin of skeletons was underway • Even some Ediacaran animals – may have had a chitinous carapace – and others appear to have had areas of calcium carbonate • The odd creature known as Kimberella – from the latest Proterozoic of Russia – had a tough outer covering similar to – that of some present-day marine invertebrates

Latest Proterozoic Kimberella • Kimberella, an animal from latest Proterozoic rocks in Russia – Exactly what Kimberella was remains uncertain – Some think it was a sluglike creature – whereas others think it was more like a mollusk

Durable Skeletons • Latest Proterozoic fossils – of minute scraps of shell-like material – and small tooth like denticles and spicules, • presumably from sponges • indicate that several animals with skeletons – or at least partial skeletons existed • However, more durable skeletons of • silica, • calcium carbonate, • and chitin (a complex organic substance) – did not appear in abundance until the beginning – of the Phanerozoic Eon 545 million years ago

Proterozoic Mineral Resources • Most of the world's iron ore comes from – Proterozoic banded iron formations • Canada and the United States have large deposits of these rocks – in the Lake Superior region – and in eastern Canada • Thus, both countries rank among – the ten leading nations in iron ore production

Iron Mine • The Empire Mine at Palmer, Michigan – where iron ore from the Early Proterozoic Negaunee Iron Formation is mined

Nickel • In the Sudbury mining district in Ontario, Canada, – nickel and platinum are extracted from Proterozoic rocks • Nickel is essential for the production of nickel alloys such as • stainless steel • and Monel metal (nickel plus copper), – which are valued for their strength and resistance to corrosion and heat • The United States must import – more than 50% of all nickel used – mostly from the Sudbury mining district

Sudbury Basin • Besides its economic importance, the Sudbury Basin, – an elliptical area measuring more than 59 by 27 km, – is interesting from the geological perspective • One hypothesis for the concentration of ores – is that they were mobilized from metal-rich rocks – beneath the basin – following a high-velocity meteorite impact

Platinum and Chromium • Some platinum – for jewelry, surgical instruments, – and chemical and electrical equipment – is exported to the United States from Canada, – but the major exporter is South Africa • The Bushveld Complex of South Africa – is a layered igneous complex containing both • platinum • and chromite – the only ore of chromium, – United States imports much of the chromium – from South Africa – It is used mostly in stainless steel

Oil and Gas • Economically recoverable oil and gas – have been discovered in Proterozoic rocks in China and Siberia, – arousing some interest in the Midcontinent rift as a potential source of hydrocarbons • So far, land has been leased for exploration, – and numerous geophysical studies have been done • However, even though some rocks – within the rift are know to contain petroleum, – no producing oil or gas wells are operating

Proterozoic Pegmatites • A number of Proterozoic pegmatites – are important economically • The Dunton pegmatite in Maine, – whose age is generally considered – to be Late Proterozoic, – has yielded magnificent gem-quality specimens – of tourmaline and other minerals • Other pegmatites are mined for gemstones as well as for – tin, industrial minerals, such as feldspars, micas, and quartz – and minerals containing such elements – as cesium, rubidium, lithium, and beryllium

Proterozoic Pegmatites • Geologists have identified more than 20, 000 pegmatites – in the country rocks adjacent – to the Harney Peak Granite – in the Black Hills of South Dakota • These pegmatites formed ~ 1. 7 billion years ago – when the granite was emplaced as a complex of dikes and sills • A few have been mined for gemstones, tin, lithium, micas, – and some of the world's largest known – mineral crystals were discovered in these pegmatites

Summary • The crust-forming processes – that yielded Archean granite-gneiss complexes – and greenstone belts – continued into the Proterozoic – but at a considerably reduced rate • Archean and Proterozoic greenstone belts – differed in detail • Early Proterozoic collisions – between Archean cratons formed larger cratons – that served as nuclei – around which Proterozoic crust accreted

Summary • One such landmass was Laurentia – consisting mostly of North America and Greenland • Important events – in the evolution of Laurentia were • Early Proterozoic amalgamation of cratons • followed by Middle Proterozoic igneous activity, • the Grenville orogeny, and the Midcontinent rift • Ophiolite sequences – marking convergent plate boundaries – are first well documented from the Early Proterozoic, – indicating that a plate tectonic style similar – to that operating now had been established

Summary • Sandstone-carbonate-shale assemblages – deposited on passive continental margins – are known from the Archean – but they are very common by Proterozoic time • The supercontinent Rodinia – assembled between 1. 3 and 1. 0 billion years ago, – fragmented, – and then reassembled to form Pannotia about 650 million years ago • Glaciers were widespread – during both the Early and Late Proterozoic

Summary • Photosynthesis continued – to release free oxygen into the atmosphere – which became increasingly oxygen rich through the Proterozoic • Fully 92% of Earth's iron ore deposits – in banded iron formations were deposited – between 2. 5 and 2. 0 billion years ago • Widespread continental red beds – dating from 1. 8 billion years ago indicate – that Earth's atmosphere had enough free oxygen – for oxidation of iron compounds

Summary • Most of the known Proterozoic organisms – are single-celled prokaryotes (bacteria) • When eukaryotic cells first appeared is uncertain, – but they may have been present by 2. 1 billion years ago • Endosymbiosis is a widely accepted theory for their origin • The oldest known multicelled organisms – are probably algae, – some of which may date back to the Early Proterozoic

Summary • Well-documented multicelled animals – are found in several Late Proterozoic localities • Animals were widespread at this time, – but because all lacked durable skeletons – their fossils are not common • Most of the world's iron ore produced – is from Proterozoic banded iron formations • Other important resources – include nickel and platinum