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[_ Old Earth _] A History of Life on Earth

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A History of Life on Earth - Written by Jon Erickson:


Chapter One:
Planet Earth:

Our sun, with its nine planets and their satellites, is a rarity among stars, one of only a small number of single, medium-sized stars in our galaxy. The inner rocky, or terrestrial, planets are much alike, with the notable exception of the Earth, the only planet with a water ocean and an oxygen atmosphere. It is also the only terrestrial planet with a rather large moon, a pairing that still defies explanation.

The atmosphere and ocean evolved during a tumultuous period of crustal formation, volcanic outgassing, and comet degassing. Numerous giant meteorites slammed into the Earth, adding unique ingredients to the boiling cauldron. Raging storms brought deluge after deluge and unimaginable electrical displays. Out of this chaos came life.


The Solar System:

Some 15 billion years ago, the universe originated with a force whose power is still hurling the farthest galaxies away from us at nearly the speed of light. The steady expansion of the beginning universe might have been temporarily sped up by a sudden inflationary bulge, as the new universe rapidly ballooned outward for an instant and then settled down to a steady growth. As the protouniverse expanded, it cooled sufficiently to allow basic units of matter to clump together to form billions of galaxies each containing billions of stars. The first galaxies evolved when the universe was about 1 billion years old and only about a tenth of its present size.

Our Milky Way galaxy has five spiral arms that peel off a central bulge. New stars originate in dense regions of interstellar gas and dust called giant molecular clouds. Several times a century, a giant star over 100 times larger than the sun explodes, producing a supernova a billion times brighter than an ordinary star. When a star reaches the supernova stage, after a very hot existence spanning several hundred million years, the previously stable nuclear reactions in its core become explosive. The star sheds its outer covering, while the core compresses to an extremely dense, hot body called a neutron star; this would be like squeezing the Earth down to about the size of a golf ball.

The expanding stellar matter from the supernova forms a nebula composed mostly of hydrogen and helium along with particulate matter that comprises all the other known elements. About a million years later, the solar nebula collapses into a star. Shock waves from nearby supernovae compress portions of the nebula, with gravitational forces causing the nebular matter to collapse upon itself, forming a protostar. As the solar nebula collapses, it rotates faster and faster, and spiral arms peel off the rapidly spinning nebula to form a protoplanetary disk. Meanwhile, the compressional heat initiates a thermonuclear reaction in the core, and a star is born.

A new star forms in the Milky Way galaxy every few years or so. About 4.6 billion years ago, our sun, an ordinary main-sequence star, ignited in one of the dusty spiral arms of the galaxy. Single, medium-sized stars like the sun are a rarity, and due to their unique evolution such stars appear to be the only ones with planets. Thus, of the myriad stars overhead, only a handful might possess a system of orbiting planets, and fewer yet might contain life.

During the sun's early developmental stages, it was ringed by a protoplanetary disk composed of several bands of coarse particles, called planetesimals, accreting from grains of dust cast off by a supernova. Some 11 trillion planetesimals orbited the sun during the Solar System's early stages of development. As they continued to collide and grow, the small rocky chunks swung around the infant sun in highly elliptical orbits along the same plane, called the ecliptic.

The constant collisions among planetesimals built larger bodies, some of which grew to over 50 miles wide, but most of the planetary mass still resided in the small planetesimals. The presence of large amounts of gas in the solar nebula slowed the planetesimals, enabling them to coalesce into planets. The planetesimals in orbit between Mars and Jupiter were unable to combine into a planet due to Jupiter's strong gravitational attraction and instead formed a belt of asteroids, many of which were several hundred miles wide.

The Solar System itself is quite large, consisting of nine known planets and their moons. The image of the original solar disk can be traced by observing the motions of the planets, all of which revolve sun in the same direction it rotates, all but one, Pluto, within 3 degrees of the ecliptic. Some 7 billion miles from the sun lies the heliopause, which marks the boundary between the sun's domain and interstellar space. About 20 billion miles from the sun is a region of gas and dust, probably remnants of the original solar nebula. Astronomers think a belt of comets lying on the ecliptic exists in this region. Several trillion miles from the sun is a shell of comets that formed from the leftover gas and ice of the original solar nebula.
 
The Protoearth:

As the Earth continued to grow by accumulating planetesimals, most of which had temperatures exceeding 1,000 degrees Celsius, its orbit began to decay due to drag forces created by leftover gases in interplanetary space. The formative planet slowly spiraled inward toward the sun, sweeping up additional planetesimals along the way like a cosmic vacuum cleaner. Eventually, the Earth's path around the sun was swept clean of interplanetary material, and its orbit stabilized near its present position.

The core and mantle segregated possibly within the first 100 million years, during a time when the Earth was in a molten state heated by radioactive isotopes and impact friction from planetesimals. The presence of magnetic rocks 2.7 billion years old suggests the Earth had a molten outer core compared to its present size at an early age. The Earth's interior was hotter, less viscous, and more vigorous, with a highly active convective flow. Heavy turbulence in the mantle with a heat flow three times greater than today produced violent agitation on the Earth's surface. This turmoil created a sea of molten and semimolten rock broken up by giant fissures, from which fountains of lava spewed skyward.

The early Earth did not possess an atmosphere to hold in the internally generated heat, and the surface rapidly cooled, forming a thin basaltic crust similar to that of Venus. Indeed, the moon and the inner planets offer clues to the Earth's early history. Among the features common to the terrestrial planets was their ability to produce voluminous amounts of basaltic lavas. The Earth's original crust has long since dissapeared, remixed into the interior by the impact of giant meteorites that were leftovers from the beginning of the Solar System.

The formative Earth was subjected to massive volcanism and meteorite bombardment that repeatedly destroyed the crust. A massive meteorite shower, consisting of thousands of 50-mile-wide impactors, bombarded the Earth and its moon between 4.2 and 3.8 billion years ago. The other inner planets and the moons of the outer planets show dense pockmarks from this invasion (Kind of like the landscape of Iraq!). The meteorite bombardment melted large portions of the Earth's crust, nearly half of which contained large impact basins up to 10 miles deep.

As the meteorites plunged into the planet's thin basaltic crust, they gouged out huge quantities of parcially solidified and molten rock. The scars in the crust quickly healed, as batches of fresh magma bled through giant fissures and poured onto the surface, creating a magma ocean. The continued destruction of the crust by heavy volcanic and meteoritic activity explains why the first half-billion years of Earth history are missing from the geologic record.
 
The Moon:

A popular explanation for the creation of the moon supposes a collision between the Earth and a large celestial body. According to this theory, soon after the Earth's formation, an asteroid about the size of Mars was knocked out of the asteroid belt either by Jupiter's strong gravitational attraction or by a collision with a wayward comet. On its way toward the inner Solar System, the asteroid glanced off the Earth, and the tangential collision, which lasted for half an hour, created a powerful explosion equivalent to the detonation of an amount of dynamite equal to the mass of the asteroid. The collision tore a huge gash in the Earth, and a large portion of its molten interior along with much of the rocky mantle of the impactor spewed into orbit, forming a ring of debris around the planet called a protolunar disk.

The force of the impact might have knocked the Earth over, tilting its rotational axis about 25 degrees. Similar collisions involving the other planets, especially Uranus, which orbits on its side like a rolling bowling ball, might explain their various degrees of tilt and elliptical orbits. The glancing blow also might have increased the Earth's angular momentum (rotational energy) and melted the planet throughout, forming a red-hot orb in orbit around the sun. The present angular momentum of the Earth suggests that other methods of lunar formation such as fission, capture, or assembly in place were unlikely.

The new satellite continued growing as it swept up debris in orbit around the Earth. In addition, huge rock fragments orbited the moon and crashed onto its surface. The massive meteorite shower that bombarded the Earth equally pounded the moon, and numerous large asteroids struck the lunar surface and broke through the thin crust. Great floods of dark basaltic lava spilled onto the surface, giving the moon a landscape of giant craters and flat lava plains called maria from the Latin word for "seas."

The moon became gravitationally locked onto its mother planet, rotating at the same rate as its orbital period, causing one side to always face the Earth. The moon exerts a force on the spinning Earth called nutation that causes the rotational axis to precess, or wobble. Many moons around other planets share similar characteristics with the Earth's moon, suggesting they formed in the same manner. Since the Earth's sister planet Venus evolved under similar circumstances and is so much like our planet, the absence of a Venusian moon is quite curious. It might have crashed into its mother planet or escaped into orbit around the sun. Perhaps Mercury, which is about the same size as Earth's moon, was once a moon of Venus.

The early moon orbited so close to the Earth it filled much of the sky. The presence of a rather large moon, the biggest in our Solar System in relation to its mother planet, the two forming a twin planetary system, might have had a major influence on the initiation of life. The unique properties of the Earth-moon system raised the tides in the ocean, and cycles of wetting and drying in tidal pools might have helped the Earth acquire life much earlier than previously thought possible.
 
The Atmosphere:

For the first half-billion years, while the Earth was still spinning wildly on its axis and surface rocks were scorching hot, the planet lacked an atmosphere and was enveloped in a near vacuum much like the moon is today. Soon after the meteorite bombardment began about 4.2 billion years ago, the Earth aquired a primordial atmosphere composed of carbon dioxide, nitrogen, water vapor, and other gases spewed out of a profusion of volcanoes. The atmosphere was so saturated with water vapor, atmospheric pressure was nearly a hundred times greater than it is today. Some meteorites that hit the Earth were stony, composed of rock and metal, others were icy, composed of frozen gases and water ice, many contained carbon (Imagine millions of tons of coal raining down from the skies). Perhaps these carbon-rich meteorites bore the seeds of life, which might have existed in the universe eons before the Earth came into being. Comets, composed of ice and rock debris, also plunged into the Earth, releasing large quantities of water vapor and gas. These cosmic gases were mostly composed of carbon dioxide, ammonia, and methane.

Most of the water vapor and gases originated within the Earth. Magma contains large quantities of volatiles, volcanic gases comprised mostly water and carbon dioxide, which make it more fluid. Tremendous pressures deep inside the Earth keep the volatiles within the magma, and when the magma rises to the surface, the drop in pressure releases the trapped water and gases, often explosively. The early volcanoes erupted violently because the Earth's interior was much hotter than it is now, and the magma contained higher amounts of volatiles.

Oxygen originated directly by the breakdown of water vapor and carbon dioxide by the sun's strong ultraviolet radiation. All oxygen generated in this manner quickly bonded to metals in the crust, much like the rusting of iron. Oxygen also recombined with hydrogen and carbon monoxide to reconstitute water vapor and carbon dioxide. A small amount of oxygen might have existed in the upper atmosphere, where it provided a thin ozone screen. This would have reduced the breakdown of water molecules by ultraviolet rays and prevented the loss of Earth's water, a fate that might have visited Venus eons ago (The Venus rift valley, which at 3 miles deep, 175 miles wide, and 900 miles long, is the largest canyon in out Solar System).

Nitrogen, which comprises about 80 percent of the present atmosphere, originated from volcanic eruptions and the breakdown of ammonia, a molecule with one nitrogen atom and three hydrogen atoms, and a major constituent of the primordial atmosphere. Unlike most other gases which have been replaced or recycled, the Earth retains much of its original nitrogen. This is because nitrogen readily transforms into nitrate, which easily dissolves in the ocean, where denitrifying bacteria return the nitrate-nitrogen back to its gaseous state. Decaying organisms also release nitrogen back into the atmosphere. Therefor, without life, the Earth would long ago lost its nitrogen and posses only a fraction of its present atmospheric pressure.
 
The Ocean:

While the atmosphere formed, the Earth's surface was constantly in chaos. Winds blew with tornadic force, and fierce dust storms on the dry surface blanketed the entire planet with suspended sediment much like the Martian dust storms of today. Huge lightning bolts flashed across the sky, and the thunder was earth-shattering as one gigantic shock wave after another reverberated over the land. Volcanoes erupted in one giant outburst after another. The sky lit up from the pyrotechnics created by the white-hot sparks of ash and the glow of constantly flowing red-hot lava. The restless Earth was rent apart as massive quakes cracked open the thin crust. Huge batches of magma flowed through the fissures and flooded the surface with voluminous amounts of lava, forming flat featureless plains.

The intense volcanism lofted millions of tons of volcanic debris into the atmosphere, where it remained suspended for long periods. Ash and dust particles scattered sunlight and gave the sky an eerie red glow like that on Mars. The dust also cooled the Earth and acted as particulate matter, upon which water vapor could coalesce. When tempuratures in the upper atmosphere lowered, water vapor condensed into clouds. The clouds were so thick and heavy they almost completely blocked out the sun, and the surface was in near darkness, dropping tempuratures even further.

As the atmosphere continued to cool, huge raindrops fell from the sky, and the Earth recieved deluge after deluge. Raging floods cascaded down steep mountain slopes and the sides of large meteorite craters and gouged out deep canyons in the rocky plain. When the rains ceased and the skies finally cleared, the Earth merged as a giant blue orb, covered by a global ocean nearly 2 miles deep and dotted with numerous volcanic islands.

Ancient marine sediments found in the metamorphosed rocks of the Isua Formation in southwestern Greenland support this scenario for the beginning of the ocean. The rocks originated in volcanic island arcs and therefor lend credence to the idea that plate tectonics operated early in the history of the Earth. They are among the oldest rocks, dating to about 3.8 billion years ago, and indicate that the planet had suface water by this time.

During the years between the end of the great meteorite bombardment and the formation of the first sedimentary rocks, vast quantities of water flooded the Earth's surface. Seawater probably began salty, due to the abundance of chlorine and sodium provided by volcanoes, but did not reach its present concentration of salts untill about 500 million years ago. The warm ocean was heated from above by the sun and from below by active volcanoes on the ocean floor, which continually supplied seawater with the elements of life.
 
The Emergance of Life:

Life arose on this planet during a period of crustal formation and volcanic outgassing of an atmosphere and ocean. It was also a time of heavy meteorite bombardment, rocky asteroids and icy comets constantly showering the early Earth, possibly providing the main source of the planet's water. Interplanetary space was littered with debris that pounded the newborn planets. Some of this space junk might provided organic compounds, from which life could evolve. The Earth is still pelted by meteorites that contain amino acids, the precursors of proteins. The meteorite impacts would most likely have made conditions very difficult for proteins to organize into living cells. The first cells might have been repeatedly exterminated, forcing life to originate again and again. Whenever primitive organic molecules attempted to arrange themselves into living matter, frequent impacts blasted them apart before they had a chance to reproduce.

Some large impactors might have generated enough heat to repeatedly evaporate most of the ocean. The vaporized ocean would have raised surface pressure over 100 times greater than the present atmosphere, and the resulting high temperatures would have sterilized the entire planet. Several thousand years would elapse before steam condensed into rain and the ocean basins refilled again, only to await the next ocean-evaporating impact. Such harsh conditions could have set back the emergence of life hundreds of millions of years.

Perhaps the only safe place for life to evolve was on the deep ocean floor, where a high density of hydrothermal vents existed. Hydrothermal vents are like geysers on the bottom of the ocean that expel mineral-laden hot water heated by shallow magma chambers resting just beneath the ocean floor. The vents might have created an environment capable of generating an immense number of organic reactions. They also would have provided evolving life forms with all the essential nutrients to sustain themselves. Indeed, such an environment exists today, home to some of the strangest creatures found on Earth. In this environement, life could have originated as early as 4.2 billion years ago.

From the very beginning, life had many common characteristics. No matter how varied life is today, from the simplest bacteria to man, its central molecular machinery is exactly the same. Every cell of every organism is constructed from the same set of 20 amino acids. All organisms use the same energy transfer mechanism for growth. All strands of DNA are left handed double helixes, and the operation of the genetic code in protein synthesis is the same for all living things.

With so much similarity, all life must have sprung from a common ancestor, and all alien forms, of which no descendants exist today, became extinct early in the history of life. Furthermore, no new life forms are being created today either because the present chemical environment is not conductive to the formation of life or living organisms prey upon the newly created life forms before they have a chance to evolve.

Since life appeared within the first half-billion years of the Earth's existence, it must have evolved from simple materials into complex organisms rather quickly. Primitive bacteria, which descended from the earliest known form of life, remain by far the most abundant organisms. Evidence that life began early in the Earth's history, when the planet was still quite hot, exists today as thermophilic (Heat-Loving) bacteria found in thermal springs and other hot-water environments throughout the world.

The existence of these organisms is evidence that thermophiles were the common ancestors of all life. The early conditions of Earth would have been ripe for the evolution of thermophilic organisms, most of which have a sulfer-based energy metabolism; sulfur compounds would have been plentiful on the hot, volcanically active planet.

It is probably fortunate the early Earth had an abundance of sulfur, which was spewed out of a profusion of volcanoes. As long as surface temperatures were hot, ring molecules of sulfur atoms in the atmosphere would block out solar ultraviolet radiation. Otherwise, the first living cells would have sizzled in the deadly rays of the sun. However, an ultraviolet shield might not have been necessary in the primordial atmosphere, because some primitive bacteria appear to tolerate high levels of ultraviolet radiation.

The first living organisms were extremely small noncellular blobs of protoplasm. The self-duplicating organisms fed on a rich broth of organic molecules generated in the primordial sea. Such a nutritional abundance set off a rapid chain reaction, resulting in a phenomenal growth. The organisms drifted freely in the ocean currents and disappeared to all parts of the world. Although the first simple organisms appear to have arrived soon after conditions on Earth became favorable, almost another billion years passed before life even remotely resembled anything living today.
 
Chapter Two:
Archean Algae


The first 4 billion years of Earth history, or about nine-tenths of geologic time, are referred to as the Precambrian, the longest and least understood of all eras. The Precambrian is divided nearly equally into the Archean and Proterozoic eons. The boundary is somewhat arbitrary and reflects major differences between the types of rocks formed during the two periods. Archean rocks are products of rapid crustal formation, while Proterozoic rocks are more representative of relatively stable modern geology.

The Archean, from 4.6 to 2.5 billion years ago, covers a time when the Earth was in great turmoil and subjected to extensive meteorite bombardment and intense volcanism. The high internal heat of the newborn planet kept the surface well agitated, destroying any semblance of a crust, which is why the first several hundred million years are absent from the geologic record. During this interval, the planet experienced a restlessness that might have been a major factor in the emergence of life do early in the history of the Earth.


The Age of Algae:

Life in the Archean consisted mostly of bacteria, unicellular or noncellular algae, and clusters of algae called stromatolites, from the Greek word stroma, meaning "stony carpet." The oldest evidence of life includes microfossils, which are remains of ancient microorganisms, and stromatolites, which are layered structures formed by the accretion of fine sediment grains by colonies of cyanobacteria or primitive blue-green algae living on the ocean floor. Stromatolites, however, are only indirect evidence of early life because they are not the remains of the microorganisms themselves but only the sedimentary structures they built.

Early stromatolite fossils exist in 3.5-billion-year-old sedimentary rocks of the Towers Formation of the Warrawoona group in North Pole, western Australia. The region was once a tidal inlet, overshadowed by tall volcanoes that erupted ash and lava, which flowed into a shallow sea. Thunderclouds hovered over the peaks, and lightning darted back and forth. Furious winds whipped up high waves that pounded the basaltic cliffs of the coastline. Farther inland, hummocks of black basalt dominated the landscape. The rotten-egg stench of sulfur was persuasive. Frequent downpours fed tidal streams that meandered onto a flat expanse of glistening gray mud before reaching the sea. Elsewhere, scattered shallow pools containing highly saline water periodically evaporated, leaving behind a variety of salts. Often, a floodtide washed across the mud flat, shifting the sediments and replenishing the brine pool.

Although the Archean spans almost half the Earth's history, its rocks represent less than 20 percent of the total area exposed at the surface. Furthermore, all known Precambrian rocks have suffered some heating episode and metamorphism. Unlike most ancient rocks in the 3.5- to 3.8-billion-year range throughout the world, only a few like those of the North Pole sequence have a history of low metamorphic temperatures. Therefore, rocks of this region have remained relatively cool throughout geologic history.

Rocks subjected to the intense heat of the Earth's interior have lost all traces of fossilized life. Even in mildly metamorphosed rocks, the existence of microfossils, which are the preserved cell walls of unicellular microorganisms, is often difficult to prove. Most of these apparent fossils are simple spheres with a few surface features, composed of inorganic carbon compounds squeezed into spheroids by the growth of mineral grains deposited around them. But some spheres were linked in pairs or in chains, and were unlikely to have been created simply by inorganic processes.

Associated with the North Pole rocks were cherts, extremely hard siliceous rocks containing microfilaments, which are small, threadlike structures of possible bacterial origin. Similar cherts with microfossils of filamentous bacteria are found at eastern Transvaal, South Africa, dating between 3.2 and 3.3 billion years old. They also exist in 2-billion-year-old chert from the Gunflint iron formation on the north shore of Lake Superior in North America. Most Precambrian cherts appear to be chemical sediments precipitated from silica-rich water in deep oceans. The abundance of chert in the Archean might serve as evidence that most of the crust was deeply submerged. However, cherts in the North Pole region appear to have a shallow-water origin.

Chert-forming silica leached out of volcanic rocks that erupted into shallow seas. The silica-rich water circulated through porous sediments, dissolving the original minerals and precipitating silica in their place. Microorganisms buried in the sediments were encased in one of nature's hardest substances, allowing the microfossils to survive the rigors of time. Modern seawater is deficient in silica because organisms like sponges and diatoms extract it to built their skeletons. Massive deposits of diatomite, also called diatomaceous earth, composed of diatom cell walls are a tribute to the great success of these organisms in the post-Precambrian era.

The North Pole stromatolites are distinctly layered accumulations of calcium carbonate with a rounded cabbagelike appearance. The size and shape of Archean-age microfossils and the form of the stromatolites suggest these microorganisms were either oxygen-releasing or sulfur-oxidizing photosynthetic life forms, dependent on sunlight for their growth.

Living stromatolites are similar to those of ancient times and comprise concentrically layered mounds of calcium carbonate built by bacteria or algae, which cement sediment grains together by secreting a jellylike ooze. Older structures at the Australian site are classified either as stromatolite fossils or as layered inorganic sedimentary structures with no biological origin. But microscopic filaments radiating outward from a central point and resembling filamentous (threadlike) bacteria also exist in the fossils, suggesting bacteria built the stromatolites.

Modern stromatolites reside in the intertidal zones above the low tide mark, and their length reflects the height of the tides, which are controlled mostly by the gravitational pull of the moon. The oldest stromatolite colonies of the North Pole region grew to great heights, with some attaining lengths of over 30 feet, which suggests that at an early age the moon orbited much closer to the Earth, and its strong gravitational attraction at this range raised tremendous tides that flooded coastal areas a long distance inland.

The early Earth spun faster on its axis, which meant days were much shorter than today. As the planet's rotation slowed due to drag forces caused by the tides (making days longer), it transferred some of its angular momentum (rotational energy) to the moon, flinging it outward into a widening orbit. Even today, the moon is still receding from the Earth.
 
The Protozoans:

Simple organisms with primitive cells called prokaryotes, from the Greek word karyon meaning "nutshell," lacked a distinct nucleus. They lived under anaerobic (lacking oxygen) conditions and depended mainly on outside sources of nutrients, typically a rich supply of organic molecules continuously created in the sea around them. Most organisms had a primitive form of metabolism called fermentation that converted nutrients into energy. It was an inefficient form of metabolism, releasing energy when enzymes broke down simple sugars such as glucose into smaller molecules.

Each tiny organism was a committee of simpler organelles that the organism incorporated into its cells in a symbiotic relationship, creating a new type of organism called a eukaryote, which was equipped with a nucleus that organized genetic material. When the cell devided, DNA in the nucleus and the organelles replicated, with half the genes remaining with the parent and the other half passed on to the daughter cell.

This process, called mitosis, increased the likelihood of genetic variation and greatly accelerated the rate of evolution as organisms encountered new environments, and were able to adapt as needed. The extraordinary variety of plant and animal life that has arisen on this planet over the last 600 million years is due exclusively to the introduction of the eukaryotic cell and its huge potential for genetic diversity.

Early single-cell animals called protistids shared many characteristics with plants. The cells contained elongated structures of mitochondria, which are bacterialike bodies that produce energy by oxidation. They also contained chloroplasts, which are packets of chlorophyll that provide energy by photosynthesis. Many protozoans secreted a tiny shell composed of calcium carbonate. When the animal died, their shells sank to the bottom of the ocean, where over time they built up impressive formations of limestone.

The ability to move about under their own power is what essentially separates animals from plants, although some animals perform this function only in the larval stage and become sedentary or fixed to the seabed as adults. Mobility enabled animals to feed on plants and other animals, establishing a new predator-prey relationship.

Some organisms moved about by a thrashing tail called a flagellum, resembling a filamentous bacterium that joined the host cell for mutual benefit. Other cells had tiny hairlike appendages called cilia that propelled the organism around by rhythmically beating the water. Many, like the amoeba, traveled by extending fingerlike protrusions outward from the main body and flowing into them.

The earliest organisms were sulfur-metabolizing bacteria similar to those living symbiotically in the tissues of tube worms, which live near sulfurous hydrothermal vents on the East Pacific Rise and also on the Gorda Ridge off the northwest Pacific coast of the United States. Sulfur was particularly abundant in the early ocean and combined easily with metals like iron to form sulfates. Since the atmosphere and ocean lacked oxygen, the bacteria obtained energy by the reduction of sulfate ions. The growth of primitive bacteria was thus limited by the amount of organic molecules produced in the ocean. Although this form of energy was satisfactory, bacteria were letting a plentiful source of energy go to waste, namely sunlight.
 
Photosynthesis:

The ratios of carbon isotopes in Archean rocks suggest that photosynthesis was in progress at an early age. The seas contained an abundance of iron, and oxygen generated by photosynthesis was lost by oxidation with this element, a fortunate circumstance since oxygen was also poisonous to primitive life forms. Abundant sulfur in the early sea provided the nutrients to sustain life without the need for oxygen, and bacteria obtained energy by the reduction (opposite of oxidation) of this important element.

A primitive form of photosynthesis probably began about 3.5 billion years ago with the first appearance of blue-green algae, or its predecessor, a photosynthetic bacteria called green sulfur bacteria. These organisms were best suited to an oxygen-poor environment. Oxygen was kept to a minimum by reacting with both dissolved metals in seawater and gases emitted from submarine hydrothermal vents.

The first green-plant photosynthesizers called proalgae were probably intermediate between bacteria and blue-green algae and could switch from fermentation, a primitive form of metabolism, to photosynthesis and back again, depending on their environment. Since sunlight penetrates seawater to a maximum depth of about 300 feet, the proalgae were confined to shallow water. Around 2.8 billion years ago, microorganisms called cyanobacteria began to use sunlight as their main energy source to drive the chemical reactions needed for sustained growth.

The development of photosynthesis was possibly the single most important step in the Evolution of life. It gave a primitive form of blue-green algae a practically unlimited source of energy. Photosynthesis utilized sunlight to split water molecules, and the hydrogen combined with carbon dioxide that was abundant in the early ocean and atmosphere to form simple sugars and proteins, liberating oxygen in the process. The growth of photosynthetic organisms was phenomenal, and the population explosion would have gotten out of hand except that oxygen, generated as a waste product of photosynthesis, was also poisonous. If not for the development of special enzymes to help organisms cope with and later use oxygen for their metabolism, life would certainly have been in jeopardy.

To create and maintain an oxygen-rich atmosphere, the carbon dioxide used in the photosynthetic process had to be buried as carbonate rock faster than the oxygen was consumed by the oxidation of carbon, metals, and volcanic gases. About 2 billion years ago, oxygen began replacing carbon dioxide in the ocean and atmosphere. Therefor, organisms had to either develop a means of shielding their nuclei and other critical sites from oxygen or use chemical pathways that removed hydrogen instead of adding oxygen. These innovations led to the evolution of the eukaryotes. In this manner, oxygen was responsible in large part for the evolution of higher forms of life.
 
Greenstone Belts:

Greenstones are ancient metamorphosed rocks unique to the Archean. About 4 billion years ago, the mantle cooled enough to form a permanent crust composed of a thin layer of basalt embedded with scattered blocks of granite called "rockbergs." The granite combined into stable bodies of basement rock, upon which all other rocks were deposited. The basement rocks became the nuclei of the continents and are presently exposed in broad, low-lying, domelike structures called shields.

The Precambrian shields are extensive uplifted areas surrounded by sediment-covered bedrock called continental platforms, which are broad, shallow depressions of basement complex (crystalline rock) filled with nearly flat-lying sedimentary rocks. The best-known areas are the Canadian Shield in North America and the Fennoscandian Shield in Europe. More than a third of Australia is Precambrian shield, and sizable shields exist in Africa, South America, and Asia. Many shields are fully exposed where flowing ice sheets eroded their cover of sediment during the last ice age.

Dispersed among and around the shields are greenstone belts, which are a mixture of metamorphosed (recrystallized) lava flows and sediments possibly derived from island arcs (chains of volcanic islands) caught between colliding continents. Although no large continents existed at this time, the foundations upon which they formed were present as protocontinents. These small landmasses were separated by marine basins that accumulated lava and sediments derived mainly from volcanic rocks that later recrystallized into greenstone belts.

Greenstone belts occupy the ancient cores of the continents. They span an area of several hundred square miles, surrounded by immense expanses of gneisses, which are the metamorphic equivalents of granites and the predominant Archean rock types. Greenstone is green because of the mineral chlorite, a greenish form of mica. The best-known greenstone belt is the Swaziland sequence in the Barberton Mountain Land of southeastern Africa. It is over 3 billion years old and is at points nearly 12 miles thick.

Caught in the Archean greenstone belts are ophiolites, the name derived from the Greek word ophis, meaning "serpent." They are slices of ocean floor shoved up on the continents by drifting plates and are as much as 3.6 billion years old. Pillow lavas, which are tubular bodies of basalt extruded undersea, also appear in the greenstone belts, signifying that the volcanic eruptions took place on the ocean floor. Thus, these deposits are among the best evidence that plate tectonics appears to have been working throughout most of the Earth's history in much the same manner as it does today.

The greenstone belts are of particular interest to geologists (and miners) because they hold most of the world's gold deposits. Archean-age ore deposits are remarkably similar worldwide. The mineralized veins are either Archean in age or they invaded Archean rocks at a much later date. Gold of Archean age is mined on every continent except Antarctica. In Africa, the best gold deposits are in rocks as much as 3.4 billion years old. Most of South African gold mines are in greenstone belts.

The Kolar greenstone belt in India, formed when two plates crashed together about 2.5 billion years ago, holds the richest gold deposits in the world. In North America, the best gold mines are in the Great Slave region of northwest Canada, where over a thousand deposits are known. Most of the gold deposits exist in greenstone belts invaded by hot magmatic solutions originating from the intrusion of granitic bodies into the greenstones, and the gold occurs in veins associated with quartz.

Because greenstone belts have no equivalent in modern geology, the geologic conditions under which they formed were very different from those observed today. Active tectonic (landform-building) forces in the mantle often broke open the thin Archean crust and injected magma into the deep crustal fracture zones. Such large-scale magmatic intrusion along with numerous large meteorite impacts characterized the unusual geology of the Archean. Since greenstone belts are geologically unique to the Archean, their absence in geologic formation after 2.5 billion years ago marks the end of the eon.
 
Archean Cratons:

Plate tectonics has played a prominent role in shaping the planet practically from the very beginning. Continents were adrift from the time they originated, within a few hundred million years after the formation of the planet. This is revealed by the presence of 4-billion-year-old Acasta gneiss, a metamorphosed granite, in the Northwest Territories of Canada, which suggests that the formation of the crust was well underway by this time.

The discovery leaves little doubt that at least small patches of continental crust existed on the Earth's surface during the first half-billion years of its history. The 3.8-billion-year-old metamorphosed marine sediments of the Isua Formation in a remote mountainous area in southwest Greenland provide evidence of an ocean. The continental crust was perhaps only about 10 percent of its present size and contained slivers of granite that drifted freely over the Earth's watery face.

Few rocks date beyond 3.7 billion years, suggesting that little continental crust was formed until afterward or was recycled into the mantle. Slices of granitic crust combined into stable bodies of basement rock called cratons, upon which all other rocks were deposited. They are composed of highly altered granite and metamorphosed marine sediments and lava flows. The rocks originated from intrusions of magma into the primitive ocean crust. Only three site in the world, in Canada, Australia, and Africa, contain rocks that were exposed on the surface during the Earth's early history and have remained essentially unchanged throughout geologic time.

Eventually, the slices of crust began to slow their erratic wanderings and combine into larger landmasses. Constant bumps and grinds from vigorous tectonic activity built the crust both inside and out. The continents continued growing rapidly until the end of the Archean 2.5 billion years ago, when they occupied up to a quarter of the Earth's surface, or about 80 percent or more of the present continental landmass. During this time, plate tectonics began to operate extensively, and much of the world as we know it took shape.

Ophiolites are best evidence for ancient plate motions. They are sections of oceanic crust that peeled off during plate collisions and were plastered onto the continents. Blueschists, which are metamorphosed rocks of subducted ocean crust, also shoved up on the continents. This resulted in a linear formation of greenish volcanic rocks along with light-colored masses of granite and gneiss, which are common igneous and metamorphic rocks that comprise the bulk of the continents.

The cratons numbered in the dozens and ranged from about a fifth the area of present-day North America to smaller than the state of Texas. The cratons were highly mobile and moved about freely on the molten rocks of the upper mantle called the asthenosphere, becoming independent mini-continents that periodically collided with and rebounded off each other. The collisions crumpled the edges of the cratons, forming small parallel mountain ranges perhaps only a few hundred feet high.

All the cratons eventually coalesced into a single large landmass several thousands of miles wide called a supercontinent. The points at which the cratons collided saw mountain ranges forced up, and the sutures joining the landmasses are still visible today as cores of ancient mountains called orogens. The original cratons formed within the first 1.5 billion years of the Earth's existence and totaled only about a tenth of the present landmass. The average rate of continental growth since then has been perhaps as much as one cubic mile a year. The constant rifting and patching of the interior along with the sediments deposited along the continental margins eventually built the supercontinent outward so that by the end of the Archean it nearly equaled the total area of today's continents.
 
Chapter Three:
Proterozoic Metazoans


Major differences exist in the character of rocks of the Proterozoic eon, from 2.5 to 0.6 billion years ago, as compared to those of the Archean. The Proterozoic featured a shift to much calmer conditions, as Earth progresses from a tumultuous adolescence to a stable adulthood. Marine life was distinct from that of the Archean and represented a considerable advancement in evolution with the development of complex organisms.

The global climate was cooler, and the Earth experienced its first major ice age more than 2 billion years ago, along with a major extinction that eliminated many primitive species attempting to evolve during this time. The Proterozoic ended about 570 million years ago after a second period of glaciation and another mass extinction. Afterward, an explosion of species, representing nearly every major group of marine organisms, set the stage for the evolution of more modern forms of life.


The Age of Worms:

Life in the Proterozoic was more advanced and complex than in the Archean. Organisms evolved very little during the first billion years on Earth because of primitive, asexual reproduction, which used simple fission, whereby species cloned themselves, offering little evolutionary change. A primitive form of metabolism also kept the organisms in a low-energy state.

The first evolutionary advancements were the development of an organized nucleus and sexual reproduction, which introduced a new breed of single-celled organisms called eukaryotes, around 1.5 billion years ago. Metabolism in eukaryotes involves respiration, indicating that substantial quantities of oxygen were present by the Proterozoic. About 2 billion years ago, when the banded iron formations, which absorbed oxygen, were no longer deposited, oxygen began to replace carbon dioxide in the ocean and atmosphere.

About 1.5 billion years ago, the previously skimpy geologic record of preserved cellular remains vastly improves as evolution suddenly sped up. However, another three-quarters of a billion years elapsed before multicellular animals called metazoans appear in the fossil record. By then, the dissolved oxygen content of the sea was about 5 to 10 percent of its present value. The increased level of oxygen also appears to have sparked the evolution of many unique animals.

The triggering mechanisms for such a rapid evolutionary phase included ecological stress, geographic isolation caused by drifting continents, and climatic changes. Organisms no longer relied entirely on surface absorption of oxygen, and gills and circulatory systems evolved when the oxygen level reached about a tenth of its present value near the end of the Proterozoic. Afterward, an explosion of species created the progenitors of all life on Earth today.

The first metazoans were a loose organization of individual cells united for a common purpose, such as locomotion, feeding, or protection. The most primitive metazoans probably comprised numerous cells, each with its own flagellum. They grouped into a small, hollow sphere, and their flagella beat the water in unison to propel the tiny animal through the sea.

From these metazoans evolved sedentary forms turned inside out and attached to the sea floor. Openings to the outside enabled the flagella now on the inside to produce a flow of water, providing a crude circulatory system for filtering food particles and ejecting wastes. These were the forerunners of the sponges, the most primitive of the metazoans. They existed in various shapes and sizes, some species possibly reaching 10 feet or more across, and grew in thickets on the ocean floor. The body consisted of three weak tissue layers, whose cells could survive independently if separated from the main body. The cells could either reattach or grow separately into adult sponges.

The next evolutionary step was the jellyfish, which had two layers of cells separated by a gelatinous substance, giving the saucerlike body a means of support. Unlike the sponges, the cells of the jellyfish were incapable of independent survival if separated from the main body. A primitive nervous system linked the cells and caused them to contract in unison, thereby providing the first simple muscles used for locomotion.

The development of muscles and other rudimentary organism including sense organs and a central nervous system to process the information, followed the evolution of primitive segmented worms. The coelomic, or hollow-bodied, worms adapted to burrowing in the ocean floor sediments and might have evolved into higher life forms of animal life. Since they were bottom-dwellers, these early worms left behind a preponderance of fossilized tracks, trail, and burrows to such an extent that the Proterozoic is often referred to as the "age of worms." Prior to about 670 million years ago, however, no track-making animals existed.

Sheetlike marine worms were less than a tenth of an inch thick but grew nearly three feet long, providing a large surface area on which to absorb oxygen and nutrients directly from seawater. Another reason for the unusual flattened bodies of many animals was the high ratio of surface are to volume it created. a result of the limited food supply available during the Proterozoic. This high ratio allowed for more effective sunlight collection for algae, which lived within the bodies of worms, helping to nourish their hosts while the worms supported them in symbiosis.

Algae also built tall stromatolite structures, composed of concentric layers of sediment, that tilted in the average direction of the sun. Stromatolite fossils in the Bitter Springs Formation in central Australia provide an 850-million-year-old record of the sun's movement across the sky. A stromatolite mound situated near the equator pointed toward one pole in the winter and the other in the summer, developing a sinuous growth pattern in the form of an S.

If a stromatolite laid down sediment layers daily, the number of layers in one full sine wave represented the number of days in a year. Analysis of stromatolites living in the late Proterozoic indicate an estimated 435 days in a single year. The results agree with counts made of growth rings of ancient coral fossils dating back to the beginning of the Cambrian period, 570 million years ago. Then, a year was about 428 days. The longer years indicates a higher rotation rate for the planet, due to the proximity of the moon, which was perhaps half its present distance from the Earth at the time it was created in the early Archean. Tidal friction gradually slowed the Earth's rotation and flung the moon into a higher orbit.

In addition, the sine wave patterns of ancient stromatolites provide information about the maximum travel of the sun across the equator. The equator forms an oblique angle to the ecliptic, controlled by the tilt of the Earth's rotational axis. The sun's maximum latitude during the peak of each season is obtained by measuring the maximum angle the sine wave deviates from the average direction of stromatolite growth. Presently, the sun travels 23.5 degrees either side of the equator from summer to winter. However, around 850 million years ago, this value was about 26.5 degrees, suggesting a much more seasonal climate.
 
The Ediacara Fauna:

During the late Proterozoic, around 800 million years ago, stromatolites underwent a marked decline in diversity possibly due to the appearance of algae-eating animals. Around 680 million years ago, thick glaciers spread over the continents during perhaps the most intense period of glaciation the Earth has ever known, when massive ice sheets overran nearly half the land surface. At this time, all continents were assembled into a supercontinent, which might have wondered over one of the poles. The ice age dealt a deathly blow to life in the ocean, and many simple organisms vanished during the worlds first mass extinction. At this point in Earth's development, animal life was still scarce, and the extinction decimated the ocean's population of acritarchs, a community of planktonic algae that were among the first organisms to develop elaborate cells with nuclei.

Not long after the ice disappeared near the end of the Proterozoic, about 670 million years ago, the great diversity of animal life culminated with the evolution of entirely new species, the likes of which had never existed before, or since, forever changing the Earth's biology. Life forms took off in all directions, producing many unique and bizarre creatures, whose fossil impressions are found in the Ediacara formation in South Australia. But many soft-bodied organisms like the late Precambrian fauna living prior to the arrival of shelled animals, did not easily enter the fossil record, another reason only a small fraction of all species the have ever lived are preserved as fossils.

The extremely flattened bodies of the Ediacaran fauna maximized the ratio of surface area to volume, enabling organisms to take in nutrients and oxygen more efficiently and to absorb light for symbiotic algae. The algae lived within the tissues of host animals, which offered protection from predators in return for nutrients and the removal of waste products. These adaptions served well for the marine conditions that prevailed in the late Proterozoic, when shallow seas were nutrient poor and oxygen levels were low.

The Earth underwent many profound physical changes near the end of the Proterozoic, promting a rapid radiation of Ediacaran fauna. At the same time, a supercontinent located on the equator rifted apart, producing intense hydrothermal activity that caused fundamental environmental changes. Furthermore, the increased marine habitat area spawned the greatest explosion of species the world has ever known, with seas that contained large populations of widespread and diverse organisms.

The dominant animals were the coelenterates, radially symmetrical invertebrate animals, including giant jellyfishlike floaters up to 3 feet wide and colonies of feathery forms, possibly predecessors of the corals, that were attached to the ocean floor and grew more than a yard long. The remaining organisms were mostly marine worms, unusual arthropodlike animals, and a tiny, curious looking, naked starfish with three rays instead of the customary five. The vase-shaped archaeocyathans resembled both sponges and corals and built the earliest limestone reefs, eventually becoming extinct in the Cambrian.

The Ediacara formation contains impressions of strange organisms. Many of these unusual creatures were the result of adaptations to highly unstable conditions during the late Proterozoic. These included an increasing oxygen supply, which made possible the evolution of large animals with vascular circulatory systems supplying the cells with blood. Over-specialization to a narrow range of environmental conditions caused a major extinctions of species at the end of the era around 570 million years ago due to the evolution of predators. Those species that survived the great extinction were markedly different from their Ediacaran ancestors.
 
Banded Iron Formations:

Mineral deposits of the Proterozoic are bedded in layers, or stratified, as opposed to the vein deposits of the Archean. Iron, the fourth most abundant element in the Earth's crust, was leached from the continents and dissolved in seawater under reducing (deoxydizing) conditions. When the iron reacted with oxygen in the ocean, it precipitated in vast deposits on shallow continental margins. Alternating bands of iron-rich and iron-poor sediments gave the ore a banded appearance, thus prompting the name banded iron formation, or BIF. These deposits, mined extensively throughout the world, provide over 90 percent of the minable iron reserves.

In effect, biologic activity was responsible for the iron deposits, since photosynthetic organisms produced the oxygen. When plants began producing oxygen, it combined with iron, keeping oxygen levels in the ocean within the limits tolerated by the early prokaryotes. Throughout the Archean, the amount of oxygen probably remained under 1 percent due to this regulating mechanism. Then, between 2.5 and 2 billion years ago, photosynthesis generated enough oxygen to react with iron on a grand scale.

BIF deposits, composed of alternating layers of iron and silica, formed about 2 billion years ago at the height of the earliest ice age. For unknown reasons, major episodes of iron deposition coincided with periods of glaciation. The average ocean temperature was probably warmer than today. When warm ocean currents rich in iron and silica flowed toward the glaciated polar regions, the suddenly cooled waters could no longer hold minerals in solution. As they became "undissolved" in the water, their precipitation formed alternating layers on the ocean floor, alternating because iron, heavier than silica, settles faster. After most of the dissolved iron was locked up in the sediments, the level of oxygen began to steadily rise, spawning the evolution of more advanced species.

Biochemical activity in the ocean was also responsible for stratified sulfide deposits. Sulfur-metabolizing bacteria living near undersea hydrothermal vents oxidizing hydrogen sulfide into elemental sulfur and various sulfates. Copper, lead, and zinc, which were much more abundant in the Proterozoic than in the Archean, also reflect a submarine volcanic origin.
 
Precambrian Glaciation:

The Proterozoic was a period of transition, when oxygen generated by photosynthesis replaced carbon dioxide. Early in the Archean, the sun's output was only about 70 percent of its present value and large amounts of atmospheric carbon dioxide, 1,000 times greater than current levels, kept the Earth's oceans from freezing solid.

When the first microscopic plants evolved, they began replacing carbon dioxide in the ocean and atmosphere with oxygen so that today the relative percentages of these two gases have completely reversed. The loss of carbon dioxide, an important greenhouse gas, caused the climate to cool, even while the sun was getting progressively hotter.

The drop in global temperatures soon after the beginning of the Proterozoic initiated the first known glacial epoch about 2.4 billion years ago, when massive sheets of ice nearly engulfed the entire landmass. The positions of the continents also had a tremendous influence on the initiation of ice ages, and landmasses wandering into the colder latitudes enabled the buildup of glacial ice. Global tectonics, featuring extensive volcanic activity and seafloor spreading, might have triggered the ice ages by drawing down the level of oxygen in the ocean and atmosphere, preserving more organic carbon in the sediments so that living organisms could not return it to the atmosphere.

Plate tectonics also began to operate more vigorously, and carbonaceous sediments were thrust deep inside the Earth along with the underlying oceanic crust. The growing continents stored large quantities of carbon in thick deposits of carbonaceous rocks such as limestone. The elimination of carbon dioxide in this manner caused the Earth to cool dramatically. Because high rates of organic carbon burial, the iron deposition and intense hydrothermal activity associated with plate tectonics furthered global cooling. Although this was the first ice age the world has ever known, it was not the worst.

The burial of carbon in the Earth's crust might have been the key to the onset of another glacial period near the end of the Proterozoic about 800 million years ago. A supercontinent located on the equator rifted apart, forming four or five major continents, the largest of which apparently wandering into the south polar region and acquired a thick blanket of ice. This was perhaps the greatest period of glaciation, and ice encased nearly half the landmass. The climate was so cold ice sheets and permafrost (permanently frozen ground) extended toward the equator. During this time, no plants grew on the land and only simple plants and animals lived in the sea.

The glacial periods are marked by deposits of tillites. Thick sequences of Precambrian tillites exist on every continent. Tillites are a mixture of boulders and clay deposited by glacial ice and cemented into solid rock. In the Lake Superior region of North America, tillites are 600 feet thick in places and range from east to west for a thousand miles. In northern Utah, tillites mount up to 12,000 feet thick. The various layers of glacially deposited sediment suggest a series of ice ages closely following each other. Similar tillites are among Precambrian rocks in Norway, Greenland, China, India, southwest Africa, and Australia.

The 680-million-year-old glacial varves in lakebed deposits north of Adelaide, South Australia might hold evidence of a solar cycle operating in the Proterozoic. The varves consist of alternating layers of silt laid down annually during the late Precambrian ice age. Each summer when the glacial ice melted, sediment-laiden meltwater discharged into a lake, and sediments settled out in a stratified deposit.

During times of intense solar activity, average global temperatures increased, and the glaciers melted more rapidly, depositing thicker varves. By counting the layers of thick and thin varves, a stratigraphic sequence is established that mimics both the 11-year sunspot cycle and the 22-year solar cycle or possibly the early lunar cycle, which today is about 19 years.

When the glacial epoch ended and the ice sheets retreated, life began to proliferate in the ocean with an intensity never experienced before or since. The rapid evolution produced three times as many phyla, groups of organisms sharing the same general body plan, as are living today, many of them unique and bizarre creatures.
 
The Continental Crust:

By the beginning of the Proterozoic, upwards of 80 percent or more of the present continental crust was already in existence. Most of the material presently locked up in sedimentary rocks was at or near the surface, and ample sources of Archean rocks were exposed to erosion and redeposition. Sediments derived directly from primary sources are called graywackes, often described as dirty sandstone and common in folded sedimentary rocks such as those in the Alps and Alaska. Most Proterozoic wackes composed of sandstones and siltstones originated from Archean greenstones. Another common rock-type was fine-grained quartzite, a metamorphic rock derived from the erosion of siliceous grainy rocks such as granite and a coarse-grained sandstone called arkose.

Conglomerates, consolidated gravel-like deposits, were also abundant in the Proterozoic. Nearly 20,000 feet of Proterozoic sediments form the Uinta Range of Utah, and the Montana Proterozoic belt system contains sediments over 11 miles thick. The Proterozoic is also known for its widespread terrestrial redbeds, named so because sediment grains were cemented with red iron oxide. Their appearance around 1 billion years ago indicates that the atmosphere had substantial levels of oxygen at this time.

The weathering of primary rocks produced solutions of calcium carbonate, magnesium carbonate, calcium sulfate, and sodium chloride, which subsequently precipitated into limestone, dolomite, gypsum, and halite. Higher carbon dioxide concentrations in the Precambrian probably account for the prevalence of dolomite over limestone. These minerals appear to be mainly chemical precipitates and not of biological origin.

In the Mackenzie Mountains of northwest Canada, dolomite deposits range up to 6,500 feet thick, and in the great Alps, massive chunks of dolomite soar skyward. Carbonate rocks such as limestone and chalk, produced chiefly by an organic process involving shells and skeletons of simple organisms, became more common during the late Proterozoic between about 700 and 570 million years ago. In contrast, they were relatively rare in the Archean due to the scarcity of lime-secreting organisms.

The continents of the Proterozoic were composed of Archean cratons. The North American continent assembled from seven cratons around 2 billion years ago, making it one of the oldest continents. The cratons welded into what is now central Canada and the north-central United States. At Cape Smith on Hudson Bay is a 2-billion-year-old slice of oceanic crust that was accreted onto the land, indicating that continents collided and closed the an ancient ocean. Arcs of volcanic rock also weave through central and eastern Canada down into the Dakotas. A region between Canada's Great Bear Lake and the Beaufort Sea holds the roots of an ancient mountain range that formed by the collision of North America and an unknown landmass between 1.2 and 0.9 billion years ago. Meanwhile, continental collisions continued to add a large area of new crust to the growing proto-North American continent.

Most of the continental crust underlying the United States from Arizona to the Great Lakes to Alabama formed in one great surge of crustal generation unequaled in North America since. The rapid buildup of new crust possibly resulted from the most energetic period of tectonic activity in Earth history. The assembled North American continent was stable enough to resist a billion years of jostling and rifting, and continued to grow by adding bits and pieces of continents and island arcs to its edges.

Large masses of volcanic rock found near the eastern edge of North America imply that the continent was the core of a larger supercontinent formed during the late Proterozoic. The interior of this landmass heated and erupted with volcanism. The warm, weakened crust consequently broke apart into possibly four or five major continents between 630 and 560 million years ago, although they were shaped differently than they are today. The breakup of the supercontinent created thousands of miles of new continental margin, which might have played a major role in the explosion of new life at the beginning of the Phanerozoic.
 
Chapter Four:
Cambrian Invertebrates


The Cambrian period from 570 to 500 million years ago was named for a mountain range in central Whales, Great Britain, that contained sediments with the earliest known fossils. Nineteenth-century geologists were often puzzled by ancient rocks that were practically devoid of fossils untill the Cambrian period, when life seemingly sprang up in great abundance the world over. The base of the Cambrian was thought to mark the beginning of life, and all time before was simply called the Precambrian.

The period was generally quiet in terms of geologic processes, with little mountian building, volcanic activity, glaciation, or extremes in climate. The breakup of the late Precambrian supercontinent and the flooding of continents of inland seas created abundant warm shallow-water habitats, prompting an explosion of new species. Never before or since have so many novel and unusual organisms existed; surprisingly, none have any counterpart in today's living world.


The Cambrian Explosion:

The Cambrian was an evolutionary heyday for species, featuring a 5-to-10-million-year explosion of the first complex animals with exoskeletons. The early Cambrian witnessed the disappearance of soft-bodied Ediacaran faunas and the proliferation of shelly faunas. The biologic proliferation peaked about 530 million years ago, filling the ocean with a rich assortment of life. Seemingly out of nowhere and in bewildering abundance, animals appeared in an astonishingly short time span cloaked in a baffling variety of exoskeletons.

The introduction of hard skeletal parts has been called the greatest discontinuity in Earth history and signaled a major evolutionary change by accelerating the developmental pace of new organisms. Nearly all major groups of modern animals appeared in the fossil record, and for the first time animals sported shells, skeletons, legs, and sensing antennae. So many new and varied life forms came into existence the age is depicted as the "Cambrian explosion."

The period follows on the heels of the great Precambrian ice age, the worst the world has ever experienced, when ice sheets covered half the landmass. It was also a time when oceanic oxygen concentrations rose to significant levels. After the ice retreated and the seas began to warm, life took off in all directions. Unique and bizarre creatures roamed the ocean depths, and the Cambrian saw a higher percentage of experimental organisms than any other interval of geologic history, with perhaps three times more phyla in existence than today.

At the beginning of the Cambrian, an ocean turnover might have brought unusual amounts of nutrient-rich bottom water to the surface. Due to an increase in seawater calcium levels, early soft-bodied creatures developed skeletons as receptacles for the disposal of excess amounts of this toxic mineral from their tissues. As concentrations of calcium in the ocean floor increased, animal skeletons became more diverse and elaborate.

Levels of atmospheric oxygen appeared to rise in concert with the skeletal revolution. The higher oxygen levels increased metabolic energy, enabling the growth of larger animals, which in turn required stronger structural supports. Skeletons also evolved as a response to an incoming wave of predators. Paradoxically, most of these predators were soft-bodied and therefor did not survive as fossils.

Soft-bodied organisms living prior to the arrival of shelled animals at the beginning of the Cambrian had a great difficulty entering the fossil record. Animals with soft body parts decayed rapidly upon death, and so only traces of their existence remain, such as imprints, tracks, and borings. Fossil impressions of soft-bodied animals in the Ediacaran Hills of southern Australia date from about 670 million years ago.

At the dawn of the Cambrian, 100 million years after the appearance of the Ediacaran fauna, most of which were evolutionary dead ends, the seascape abruptly changed with the sudden arrival of animals with hard skeletal parts. Most phyla of living organisms appeared almost simultaneously, many of which had their origins in the latter part of the Precambrian. Body styles that evolved in the Cambrian largely served as blueprints for modern species, with few new radical body plans appearing since then.

When skeletons evolved, the number of organisms preserved in the fossil record jumped dramatically. All known animal phyla that readily fossilized appeared in the Cambrian, after which the number of new classes decreased sharply. Fossils became so abundant at the beginning of the period for several reasons: the development of hard body parts that fossilize by replacement with calcium carbonate or silica, rapid burial that prevents attack by scavengers and decay by oxidation, long periods of deposition with little erosion, and large populations of species.
 
The Age of Trilobites:

The Cambrian is best known for a famous group of invertebrates called trilobites, which were primitive crustaceans and a favorite among fossil collectors. The trilobite body has three lobes (hence the name), consisting of a central axial lobe containing the creature's essential organs and two side, or pleural, lobes. Since trilobites were so widespread and lived throughout the Paleozoic, their fossils are important markers for dating rocks of this era. They appeared at the very base of the Cambrian and became the dominant invertebrates of the early Paleozoic, diversifying into about 10,000 species before declining and becoming extinct after some 340 million years of highly successful existence.

About 10 million years into the Cambrian, a wave of extinctions decimated a huge variety of newly evolved species. The mass extinction eliminated more than 80 percent of all marine animal genera and is numbered among the worst in Earth history. It coincided with a drop in sea level that followed continental collisions. The die-offs paved the way for the ascendancy of the trilobites, which were among the first organisms to grow hard shells and the dominant species for the next 100 million years. They were small, oval-shaped arthropods ancestors to the horseshoe crab, their only remaining direct descendant. Only the giant paradoxides was truly a paradox among trilobites, extending nearly 2 feet in length, while most trilobites were less than 4 inces long.

The trilobites occupied the shallow waters near the shores of ancient seas the flooded inland areas, providing extensive continental margins from the coastline to the abyss. In addition, a stable environment enabled marine life to flourish and proliferate. The flora included cyanobacteria, red and green algae, and acritarchs, a form of plankton that supported early Paleozoic food chains.

Curiously, many trilobite fossils have bite scars predominantly on the right side. Predators might have attacked from the right because when the trilobite curled up to protect itself, it exposed the right side of its body. (Trilobite fossils are often found with their bodies completely curled up.) However, if the trilobite had a vital organ on its left side and an attack occurred there, it stood a good chance of being eaten, leaving no fossil. Therefor, those attacked on the right side stood a better chance of entering the fossil record. Trilobites shed their exoskeletons as they grew, and in this manner an individual could have left several incomplete fossils, which explains why whole fossils are rare.

The population of trilobites peaked during the late Cambrian around 520 million years ago, when they accounted for about two-thirds of all marine species. By the mid-Ordovician, about 460 million years ago, that fraction dropped to one-third because of the rise of mollusks and other competing animals. Later, the trilobites left the nearshore for the offshore, possibly due to environmental changes. The decline of the trilobites also might be connected to the arrival of the jawed fishes.
 
Cambrian Paleontology:

The coelenterates, which were well represented in the Cambrian seas, are the most primitive of animals and include jellyfish, sea anemones, sea pens, and corals. Most coelenterates are radically symmetrical, with body parts radiating outward from a central point. They have a saclike body and a mouth surrounded by tentacles. The corals posses a large variety of skeletal forms, and successive generations built thick limestone reefs. Corals began constructing reefs in the lower Paleozoic, forming chains of islands and barrier reefs along the shorelines of the continents.

The archaeocyathans resembled both corals and sponges but have no close relationship to any living group and thus belong in their own unique phylum. They formed the earliest reefs, eventually becoming extinct in the Cambrian. Many corals declined and were replaced by sponges and algae in the late Paleozoic due to the recession of the seas in which they once thrived.

The coral polyp is a soft-bodied creature that is essentially a contractible sac, crowned by a ring of tentacles. The tentacles surround a mouth-like opening and are tipped with poisonous stingers. The polyps live in individual skeletal cups composed of calcium carbonate. They extend their tentacles to feed at night and withdraw during the day or at low tide to keep from drying in the sun. The corals coexist symbiotically (in conjunction) with zooxantellae algae, which live within the polyp's body. The algae consume the coral's waste products and produce organic materials that nourish the polyp. Because the algae require sunlight for photosynthesis, corals must live in warm, shallow water.

The echinoderms, meaning "spiny skin," were perhaps the strangest animals ever preserved in the fossil record of the early Paleozoic. Their five-fold radial symmetry with arms radiating outward from the center of the body makes them unique among the more complex animals. They are the only species possessing a water vascular system composed of internal canals that operate a series of tube feet, or podia, used for locomotion, feeding, and respiration. The echinoderms have more classes than any phylum both living or extinct. The major classes of living echinoderms include starfish, brittle stars, sea urchins, sea cucumbers, and crinoids, known as sea lilies because of their plantlike appearance.

The brachiopods, also called lamp shells, were once the most abundant and diverse organisms, with more than 30,000 species cataloged in the fossil record. They had two saucerlike shells called valves the fitted face to face and opened and closed using simple muscles. More advanced species, including living brachiopods called articulates, had ribbed shells with interlocking teeth that opened and closed along a hinge. The brachiopods were fixed to the ocean floor by a rootlike appendage and filtered food particles through opened shells that closed to protect the animals against predators. The appearance of abundant brachiopod fossils indicates the existence of seas of moderate to shallow depth.

The mollusks are a highly diverse group and left the most impressive fossil record of all marine animals. The phylum is so diverse it is often difficult to find common features among its members. The three major groups are the snails, clams, and cephalopods. Snails and the related slugs comprise the largest group and ranged from the Cambrian onward. The cephalopods, which include squid, cuttlefish, octopus, and nautilus, traveled by jet propulsion. They sucked water into a cylindrical cavity through openings on each side of the head and expelled it under pressure through a funnel-like appendage. Their straight, streamline shells, up to 30 feet and more in length, made the nautiloids among the swiftest animals of the ancient seas. The ammonoids were the most spectacular of marine predators, with a large variety of coiled shell forms.

The arthropods are the largest phylum of living organisms, comprising roughly a million species, or about 80 percent of all known animals. Three-foot-long arthropods represent one of the largest of all Cambrian invertebrates. Among the first and best known of the ancient arthropods were the trilobites. The arthropod body is segmented, suggesting a relationship to the annelid worms. Paired, jointed limbs generally present on most segments were modified for sensing, feeding, walking, and reproduction. The crustaceans are primarily aquatic arthropods and include shrimp, lobsters, crabs, and barnacles.

The conodonts are fossilized tiny jawlike appendages, commonly occurring in the marine rocks ranging from the Cambrian to the Triassic and important for dating Paleozoic rocks. They are among the most puzzling of all fossils and are thought to be parts of an unusual, soft-bodied animal, perhaps the most primitive of the vertebrates. Graptolites were colonies of cupped organisms that resembled stems and leaves, appearing much like plants but actually being animals. They either clung to the seafloor like small shrubs, floated freely near the surface, resembling tiny hacksaw blades, or attached themselves to seaweed.

Large numbers of graptolites buried in the bottom mud produced organic-rich black shales that indicate poor oxygen conditions and are important markers for correlating rock units of the lower Paleozoic. They were thought to have gone extinct in the late Carboniferous about 300 million years ago, but the discovery of living pterobrachs, possible counterparts of graptolites, suggests these might be "living fossils."
 
The Burgess Shale Fauna:

The Burgess Shale Fauna Formation in British Columbia, Canada contains the remains of bizarre soft-bodied animals that appeared about 530 million years ago, soon after the emergence of complex creatures. Some organisms might be surviving Ediacaran fauna, most of which became extinct near the end of the Precambrian. Indeed, the so-called Cambrian explosion might have been triggered in part by the availability of habitats vacated when the Ediacaran species departed. Though many mass extinctions of marine organisms have occurred since then, no fundamentally new body plans have appeared during the past 500 million years.

The Burgess Shale invertebrates have specialized adaptations and are surprisingly complex. However, most species became extinct later in the Cambrian, and only a few gave rise to anything living today. Many of these bizarre animals, some of which were possible carryovers from the upper Precambrian, never made it beyond the middle Paleozoic. They were so strange, they continue to defy efforts to classify them into existing taxonomic groups.

One peculiar animal appropriately named hallucigenia propelled itself across the seafloor on seven pairs of pointed stilts. Seven tentacles arose from the upper body, and each appears to have had its own individual mouth. Another curious Burgess Shale animal called wiwaxia was a spiny creature about an inch long, possibly related to a modern scaleworm known as a sea mouse. An unusual worm had enormous eyes and prominent fins. Another odd creature had five eyes arranged across its head, a vertical tail fin to help steer it across the seafloor, and a grasping organ projected forward for catching prey.

An extraordinary arthropod called anomalocarids was possibly the largest of the Cambrian predators, reaching 3 feet in length. Its mouth was surrounded by spiked plates and flanked by a pair of jointed appendages apparently evolved for holding and crushing the armored plates of invertebrates. The animal appears to have been well equipped for devouring crustaceans and was appropriately dubbed the "terror of trilobites." Several trilobite species evolved long spines that might have served as protection against anomalocarid attacks.

The Burgess Shale faunas originated in shallow water on a gigantic coral reef covered with mud that surrounded the continent of Laurentia, which included the present United States, Canada, and Greenland. Most came form the western Cordillera of North America, an ancient mountain range that faced open ocean in the middle Cambrian. Their widespread distribution around other continents suggests that many members could swim. Most of the Burgess Shale faunas abruptly went extinct at the end of the Cambrian, and only a few archaic forms survived to the middle Devonian.
 
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