8 Earth History

Entire courses and careers have been based on the wide-ranging topics covering Earth’s history. Throughout the long history of Earth, change has been the norm. Looking back in time, an untrained eye would see many unfamiliar life forms and terrains. The main topics studied in Earth history are paleogeography, paleontology, paleoecology, and paleoclimatology—respectively, past landscapes, past organisms, past ecosystems, and past environments. This chapter will cover briefly the origin of the universe and the 4.6-billion-year history of Earth. This Earth history will focus on the major physical and biological events in each eon and era.

8.1 Hadean Eon

Geoscientists use the geological timescale to assign relative age names to events and rocks, separating major events in Earth’s history based on significant changes recorded in rocks and fossils. This section summarizes the most notable events of each major time interval. For a breakdown on how these time intervals are chosen and organized, see Chapter 7.

The Hadean Eon, named after the Greek god and ruler of the underworld Hades, is the oldest eon and dates from 4.5–4.0 billion years ago.

This time represents Earth’s earliest history, during which the planet was characterized by a partially molten surface, volcanism, and asteroid impacts. In addition to these impacts, several other mechanisms made the newly forming Earth incredibly hot, including gravitational compression and radioactive decay. Most of this initial heat still exists inside the Earth. The Hadean was originally defined as the birth of the planet occurring 4.0 billion years ago and preceding the existence of many rocks and life forms. However, geologists have dated minerals at 4.4 billion years, with evidence that liquid water was present. There is possibly even evidence of life existing over 4.0 billion years ago. However, the most reliable record for early life, the microfossil record, starts at 3.5 billion years ago.

8.1.1 Origin of Earth’s Crust

As Earth cooled from its molten state, minerals started to crystallize and settle, resulting in a separation of minerals based on density and the creation of the crust, mantle, and core. The earliest Earth was chiefly molten material and would have been rounded by gravitational forces, causing it to resemble a ball of lava floating in space. As the outer part of the Earth slowly cooled, the high melting-point minerals (see Bowen’s reaction series in Chapter 4) formed solid slabs of early crust. These slabs were probably unstable and easily reabsorbed into the liquid magma until the Earth cooled enough to allow numerous larger fragments to form a thin, primitive crust. Scientists generally assume this crust was oceanic and mafic in composition and that it was littered with impacts, much like the Moon’s current crust. There is still some debate over when plate tectonics started, which would have led to the formation of continental and felsic crust. Regardless of this, as Earth cooled and solidified, less-dense felsic minerals floated to the surface of the Earth to form the crust, while the denser mafic and ultramafic materials sank to form the mantle and the highest-density iron and nickel sank into the core. This differentiated the Earth from a homogeneous planet, making it heterogeneous, with layers of felsic crust, mafic crust, ultramafic mantle, and iron and nickel core.

8.1.2 Origin of the Moon

Several unique features of Earth’s Moon have prompted scientists to develop the current hypothesis about its formation. The Earth and Moon are tidally locked, meaning that, as the Moon orbits, one side always faces the Earth and the opposite side is not visible to us. Most importantly, the chemical compositions of the Earth and Moon show nearly identical isotope ratios and volatile content. Apollo missions returned from the Moon with rocks that allowed scientists to conduct very precise comparisons between Moon and Earth rocks. Other bodies in the Solar System and meteorites do not share the same degree of similarity and show much higher variability. If the Moon and Earth formed together, this would explain why they are so chemically similar.

Many ideas have been proposed for the origin of the Moon. The Moon could have been captured from another part of the Solar System and formed in place together with the Earth, or the Moon could have been ripped out of the early Earth. No proposed explanations have been able to account for all the evidence. The currently prevailing hypothesis is the giant impact hypothesis, which proposes a body about half of Earth’s size must have shared at least parts of Earth’s orbit and collided with it, resulting in a violent mixing and scattering of material from both objects. Both bodies would be composed of a combination of materials, with more of the lower-density splatter coalescing into the Moon. This may explain why the Earth has a higher density and thicker core than the Moon.

8.1.3 Origin of Earth’s Water

Explanations for the origin of Earth’s water include volcanic outgassing, comets, and meteorites. The volcanic outgassing hypothesis states that Earth’s water originated from inside the planet and emerged via tectonic processes as vapor associated with volcanic eruptions. Since all volcanic eruptions contain some water vapor, at times more than 1% of the volume, these alone could have created Earth’s surface water. Another likely source of water was space. Comets are a mixture of dust and ice, with some or most of that ice being frozen water. Seemingly dry meteors can contain small but measurable amounts of water, usually trapped in their mineral structures. During heavy bombardment periods later in Earth’s history, its cooled surface was pummeled by comets and meteorites, which could be why so much water exists above ground. There isn’t a definitive answer for what process resulted in ocean water. Earth’s water isotopically matches water found in meteorites much better than water from comets. However, it is hard to know if Earth processes could have changed the water’s isotopic signature over the last four-plus billion years. It is possible that all three sources contributed to the origin of Earth’s water.

8.2 Archean Eon

The Archean Eon, which lasted from 4.0–2.5 billion years ago, is named after the Greek word for beginning. This eon represents the beginning of the rock record. Although there is current evidence that rocks and minerals existed during the Hadean Eon, the Archean has a much more robust rock and fossil record.

8.2.1 Late Heavy Bombardment

Objects were chaotically flying around at the start of the Solar System, building the planets and moons. There is evidence that after the planets formed about 4.1–3.8 billion years ago, a second large spike of asteroids and comets impacted the Earth and Moon in an event called the Late Heavy Bombardment. Meteorites and comets in stable or semi-stable orbits became unstable and started impacting objects throughout the Solar System; as this is when the Moon received most of its craters, this event is also known as the lunar cataclysm. During the Late Heavy Bombardment, the Earth, Moon, and all planets in the Solar System were pummeled by material from the Asteroid Belt and Kuiper Belt. Evidence of this bombardment was found within samples collected from the Moon.

It is universally accepted that the Solar System experienced extensive asteroid and comet bombardment at its start; however, some other process must have caused the second increase in impacts hundreds of millions of years later. A leading theory blames gravitational resonance between Jupiter and Saturn for disturbing orbits within the Asteroid and Kuiper Belts, a notion based on a similar process observed in the Eta Corvi star system.

8.2.2 Origin of the Continents

Continents are a crucial part of plate tectonics working as they do currently. However, the easiest way to create continental material is via assimilation and differentiation of existing continents (see Chapter 4). This chicken-and-egg quandary over how continents were made in the first place is not easily answered because of the great age of continental material and how much evidence has been lost during tectonics and erosion. While the timing and specific processes are still debated, volcanic action must have brought the first continental material to the Earth’s surface during the Hadean, 4.4 billion years ago. This model does not solve the problem of continent formation, since magmatic differentiation seems to need thicker crust. Nevertheless, the continents formed by some incremental process during the early history of Earth. The leading idea is that density differences allowed lighter felsic materials to float upward and heavier ultramafic materials and metallic iron to sink. These density differences led to the layering of the Earth, adding the layers that are now detected by seismic studies. Early protocontinents accumulated felsic materials as developing plate-tectonic processes brought lighter material from the mantle to the surface.

The first solid evidence of modern plate tectonics is found at the end of the Archean, indicating that at least some continental lithosphere must have been in place. This evidence does not necessarily mark the starting point of plate tectonics; remnants of earlier tectonic activity could have been erased by the rock cycle.

The stable interiors of the current continents are called cratons and were mostly formed in the Archean Eon. A craton has two main parts: the shield, which is crystalline basement rock near the surface, and the platform made of sedimentary rocks covering the shield. Most cratons have remained relatively unchanged, with most tectonic activity having occurred around cratons instead of within them. Whether they were created by plate tectonics or another process, Archean continents gave rise to the Proterozoic continents that now dominate our planet.

The general guidelines as to what constitutes a continent and differentiates oceanic from continental crust are under some debate. At passive margins, continental crust grades into oceanic crust at passive margins, making a distinction difficult. Even island-arc and hotspot material can seem more closely related to continental crust than oceanic. Continents usually have a craton in the middle with felsic igneous rocks. There is evidence that submerged masses like Zealandia, which includes present-day New Zealand, would be considered a continent. Continental crust that does not contain a craton is called a continental fragment, such as the island of Madagascar off the east coast of Africa.

8.2.3 First Life on Earth

Life most likely started during the late Hadean or early Archean Eons. The earliest evidence of life are chemical signatures, microscopic filaments, and microbial mats. Carbon found in 4.1-billion-year-old zircon grains have a chemical signature suggesting an organic origin. Other evidence of early life are 3.8–4.3 billion-year-old microscopic filaments from a hydrothermal vent deposit in Quebec, Canada. While the evidence of chemical and microscopic filaments is not as robust as fossils, there is significant fossil evidence for life at 3.5 billion years ago. These first well-preserved fossils are photosynthetic microbial mats called stromatolites, found in Australia.

Although the origin of life on Earth is unknown, hypotheses include a chemical origin in the early atmosphere and ocean, deep-sea hydrothermal vents, and delivery to Earth by comets or other objects. One hypothesis is that life arose from the chemical environment of the Earth’s early atmosphere and oceans, which was very different than today. The oxygen-free atmosphere produced a reducing environment with abundant methane, carbon dioxide, sulfur, and nitrogen compounds. This is what the atmosphere is like on other bodies in the Solar System. In the famous Miller-Urey experiment, researchers simulated early Earth’s atmosphere and lightning within a sealed vessel. After igniting sparks within the vessel, they discovered the formation of amino acids, the fundamental building blocks of proteins. In 1977, when scientists discovered an isolated ecosystem around hydrothermal vents on a deep-sea mid-ocean ridge (see Chapter 4), it opened the door for another explanation of the origin of life. The hydrothermal vents have a unique ecosystem of life forms that use chemosynthesis as the foundation of the food chain instead of photosynthesis. The ecosystem derives its energy from hot chemical-rich waters pouring out of underground towers. This suggests that life could have started on the deep ocean floor and derived energy from the heat of the Earth’s interior via chemosynthesis. Scientists have since expanded the search for life to more unconventional places, like Jupiter’s icy moon Europa.

Another possibility is that life or its building blocks came to Earth from space, carried aboard comets or other objects. Amino acids, for example, have been found within comets and meteorites. This intriguing possibility also implies a high likelihood of life existing elsewhere in the cosmos.

8.3 Proterozoic Eon

The Proterozoic ("earlier life") Eon comes after the Archean Eon and lasted from 2.5 billion years ago to 541 million years ago. During this time, most of the central parts of the continents had formed and plate tectonic processes had started. Photosynthesis by microbial organisms, such as single-celled cyanobacteria, had been slowly adding oxygen to the oceans. As cyanobacteria evolved into multicellular organisms, they completely transformed the oceans and later the atmosphere by adding massive amounts of free oxygen gas (O₂) and initiated what is called the Great Oxygenation Event (GOE). This drastic environmental change decimated the anaerobic bacteria, which could not survive in the presence of free oxygen. On the other hand, aerobic organisms could thrive in ways they could not earlier.

An oxygenated world also changed the chemistry of the planet in significant ways. For example, iron remained in solution in the nonoxygenated environment of the earlier Archean Eon. In chemistry, this is known as a reducing environment. Once the environment was oxygenated, iron combined with free oxygen to form solid precipitates of iron oxide, such as the minerals hematite or magnetite. These precipitates accumulated into large mineral deposits with red chert known as banded-iron formations, which are dated at about two billion years.

The formation of iron oxide minerals and red chert (see Figure 8.18) in the oceans lasted a long time and prevented oxygen levels from increasing significantly since precipitation took the oxygen out of the water and deposited it into the rock strata. As oxygen continued to be produced and mineral precipitation leveled off, dissolved oxygen gas eventually saturated the oceans and started bubbling out into the atmosphere. Oxygenation of the atmosphere is the single biggest event that distinguishes the Archean and Proterozoic environments. In addition to changing mineral and ocean chemistry, the GOE is also identified as triggering Earth’s first glaciation event around 2.1 billion years ago, the Huron glaciation. Free oxygen reacted with methane in the atmosphere to produce carbon dioxide. Carbon dioxide and methane are called greenhouse gases because they trap heat within the Earth’s atmosphere, like the insulated glass of a greenhouse. Methane is a more effective insulator than carbon dioxide, so as the proportion of carbon dioxide in the atmosphere increased, the greenhouse effect decreased and the planet cooled.

8.3.1 Rodinia

By the Proterozoic Eon, lithospheric plates had formed and were moving according to plate tectonic forces that were similar to current times. As the moving plates collided, the ocean basins closed to form a supercontinent called Rodinia. The supercontinent formed about one billion years ago and broke up about 750 to 600 million years ago, at the end of the Proterozoic. One of the resulting fragments was a continental mass called Laurentia that would later become North America. Geologists have reconstructed Rodinia by matching and aligning ancient mountain chains, assembling the pieces like a jigsaw puzzle and using paleomagnetics to orient to magnetic north.

The disagreements over these complex reconstructions is exemplified by geologists proposing at least six different models for the breakup of Rodinia to create Australia, Antarctica, parts of China, the Tarim craton north of the Himalayas, Siberia, or the Kalahari craton of Eastern Africa. This breakup created lots of shallow-water, biologically favorable environments that fostered the evolutionary breakthroughs marking the start of the next eon, the Phanerozoic.

8.3.2 Life Evolves

Life in the Archean and before is poorly documented in the fossil record. Based on chemical evidence and evolutionary theory, scientists propose this life would have been single-celled photosynthetic organisms, such as the cyanobacteria that created stromatolites. Cyanobacteria produced free oxygen in the atmosphere through photosynthesis. Cyanobacteria, archaea, and bacteria are prokaryotes—primitive organisms made of single cells that lack cell nuclei and other organelles.

A large evolutionary step occurred during the Proterozoic Eon with the appearance of eukaryotes around 2.1 to 1.6 billion years ago. Eukaryotic cells are more complex, having nuclei and organelles. The nuclear DNA is capable of more complex replication and regulation than that of prokaryotic cells. The organelles include mitochondria for producing energy and chloroplasts for photosynthesis. The eukaryote branch in the tree of life gave rise to fungi, plants, and animals.

Another important event in Earth’s biological history occurred about 1.2 billion years ago when eukaryotes began sexual reproduction. Sharing genetic material from two reproducing individuals, male and female, greatly increased genetic variability in their offspring. This genetic mixing accelerated evolutionary change, contributing to more complexity among individual organisms and within ecosystems (see Chapter 7).

Proterozoic land surfaces were barren of plants and animals, and geologic processes actively shaped the environment differently because land surfaces were not protected by leafy and woody vegetation. For example, rain and rivers would have caused erosion at much higher rates on land surfaces devoid of plants. This resulted in thick accumulations of pure quartz sandstone from the Proterozoic Eon, such as the extensive quartzite formations in the core of the Uinta Mountains in Utah.

Fauna during the Ediacaran Period, 635.5 to 541 million years ago, are known as the Ediacaran fauna and offer a first glimpse at the diversity of ecosystems that evolved near the end of the Proterozoic. These soft-bodied organisms were among the first multicellular life forms and were probably wormlike or similar to jellyfish. Ediacaran fauna did not have hard parts like shells and were not well preserved in the rock records. However, studies suggest they were widespread in the Earth’s oceans. Scientists still debate how many species were evolutionary dead-ends that became extinct and how many were ancestors of modern groupings. The transition of soft-bodied Ediacaran life forms to life forms with hard body parts occurred at the end of the Proterozoic and beginning of the Phanerozoic Eons. This evolutionary explosion of biological diversity made a dramatic difference in scientists’ ability to understand the history of life on Earth.

8.4 Phanerozoic Eon: Paleozoic Era

The Phanerozoic Eon is the most recent, spanning 541 million years ago to today. Its name means “visible life” because the Phanerozoic rock record is marked by an abundance of fossils. Phanerozoic organisms had hard body parts like claws, scales, shells, and bones that were more easily preserved as fossils. Rocks from the older Precambrian time are less commonly found and rarely include fossils because those organisms had soft body parts. Phanerozoic rocks are younger, more common, and contain the majority of extant fossils. The study of rocks from this eon yields much greater detail. The Phanerozoic is subdivided into three eras; from oldest to youngest, they are Paleozoic (“ancient life”), Mesozoic (“middle life”), and Cenozoic (“recent life”), and the remaining three sections in this chapter cover three important eras.

Life in the early Paleozoic Era was dominated by marine organisms, but by the middle of the era, plants and animals evolved to live and reproduce on land. Fish evolved jaws, and fins evolved into jointed limbs. The development of lungs allowed animals to emerge from the sea and become the first air-breathing tetrapods (four-legged animals), such as amphibians. From amphibians evolved reptiles with the amniotic egg. From reptiles evolved an early ancestor to birds and mammals, and their scales became feathers and fur. Near the end of the Paleozoic Era, the Carboniferous Period had some of the most extensive forests in Earth’s history. Their fossilized remains became the coal that powered the Industrial Revolution.

8.4.1 Paleozoic Tectonics and Paleogeography

During the Paleozoic Era, sea levels rose and fell four times. With each sea level rise, the majority of North America was covered by a shallow tropical ocean. Evidence of these submersions is found in the abundant marine sedimentary rocks such as limestone with fossils corals and ooids. Extensive sea level falls are documented by widespread unconformities. Today, the midcontinent has extensive marine sedimentary rocks from the Paleozoic and western North America has thick layers of marine limestone on block-faulted mountain ranges such as Mount Timpanogos near Provo, Utah.

The assembly of the supercontinent Pangea, sometimes spelled Pangaea, was completed by the late Paleozoic Era. The name Pangea was originally coined by Alfred Wegener and means “all land.” Pangea describes when all of the major continents were grouped together as one by a series of tectonic events including subduction island-arc accretion, continental collisions, and ocean-basin closures. In North America, these tectonic events occurred on the east coast and are known as the Taconic, Acadian, Caledonian, and Alleghanian orogenies. The Appalachian Mountains are the erosional remnants of these mountain-building events in North America. Surrounding Pangea was a global ocean basin known as the Panthalassa. Continued plate movement extended the ocean into Pangea, forming a large bay called the Tethys Sea that eventually divided the land mass into two smaller supercontinents, Laurasia and Gondwana. Laurasia consisted of Laurentia and Eurasia, and Gondwana consisted of the remaining continents of South America, Africa, India, Australia, and Antarctica.

While the east coast of North America was tectonically active during the Paleozoic Era, the west coast remained mostly inactive as a passive margin during the early Paleozoic. The western edge of the North American continent was near the present-day Nevada-Utah border and was an expansive, shallow continental shelf near the paleoequator. However, by the Devonian Period, the Antler orogeny started on the west coast and lasted until the Pennsylvanian Period. The Antler orogeny was a volcanic island arc that was accreted onto western North America, with the subduction direction away from North America. This created a mountain range on the west coast of North America called the Antler highlands and was the first part of building the land in the west that would eventually make most of the California, Oregon, and Washington states. By the late Paleozoic, the Sonoma orogeny began on the west coast and was another collision of an island arc. The Sonoma orogeny marks the change in subduction direction to be toward North America with a volcanic arc along the entire west coast of North America by the late Paleozoic to early Mesozoic Eras.

By the end of the Paleozoic Era, the east coast of North America had a very high mountain range due to continental collision and the creation of Pangea. The west coast of North America had smaller and isolated volcanic highlands associated with island arc accretion. During the Mesozoic Era, the size of the mountains on either side of North America would flip, with the west coast being a more tectonically active plate boundary and the east coast changing into a passive margin after the breakup of Pangea.

8.4.2 Paleozoic Evolution

The beginning of the Paleozoic Era is marked by the first appearance of hard body parts like shells, spikes, teeth, and scales; it is also marked by the appearance in the rock record of most animal phyla known today. That is, most basic animal body plans appeared in the rock record during the Cambrian Period. This sudden appearance of biological diversity is called the Cambrian explosion. Scientists debate whether this sudden appearance is more from a rapid evolutionary diversification as a result of a warmer climate following the late Proterozoic glacial environments, from the better preservation and fossilization of hard parts, or due to artifacts of a more complete and recent rock record. For example, fauna may have been diverse during the Ediacaran Period, setting the stage for the Cambrian explosion, but they lacked hard body parts and would have left few fossils behind. Regardless, the Cambrian Period 541–485 million years ago marked the appearance of most animal phyla.

One of the best fossil sites for the Cambrian explosion was discovered in 1909 by Charles Walcott (1850–1927) in the Burgess Shale in Western Canada. The Burgess Shale is a Lagerstätte, a site of exceptional fossil preservation that includes impressions of soft body parts. This discovery allowed scientists to study Cambrian animals in immense detail because soft body parts are not normally preserved and fossilized. Other Lagerstätte sites of similar age in China and Utah have allowed scientist to form a detailed picture of Cambrian biodiversity. The biggest mystery surrounds animals that do not fit existing lineages and are unique to that time. This includes many famous fossilized creatures: the first compound-eyed trilobites; Wiwaxia, a creature covered in spiny plates; Hallucigenia, a walking worm with spikes; Opabinia, a five-eyed arthropod with a grappling claw; and Anomalocaris, the alpha predator of its time, complete with grasping appendages and circular mouth with sharp plates. Most notably appearing during the Cambrian is an important ancestor to humans. A segmented worm called Pikaia is thought to be the earliest ancestor of the Chordata phylum that includes vertebrates, animals with backbones.

By the end of the Cambrian, mollusks, brachiopods, nautiloids, gastropods, graptolites, echinoderms, and trilobites covered the seafloor. Although most animal phyla appeared by the Cambrian, the biodiversity at the family, genus, and species levels was low until the Ordovician Period. During the Great Ordovician Biodiversification Event, vertebrates and invertebrates (animals without backbone) became more diverse and complex at the family, genus, and species levels. The cause of the rapid speciation event is still debated, but some likely causes are a combination of warm temperatures, expansive continental shelves near the equator, and more volcanism along the mid-ocean ridges. Some have shown evidence that an asteroid breakup event and consequent heavy meteorite impacts correlate with this diversification event. The additional volcanism added nutrients to ocean water, helping support a robust ecosystem. Many life forms and ecosystems that would be recognizable in current times appeared at this time. Mollusks, corals, and arthropods in particular multiplied to dominate the oceans.

One important evolutionary advancement during the Ordovician Period was reef-building organisms, mostly colonial coral. Corals took advantage of the ocean chemistry, using calcite to build large structures that resembled modern reefs like the Great Barrier Reef off the coast of Australia. These reefs housed thriving ecosystems of organisms that swam around, hid in, and crawled over them. Reefs are important to paleontologists because of their preservation potential, massive size, and in-place ecosystems. Few other fossils offer more diversity and complexity than reef assemblages.

According to evidence from glacial deposits, a small ice age caused sea levels to drop and led to a major mass extinction by the end of the Ordovician. This is the earliest of five mass extinction events documented in the fossil record. During this mass extinction, an unusually large number of species abruptly disappear in the fossil record (see video below).

Life bounced back during the Silurian Period. The period’s major evolutionary event was the development of jaws from the forward pair of gill arches in bony fishes and sharks. Hinged jaws allowed fish to exploit new food sources and ecological niches. This period also included the introduction of armored fishes, known as the placoderms. In addition to fish and jaws, Silurian rocks provide the first evidence of terrestrial, or land-dwelling, plants and animals. The first vascular plant, Cooksonia, had woody tissues, pores for gas exchange, and veins for water and food transport. Insects, spiders, scorpions, and crustaceans began to inhabit moist, freshwater terrestrial environments.

The Devonian Period is called the Age of Fishes due to the rise in plated, jawed, and lobe-finned fishes. The lobe-finned fishes, which were related to the modern lungfish and coelacanth, are important for their eventual evolution into tetrapods—four-limbed vertebrate animals that can walk on land. The first lobe-finned land-walking fish, named Tiktaalik, appeared about 385 million years ago and serves as a transition fossil between fish and early tetrapods. Though Tiktaalik was clearly a fish, it had some tetrapod structures as well. Several fossils from the Devonian are more tetrapodlike than fishlike, but these weren’t fully terrestrial. The first fully terrestrial tetrapod arrived in the Mississippian (early Carboniferous) Period. By the Mississippian Period, tetrapods had evolved into two main groups, amphibians and amniotes, from a common tetrapod ancestor. The amphibians were able to breathe air and live on land but still needed water to nurture their soft eggs. The first reptile (an amniote) could live and reproduce entirely on land with hard-shelled eggs that wouldn’t dry out.

Land plants had also evolved into the first trees and forests. Toward the end of the Devonian, another mass extinction event occurred. This extinction, while severe, is the least temporally defined, with wide variations in the timings of the event or events. Reef-building organisms were the hardest hit, leading to dramatic changes in marine ecosystems.

The next time period is called the Carboniferous (North American geologists have subdivided this into the Mississippian and Pennsylvanian Periods). This period saw the highest levels of oxygen ever known, with forests (e.g., ferns, club mosses) and swamps dominating the landscape. This helped cause the largest arthropods ever, like the millipede Arthropleura, at 2.5 meters (6.4 feet) long! It also saw the rise of a new group of animals, the reptiles. The evolutionary advantage that reptiles have over amphibians is the amniote egg (egg with a protective shell), which allows them to rely on nonaquatic environments for reproduction. This widened the terrestrial reach of reptiles compared to amphibians. This period’s booming life, especially plant life, created cooling temperatures as carbon dioxide was removed from the atmosphere. By the middle Carboniferous, these cooler temperatures led to an ice age (called the Karoo glaciation) and less-productive forests. The reptiles fared much better than the amphibians, leading to their diversification. This glacial event lasted into the early Permian.

By the Permian, the now-assembled Pangea supercontinent led to a dryer climate and even more diversification and domination by the reptiles. The groups that developed in this warm climate eventually radiated into dinosaurs. Another group, known as the synapsids, eventually evolved into mammals. Synapsids, including the famous sail-backed Dimetrodon, are commonly confused with dinosaurs. Pelycosaurs (of the Pennsylvanian to early Permian, like Dimetrodon) are the first group of synapsids that exhibit the beginnings of mammalian characteristics, such as well-differentiated dentition: incisors, highly developed canines in lower and upper jaws, and cheek teeth, premolars, and molars. Starting in the late Permian, a second group of synapsids, called the therapsids (or mammal-like reptiles) evolve and become the ancestors to mammals.

Permian Mass Extinction

The end of the Paleozoic Era is marked by the largest mass extinction in Earth history. The Paleozoic Era had two smaller mass extinctions, but these were not as large as the Permian Mass Extinction, also known as the Permian-Triassic Extinction Event. It is estimated that up to 96% of marine species and 70% of land-dwelling (terrestrial) vertebrates went extinct. Many famous organisms, like sea scorpions and trilobites were never seen again in the fossil record. What caused such a widespread extinction event? The exact cause is still debated, though the leading idea relates to extensive volcanism associated with the Siberian Traps, which are one of the largest deposits of flood basalts known on Earth, dating to the time of the extinction event. The eruption size is estimated at over three million cubic kilometers—approximately four million times larger than the famous 1980 Mount St. Helens eruption in Washington. The unusually large volcanic eruption would have contributed a large amount of toxic gases, aerosols, and greenhouse gases into the atmosphere. Further, some evidence suggests that the volcanism burned vast coal deposits, releasing methane (a greenhouse gas) into the atmosphere. As discussed in Chapter 15, greenhouse gases cause the climate to warm. This extensive addition of greenhouse gases from the Siberian Traps may have caused a runaway greenhouse effect that rapidly changed the climate, acidified the oceans, disrupted food chains, disrupted carbon cycling, and caused the largest mass extinction.

8.5 Phanerozoic Eon: Mesozoic Era

Following the Permian Mass Extinction, the Mesozoic (“middle life”) was from 252 million years ago to 66 million years ago. As Pangea started to break apart, mammals, birds, and flowering plants developed. The Mesozoic is probably best known as the Age of Reptiles, most notably, the dinosaurs.

8.5.1 Mesozoic Tectonics and Paleogeography

Pangea started breaking up (in a region that would become eastern Canada and United States) around 210 million years ago in the Late Triassic. Clear evidence for this includes the age of the sediments in the Newark Supergroup rift basins and the Palisades sill of the eastern part of North America and the age of the Atlantic ocean floor. Due to seafloor spreading, the oldest rocks on the Atlantic’s floor are along the coast of northern Africa and the east coast of North America, while the youngest are along the mid-ocean ridge.

This age pattern shows how the Atlantic Ocean opened as the young Mid-Atlantic Ridge began to create the seafloor. This means the Atlantic Ocean started opening and was first formed here. The southern Atlantic opened next, with South America separating from Central and Southern Africa. Finally, after the Mesozoic ended, Greenland and Scandinavia parted ways in the northernmost Atlantic. The breaking points of each rifted plate margin eventually turned into the passive plate boundaries of the east coast of the Americas today.

In western North America, an active plate margin had started with subduction, controlling most of the tectonics of that region in the Mesozoic. Another possible island-arc collision created the Sonoman orogeny in Nevada during the latest Paleozoic to the Triassic. In the Jurassic, another island-arc collision caused the Nevadan orogeny, a large Andean-style volcanic arc and thrust belt. The Sevier orogeny followed in the Cretaceous, which was mainly a volcanic arc to the west and a thin-skinned fold and thrust belt to the east, meaning stacks of shallow faults and folds built up the topography. Many of the structures in the Rocky Mountains today date from this orogeny.

Tectonics had an influence in one more important geographic feature in North America: the Cretaceous Western Interior Foreland Basin, which flooded during high sea levels, forming the Cretaceous Interior Seaway. Subduction from the west was the Farallon plate, an oceanic plate connected to the Pacific plate (seen today in remnants such as the Juan de Fuca plate, off the coast of the Pacific Northwest). Subduction was shallow at this time because a very young, hot, and less dense portion of the Farallon plate was subducted. This shallow subduction caused a downwarping in the central part of North America. Additional contributors to the high sea levels include shallow subduction and increasing rates of seafloor spreading and subduction, high temperatures, and melted ice. These factors allowed a shallow epicontinental seaway that extended from the Gulf of Mexico to the Arctic Ocean, dividing North America into two separate land masses, Laramidia to the west and Appalachia to the east, for 25 million years. Many of the coal deposits in Utah and Wyoming formed from swamps along the shores of this seaway. By the end of the Cretaceous, cooling temperatures caused the seaway to regress.

8.5.2 Mesozoic Evolution

The Mesozoic Era was dominated by reptiles—more specifically, the dinosaurs. The Triassic saw devastated ecosystems that took over 30 million years to fully re-emerge after the Permian Mass Extinction. The first appearance of many modern groups of animals that would later flourish occurred at this time. This includes frogs (amphibians), turtles (reptiles), marine ichthyosaurs and plesiosaurs (marine reptiles), mammals, and the archosaurs. The archosaurs (“ruling reptiles”) include ancestral groups that went extinct at the end of the Triassic, as well as the flying pterosaurs, crocodilians, and the dinosaurs. Archosaurs, like the placental mammals after them, occupied all major environments: terrestrial (dinosaurs), airborne (pterosaurs), aquatic (crocodilians) and even fully marine habitats (marine crocodiles). The pterosaurs, the first vertebrate group to take flight, started small in the Triassic, like the dinosaurs and mammals.

At the end of the Triassic, another mass extinction event occurred, the fourth major mass extinction in the geologic record. This was perhaps caused by the Central Atlantic Magmatic Province flood basalt. The end-Triassic extinction made certain lineages go extinct and helped spur the evolution of survivors like mammals, pterosaurs (flying reptiles), ichthyosaurs/plesiosaurs/mosasaurs (marine reptiles), and dinosaurs.

Mammals, as previously mentioned, got their start from a reptilian synapsid ancestor that possible lived in the late Paleozoic. Mammals stayed small, in mainly nocturnal niches, with insects being their largest prey. The development of warm-blooded circulation and fur may have been a response to this lifestyle.

In the Jurassic, species that were previously common flourished due to a warmer and more tropical climate. The dinosaurs were relatively small animals in the Triassic Period of the Mesozoic but became truly massive in the Jurassic. Dinosaurs are split into two groups based on their hip structure, i.e., orientation of the pubis and ischium bones in relationship to each other. This is referred to as the “reptile-hipped” saurischians and the “bird-hipped” ornithischians. This has recently been brought into question by a new idea for dinosaur lineage.

Most of the dinosaurs of the Triassic were saurischians, but all of them were bipedal. The major adaptive advantage of dinosaurs was changes in the hip and ankle bones, enabling the tucking of the legs under the body for improved locomotion as opposed to the semi-erect gait of crocodiles or the sprawling posture of reptiles. In the Jurassic, limbs (or a lack thereof) were also important to another group of reptiles, leading to the evolution of Eophis, the oldest snake.

There is a paucity of dinosaur fossils from the Early and Middle Jurassic, but by the Late Jurassic, they were dominating the planet. The saurischians diversified into giant herbivorous (plant-eating) long-necked sauropods weighing up to 100 tons and into theropods that were bipedal and carnivorous, with the possible exception of the genus Therizinosaurus. All of the ornithischians (e.g., Stegosaurus, Iguanodon, Triceratops, Ankylosaurus, Pachycephhlosaurus) were herbivorous, with a strong tendency to have a “turtle-like” beak at the tips of their mouths.

The pterosaurs grew and diversified in the Jurassic, and another notable arial organism developed and thrived in the Jurassic: birds. When the seeming dinosaur-bird hybrid Archeopteryx was found in the Solnhofen Lagerstätte of Germany, it started the conversation on the origin of birds. The idea that birds evolved from dinosaurs occurred very early in the history of research into evolution, only a few years after Darwin’s On the Origin of Species. This study used a remarkable fossil of Archeopteryx from a transitional animal between dinosaurs and birds. Small meat-eating theropod dinosaurs were likely the branch that became birds due to their similar features. A significant debate still exists over how and when powered flight evolved. Some have stated a running-start model, while others have favored a tree-leaping gliding model or even a semi-combination: flapping to aid in climbing.

The Cretaceous saw a further diversification, specialization, and domination of the dinosaurs and other fauna. One of the biggest changes on land was the transition to angiosperm-dominated flora. Angiosperms, which are plants with flowers and seeds, had originated in the Cretaceous, switching many plains to grasslands by the end of the Mesozoic. By the end of the period, they had replaced gymnosperms (evergreen trees) and ferns as the dominant plant in the world’s forests. Haplodiploid eusocial insects (bees and ants) are descendants of wasplike ancestors from the Jurassic that co-evolved with the flowering plants during this time period. The breakup of Pangea not only shaped our modern world’s geography but biodiversity at the time as well. Throughout the Mesozoic, animals on the isolated, now-separated island continents (formerly parts of Pangea) took strange evolutionary turns. This includes giant titanosaurian sauropods (Argentinosaurus) and theropods (Giganotosaurus) from South America.

K-T Extinction

Similar to the end of the Paleozoic Era, the Mesozoic Era ended with the K-Pg Mass Extinction (previously known as the K-T extinction) 66 million years ago. This extinction event was likely caused by a large bolide (an extraterrestrial impactor such as an asteroid, meteoroid, or comet) that collided with Earth. Ninety percent of plankton species, 75% of plant species, and all the dinosaurs went extinct at this time.

One of the strongest pieces of evidence comes from the element iridium. Quite rare on Earth and more common in meteorites, it has been found all over the world in higher concentrations at a particular layer of rock that formed at the time of the K-T boundary. Soon other scientists started to find evidence to back up the claim. Melted rock spheres—a special type of “shocked” quartz called stishovite, found only at impact sites—were found in many places around the world. The huge impact created a strong thermal pulse that could be responsible for global forest fires, strong acid rains, a corresponding abundance of ferns, the first colonizing plants after forest fires, enough debris thrown into the air to significantly cool temperatures afterward, and a two-kilometer-high tsunami inferred from deposits found from Texas to Alabama.

Still, with all this evidence, one large piece remained missing: the crater where the bolide impacted. It was not until 1991 that the crater was confirmed using petroleum company geophysical data. Even though it is the third-largest confirmed crater on Earth at roughly 180 km wide, the Chicxulub crater was hard to find due to being partially underwater and partially obscured by the dense forest canopy of the Yucatan Peninsula. Coring of the center of the impact, called the peak ring, contained granite, indicating the impact was so powerful that it lifted basement sediment from the crust several miles toward the surface. In 2010, an international team of scientists reviewed 20 years of research and blamed the impact for the extinction.

With all of this information, it seems like the case would be closed. However, there are other events at this time which could have partially aided the demise of so many organisms. For example, sea levels are known to have been slowly decreasing at the time of the K-T event, which is tied to marine extinctions, though any study on gradual vs. sudden changes in the fossil record is flawed due to the incomplete nature of the fossil record. Another big event at this time was the Deccan Traps flood basalt volcanism in India. At over 1.3 million cubic kilometers of material, it was certainly a large source of material hazardous to ecosystems at the time, and it has been suggested to be at least partially responsible for the extinction. Some have found the impact and eruptions too much of a coincidence and have even linked the two together.

8.6 Phanerozoic Eon: Cenozoic Era

The Cenozoic, meaning “new life,” is known as the Age of Mammals because it is in this era that mammals came to be a dominant and large life form, including human ancestors. Birds also flourished in the open niches left by the dinosaur’s demise. Most of the Cenozoic has been relatively warm, with the main exception being the ice age that started about 2.558 million years ago and (despite recent warming) continues today. Tectonic shifts in the west caused volcanism but eventually changed the long-standing subduction zone into a transform boundary.

8.6.1 Cenozoic Tectonics and Paleogeography

In the Cenozoic, the plates of the Earth moved into more familiar places, with the biggest change being the closing of the Tethys Sea with collisions such as the Alps, Zagros, and Himalayas, a collision that started about 57 million years ago and continues today. Maybe the most significant tectonic feature that occurred in the Cenozoic of North America was the conversion of the west coast of California from a convergent boundary subduction zone to a transform boundary. Subduction off the coast of the western United States, which had occurred throughout the Mesozoic, had continued in the Cenozoic. After the Sevier orogeny in the late Mesozoic, a subsequent orogeny called the Laramide orogeny, occurred in the early Cenozoic. The Laramide was thick-skinned, different than the Sevier orogeny. It involved deeper crustal rocks and produced bulges that would become mountain ranges like the Rockies, Black Hills, Wind River Range, Uinta Mountains, and the San Rafael Swell. Instead of descending directly into the mantle, the subducting plate shallowed out and moved eastward beneath the continental plate, affecting the overlying continent hundreds of miles east of the continental margin and building high mountains. This occurred because the subducting plate was so young and near the spreading center, meaning the density of the plate was low and subduction was hindered.

As the mid-ocean ridge itself started to subduct, the relative motion had changed. Subduction caused a relative convergence between the subducting Farallon plate and the North American plate. On the other side of the mid-ocean ridge from the Farallon plate was the Pacific plate, which was moving away from the North American plate. Thus, as the subduction zone consumed the mid-ocean ridge, the relative movement became transform instead of convergent, which went on to become the San Andreas fault system. As the San Andreas grew, it caused east-west directed extensional forces to spread over the Western United States, creating the Basin and Range Province. The transform fault switched position over the last 18 million years, twisting the mountains around Los Angeles, and new faults in the southeastern California deserts may become a future San Andreas-style fault. During this switch from subduction to transform, the nearly horizontal Farallon slab began to sink into the mantle. This caused magmatism as the subducting slab sank, allowing asthenosphere material to rise around it. This event is called the Oligocene ignimbrite flare-up, which was one of the most significant periods of volcanism ever, including the largest single confirmed eruption, the 5,000 cubic kilometer Fish Canyon Tuff.

8.6.2 Cenozoic Evolution

There are five groups of early mammals in the fossil record, based primarily on fossil teeth, the hardest bone in vertebrate skeletons. For the purpose of this text, the most important group are the Eupantotheres, which diverged into the two main groups of mammals, the marsupials (like Sinodelphys) and placentals or eutherians (like Eomaia) in the Cretaceous and then diversified in the Cenozoic. The marsupials dominated on the isolated island continents of South America and Australia, and many went extinct in South America with the introduction of placental mammals. Some well-known mammal groups have been highly studied with interesting evolutionary stories in the Cenozoic. For example, horses started small, with four toes, and ended up larger, with just one toe. Cetaceans (marine mammals like whales and dolphins) started on land from small bearlike (mesonychids) creatures in the early Cenozoic and gradually took to water. However, no form of evolution has been more studied than human evolution. Hominids, the name for humanlike primates, started in Eastern Africa several million years ago.

The first critical event in this story is an environmental change from jungle to more of a savanna, probably caused by changes in Indian Ocean circulation. While bipedalism is known to have evolved before this shift, it is generally believed that our bipedal ancestors (like Australopithecus) had an advantage by covering ground more easily in a more open environment compared to their nonbipedal evolutionary cousins. There is also a growing body of evidence, including the famous Australopithecine fossil “Lucy,” that our early ancestors lived in trees. Arboreal animals usually demand a high intelligence to navigate through a three-dimensional world. It is from this lineage that humans evolved, using the capability of endurance running as a means to acquire more resources and possibly even hunt. This can explain many uniquely human features, from our long legs, strong achilles, lack of lower gut protection, and our wide range of running efficiencies.

Once the hands were freed up, the next big step was a large brain. There have been arguments that a switch to eating more meat, cooking with fire, tool use, and even the construct of society itself can explain this increase in brain size. Regardless of how, it was this increased cognitive power that allowed humans to reign as their ancestors moved out of Africa and explored the world, ultimately entering the Americas through land bridges like the Bering land bridge. The details of this worldwide migration and the different branches of the hominid evolutionary tree are very complex and best reserved for their own course.

Anthropocene and Extinction

Humans have had an influence on the Earth, its ecosystems, and climate. Yet human activity cannot explain all of the changes that have occurred in the recent past. The start of the Quaternary Period, the last and current period of the Cenozoic, is marked by the start of our current ice age 2.58 million years ago. During this time period, ice sheets advanced and retreated, most likely due to Milankovitch cycles (see Chapter 15). Also at this time, various cold-adapted megafauna emerged (like giant sloths, saber-tooth cats, and woolly mammoths), and most of them went extinct as the Earth warmed from the most recent glacial maximum. A long-standing debate concerns the cause of these and other extinctions. Is climate warming to blame, or were they caused by humans? Certainly, we know of recent human extinctions of animals like the dodo or passenger pigeon. Can we connect modern extinctions to extinctions in the recent past? If so, there are several ideas as to how this happened. Possibly the most widely accepted and oldest is the hunting/overkill hypothesis. The idea behind this hypothesis is that humans hunted large herbivores for food, then as a result, carnivores could not find food; human arrival times in locations have been shown to be tied to increased extinction rates in many cases.

Modern human impact on the environment and the Earth as a whole is unquestioned. In fact, many scientists are starting to suggest that the rise of human civilization ended and/or replaced the Holocene Epoch and defines a new geologic time interval: the Anthropocene. Evidence for this change includes extinctions, increased tritium (hydrogen with two neutrons) due to nuclear testing, rising pollutants like carbon dioxide, the emergence of more than 200 never-before seen mineral species that have occurred only in this epoch, materials such as plastic and metals which will be long-lasting “fossils” in the geologic record, and large amounts of earthen material moved. The biggest scientific debate with this topic is the starting point. Some say that humans’ invention of agriculture would be recognized in geologic strata and that should be the starting point, around 12,000 years ago. Others link the start of the Industrial Revolution and the subsequent addition of vast amounts of carbon dioxide in the atmosphere. Either way, the idea is that alien geologists visiting Earth in the distant future would easily recognize the impact of humans on the Earth as the beginning of a new geologic period.

Summary

The changes that have occurred since the inception of Earth are vast and significant. From the oxygenation of the atmosphere, the progression of life forms, and the assembly and deconstruction of several supercontinents, to the extinction of more life forms than exist today, having a general understanding of these changes can put present change into a more rounded perspective.