8 Earth History
By the end of this chapter, students should be able to:
- Explain the Big Bang and the origin of the elements.
- Explain the solar system’s origin and the consequences for Earth.
- Describe the turbulent beginning of Earth during the Hadean and Archean eons.
- Understand the transition to modern atmosphere, plate tectonics, and evolution that occurred in the Proterozoic eon.
- Describe the three eras of the Phanerozoic: The Paleozoic, which established many of the common life forms alive today and our last supercontinent, Pangea; The Mesozoic, which was the age of dinosaurs and the time of the breakup of Pangea; and the Cenozoic, the time of mammalian and bird dominance and the shaping of the modern world.
Earth history is a wide ranging topic that entire courses and scientific careers are based on. Throughout the long history of Earth, change has been the norm. If travelling back in time, many life forms and terrains would be unfamiliar to the untrained eye. The main topics that are studied in Earth History are the past landscapes (paleogeography), organisms (paleontology), and environments (paleoecology and paleoclimatology). This chapter will focus on the basic ideas that are part of the study of the Earth, including the start of the Universe. Please note 8.1 and 8.2 are topics perhaps more suited for an astronomy text, but are included here to give context to the Earth.
8.1 Origin of the Universe
In the beginning, time and space were initiated in the Big Bang and the universe formed from an infinitely dense and hot core of material. The Universe appears to have a infinite reach and an infinite number of stars in galaxies reaching far and wide, of which our galaxy and solar system occupy a seemingly obscure and insignificant section of the entirety. The details of the origin of the Universe, and especially any details of events before the moment of its creation, are subject to great debate and mystery, but are detailed in The Big Bang Theory. Though the ideas behind the Big Bang Theory feel almost mystical, they are helped by complex ideas in Einstein’s theory of general relativity . The evidence that has supported this theory, however, is grounded in straightforward empirical observations.
8.1.1 Big Bang Theory
The Big Bang theory is the idea that after the beginning of the universe (the ‘bang’ in the title), all matter and space (called space-time by Einstein) expanded explosively outward, and has continued to expand in the 13.8 billion years since. Recent observations have suggested that the universe is continuing to expand, only at a more and more rapid rate . Shortly after the Big Bang, atoms were also created, and most of these were hydrogen. Hydrogen still makes up about 74% of all matter observed. The reason we know this comes from spectroscopy.
Spectroscopy is the study of the details of light. Visible light is just one range of the many types of light that exist, including x-rays, microwaves, and radio waves. These different waves make up the electromagnetic spectrum. Each beam of light is actually a unique mixture of a suite of wavelengths across the spectrum that combine to make the color we see. This signature of wavelengths of light is created or absorbed as light is made inside atoms, and this can be matched to specific elements. Even white light from the sun, which seems like a solid continuum, has strong wavelengths, weak wavelengths, and gaps. The breaks correspond to elements present in Earth’s or the sun’s atmosphere which act as filters for their specific wavelengths of light. These ‘missing’ wavelengths were famously observed by Josef von Fraunhofer in the early 1800s , but it took decades before scientists were able to relate the missing wavelengths to atmospheric filtering, either in the sun’s atmoshpere or the Earth’s . Using spectroscopy, we can tell that the sun is mostly hydrogen and helium. Extrapolating this process to the light that has traveled to us from distant stars, we can calculate the abundance of elements in any specific star and in the visible universe as a whole. Also, this information can be used as an interstellar speedometer.
The Doppler Effect is the same wavelength process that changes the pitch of the sound of a car or ambulance from high to low as it approaches then passes. When an object emits waves, such as sound waves, but also moves toward an observer, wavelengths get compressed (higher pitch as with sound). When an object moves away from an observer, the wavelengths become longer (lower pitch as with sound). This effect is used on the light emitted from stars and galaxies to determine their speed and direction of travel . Scientists, including Slipher and Hubble , examined galaxies both near and far, and found that almost all galaxies outside of our galaxy are moving away from each other and us. Because the wavelengths of receding objects are extended, this shifts visible light toward the red end of the spectrum and is called redshift. Not only that, Hubble noticed the further away galaxies are, the larger the redshift, and thus, the faster they are traveling away from us. The only way to reconcile this information is to deduce the universe itself as expanding. This is the origin of the Big Bang Theory, and the reverse of this expansion gives scientists the 13.8 billion year age of the universe.
Cosmic Microwave Background Radiation
Another strong indication of the big bang event is cosmic microwave background radiation. This was accidentally discovered by Arno Penzias and Robert Wilson when they were trying to eliminate background noise from a communication satellite. They discovered a very faint energy, or heat, that is omnipresent across the universe. This is energy left behind from the big bang itself, similar to an echo.
8.1.2 Stellar Evolution
It is thought the big bang created mostly hydrogen, and smaller amounts of elements Helium, Lithium and Beryllium. Cosmic ray interactions can also explain other amounts of lighter elements like Lithium and Beryllium. Another process must be responsible for the nearly 90 other natural elements on Earth. The current model of stellar evolution explains the origins of the other heavier elements.
Birth of a star
Stars start their lives as elements floating in cold, spinning clouds of gas and dust known as nebulas. Gravitational attraction or perhaps a nearby stellar explosion causes the elements to start to condense and spin into disk shape. In the center of this disk shape a new star is born. Under the force of gravity, the spinning whirlpool of material concentrates in the middle, making the gravitational forces collect even more mass. Eventually, the immense concentration of mass reaches a critical point where the heat and pressure are so high that fusion initiates. Fusion is a nuclear reaction in which two or more nuclei (center of an atom) are forced together and combine. This gives off a tremendous amount of energy. This energy is often observed as light and solar radiation.
Fusion is the process in which smaller elements combine into larger ones, and is the “lifeblood” of a star. This is not a chemical reaction; this is an element, such as hydrogen, combining (fusing) with other hydrogen atoms to become a new element, in this case, helium, in the core of the star. Another product of this process is energy. It is this energy that leaves the sun and comes to the Earth as light and heat. It is steady and predictable, which is why we call this part of a star’s life the main phase. In the main phase, stars turn hydrogen into helium. Since hydrogen is so plentiful, this part of a star’s life can last for billions of years, and the size and energy output remains relatively steady for a very long time.
The giant phase in a star’s life occurs when the star runs out of hydrogen to fuse. Large enough stars have the heat and pressure to now start fusing helium into heavier elements. This style of fusion is more energetic, and the increased energy expands the size of the star with the help of increasing temperatures, ballooning it to a much larger size and brightness. This is predicted to happen to our sun in another few billion years, with the giant phase radius of the sun being at or near the radius of Earth’s orbit, rendering life on Earth impossible. The mass reached at the main phase is the primary factor determining how the star will evolve. If the star has a large enough mass, it reaches a point at which a new element (such as helium) undergoing fusion is exhausted, and a new element starts the fusion process. This occurs over and over (in stars large enough) forming progressively larger elements like carbon and oxygen. Eventually, this process reaches its limit as iron and nickel are formed. This explains the abundance of iron and nickel in rocky objects (like Earth) within the Solar System. At this point, any further fusion absorbs energy instead of giving it off, starting the beginning of the end for the life of the star .
Death of a Star
The death of a star can range from the spectacular to the other-worldly (see above figure). In stars like the sun, a planetary nebula is formed, which is nothing more than the collapse of the outer layers of the star, not too unlike an implosion of a building. In the eternal tug-of-war of forces between gravity’s pull inwards and fusion’s outward pressure, when fusion ends, gravity instantly takes over, with outer gasses puffing away. More massive stars do this as well, but in their case, the collapse is much more energetic, and this starts another type of energy release mixed with element creation known as a supernova. In a supernova, the sudden halt in the collapse of the core creates an outward propagating shock wave. A supernova is the most energetic explosion in the universe short of the big bang; the energy is so high that every element up through uranium can be made, despite the energy loss that occurs in the fusion process .
After the events of the death of the star, the aftermath results in several strange objects. Stars like the sun turn into white dwarfs when they are dead . White dwarfs are hot star “embers.” The white dwarf that forms after the sun’s demise will take most of the mass within the sun and pack it into a very small, dense object about the size of Earth. Larger stars, after their supernovas, pack even tighter into neutron stars, objects so dense that protons capture electrons, turning into neutrons. The largest stars collapse even further and become denser, into objects so dense, that light cannot escape their grasp. These are the infamous black holes. The details of the physics of what occurs in these objects are still up for debate.
8.2 Origin of the Solar System: The Nebular Hypothesis
The origin of a star, of course, is also the origin of a solar system. The nebular hypothesis is the idea that a spinning cloud of dust (called a nebula, with mostly light elements) flattened into a disk, known as the protoplanetary disk, and created a star with orbiting planets in a solar systems . The vast majority of material collects in the center of the spinning nebula which is why the sun accounts for over 99% of the mass in our solar system.
8.2.1 Planet Arrangement and Segregation
As our solar system formed, it developed distinct zones of temperature within a cloud of dispersed particles. Close to the center, the temperatures were very high, allowing only items with very high temperatures to condense such as metals and silicate minerals. Farther from the sun where temperatures were lower, lighter gaseous molecules such as methane, ammonia, carbon dioxide, and water condensed . Thus, the inner four planets are rocky and the outer four planets are gaseous (also known as gas giants).
Both rocky and gas styles of planets have a similar model for growth. Particles of dust, floating in the disc, began to be attracted to each other by static charges, and eventually, gravity. Soon, as clumps of dust get bigger and bigger, they began to interact with each other and collide, sticking together, forming even larger planet-starting blocks. Eventually, over the course of many thousands or millions of years, planets were formed as the material in the protoplanetary disc was attracted to the growing planets. Both rocky and gaseous planets started with a solid core, but rocky planets built on that core with more rock, while gas planets added more gas and ice. Ice giants formed later, got less gas, and therefore, more ice. That is why larger Jupiter and Saturn (gas giants) are mostly hydrogen and helium (>90%), and ice giants Uranus and Neptune are mostly ices of methane, and only about 20% H and He.
It is clear that planetary composition is different, but size is just as dramatically different. The gas giant planets are big for two reasons: First, there is more material in a planetary nebula to form gas and ices than metals and rocks (i.e. lots of H, C, O, N, less Si, Fe, etc.) and therefore the outer planets could get larger overall. Second, as they got larger, their gravitational pull increased, allowing them to collect large quantities of hydrogen and helium which could not be collected by the smaller inner planets’ smaller gravities.
Jupiter’s gravity was able to shape the solar system even more. As material started to coalesce into planets, material close to Jupiter was accelerated, and the collisions here did more destruction than constructive “gluing” together . This is the reason behind the asteroid belt, an “unfinished” planet between Mars and Jupiter. It can also explain Mars’ smaller mass, which Jupiter may have consumed as it migrated into different positions . The asteroid belt is the source of most meteorites that fall to the surface today. We use these planetary building blocks for two primary purposes: to find the age of the Earth and the Earth’s overall composition.
Pluto and planet definition
The outermost part of the solar system is known as the Kuiper belt, which is a scattering of rocky and icy bodies. Beyond that is the Oort cloud, a zone filled with small and dispersed traces of ices. These are the locations where most comets formed and continue to orbit, as objects in both have relatively irregular orbits compared to the rest of the solar system . The former ninth planet, Pluto, is located in this region of space. Why did the XXVIth General Assembly of the International Astronomical Union strip Pluto’s planetary status? In 2005, an object (Eris) more massive than Pluto was discovered, and a decision was made to not include it as a planet, and therefore, exclude Pluto as well. Pluto is currently classified as a dwarf planet. In 2009, this international body of astronomers decided to narrow the definition of a planet to three criteria: 1) enough mass to have gravitational forces that force it to be rounded, 2) not massive enough to create fusion, and 3) large enough to be in a cleared orbit, free of other planetesimals that should have been incorporated at the time the planet formed. Pluto passed the first two parts of the definition, but not the third.
The geological time scale is a way that geoscientists assign relative age names to events and rocks, separating major events in Earth’s history based on significant changes as recorded in the rocks and fossils. The following sections summarize the most notable events during each major time episode. For a breakdown on how these time intervals are chosen and organized, see chapter 7.
This time represents the earliest earth characterized by a partially molten surface, volcanism, and asteroid impacts. It was originally defined as the birth of the planet through 4 billion years ago and initially thought to be a time before many rocks and life forms existed. However, geologists have found some minerals to be as old as 4.4 billion years old with evidence there was liquid water present, , and possibly even evidence of life over 4 billion years ago . However, the most reliable record for early life, the microfossil record, starts at least 3.5 billion years ago .
8.3.1 Origin of the Crust
As the Earth formed, it was incredibly hot from gravitational compression heating (via the Kelvin–Helmholtz mechanism), radioactive decay (which was higher at the start of Earth), and heat from impacts. As an aside, the majority of this initial heat (dubbed Original Heat) still exists on Earth today. The earliest Earth, chiefly molten material, would have been rounded by the force of gravity, and would have resembled a ball of lava floating in space, with high amounts of volcanism. Slowly, the outer part of the Earth cooled and solid slabs of “protocrust” (a crust precursor) from high melting point minerals formed (remember the Bowen’s Reaction Series in Chapter 4). They probably were unstable and reabsorbed easily into the liquid magma until further cooling allowed for larger and more numerous fragments to form thin primitive crust. It is generally assumed that this crust would be oceanic and mafic in composition, and littered with impacts, much like the Moon’s current crust. The exact start of plate tectonics, which presumably would have led to the forming of more felsic (continental) crust, is still under some debate, as is the timing . It is reasonable to think that an early Earth had less dense felsic materials floating to the surface and denser mafic and ultramafic materials sinking, with iron and nickel sinking to the center from its greater density. Starts of convection, separation of the thin crustal slabs by volcanism, and early forms of plate tectonics are still debated. The earth thus differentiated from a homogenous planet into a heterogenous planet with layers of felsic crust, mafic crust, ultramafic mantle, and iron and nickel core, but the details of the modern origins of crust and tectonics is still mysterious.
8.3.2 Origin of the Moon
Earth’s moon has several unique features that have resulted in the current hypothesis for how it formed. First, the Earth and Moon are tidally locked, meaning the moon rotates such that one side of the Moon always faces the Earth, and the other side is “dark” (to us) because Earth cannot see it. Also, and most importantly, the chemical composition of the Earth and the Moon were able to be compared after the Apollo missions returned moon rocks. Very precise analysis of details in Moon rocks and Earth rocks show isotope ratios and volatile content are nearly identical from each body. Other bodies in the solar system and meteorites do not share the same amount of similarity and have a much higher variability. If the Moon and Earth formed together, then it is easy to explain why they are so similar chemically.
Many ideas have been proposed for the origin of the Moon: The Moon could have been captured from another part of the solar system, the Earth and Moon could have formed in place together, or the Moon could have been ripped out of the early Earth. None of these hypotheses explain all of the evidence presented. The “best” hypothesis currently is the Giant Impact Hypothesis . A hypothetical body about half of Earth’s current size must have shared at least parts of Earth’s orbit and collided with it, resulting in a violent mixing of both objects. Material scattered in the collision eventually coalesced into the Moon. The composition of both bodies would have been combined, with more of the splatter material being lower density. This may also explain a higher density Earth (with a thicker core) and a lower density Moon.
Computer simulation of the impact hypothesis for the origin of the Moon (40 seconds).
8.3.3 Earth’s Water
Many different ideas for the origin of Earth’s water have been proposed. One hypothesis is that water originated from inside the Earth itself, via tectonic processes and volcanic outgassing . Since all volcanic eruptions contain some water vapor, at times more than 1% of the volume, this alone could be an explanation of Earth’s water. The other likely source of water is from space. Comets are a mixture of dust and ice, with some or most of that ice being water. Meteors, while seemingly dry, can contain a small but measurable amount of water, usually trapped in mineral structures . During heavy bombardment (see below), the Earth’s cooled surface would have been pummeled by both comets and meteorites, and this could be why so much water is found on the surface. So which source matches ocean water? The answer is not definitive. Isotopically, water on Earth matches water in meteorites much better than comets . However, it is hard to know if processes on Earth could have changed the isotopic signature of water over the last 4+ billion years . Most likely, all three sources contributed to Earth’s water in the past, and continue to add trace quantities of water today.
The Archean eon, from 4 billion years ago to 2.5 billion years ago, is named after the Greek word for beginning, representing the supposed beginning of the rock record. Although now there is evidence of rocks and minerals from the Hadean eon , but the Archean has a much more robust rock and fossil record.
8.4.1 Late Heavy Bombardment
At the start of the solar system, objects were chaotically flying around, building up the planets and moons. Well after the planets were made, about 4.1 to 3.8 billion years ago, there is evidence of a second, later large spike of impacts from asteroids and comets on Earth and the Moon in an event called the Late Heavy Bombardment . At this time, meteorites and comets, which were presumably in stable or semi-stable orbits, began to become unstable and started impacting objects throughout the solar system. This idea is also known as the lunar cataclysm, though this leads to the biggest criticism of the theory: limited lunar sampling. During the Late Heavy Bombardment, Earth, the Moon, and all the planets were pummeled by a large amount of material from the Asteroid and Kuiper Belts, and evidence of this was found within lunar samples returned from the Moon.
While it is universally accepted that bombardment from asteroids and comets would have been extensive at the start of the solar system, some other process must have caused an increase in impacts hundreds of millions of years later. A leading theory blames gravitational resonance between Jupiter and Saturn which disturbed orbits within the asteroid belt and the Kuiper belt . This process has even been hypothesized from observations of the star system Eta Corvi .
8.4.2 Origin of the Continents
In order to have plate tectonics (as the process works today), it is necessary to have continents. However, the easiest way for continental material to be made is using existing continents via assimilation and differentiation (see chapter 4). This becomes a chicken-and-egg situation to know how continents were made in the first place, and the answers are not easy to find because of the great age of continental material and how much evidence has been lost during subsequent tectonics and erosion. Somehow, with a timing and process that is still debated, volcanic action must have brought the first continental material to the surface during the Hadean, 4.4 billion years ago . This does not solve the problem of forming continents, since magmatic differentiation seems to need thicker crust. Nevertheless, it happened somehow by some incremental process . The best idea is that density differences in the materials of the early earth allowed lighter felsic materials to float upwards and heavier ultramafic materials and metallic iron to sink. These density differences led to the layering of the earth that is now detected by seismic studies. Early proto-continents added additional felsic material as plate tectonic processes developed and brought more material from the mantle to near the surface .
The first solid evidence of modern plate tectonics is not found until the end of the Archean, meaning the beginnings of continental lithosphere must have been in place by then. This does not necessarily mean that the starting point of plate tectonics occurred then; it could mean that earlier evidence was erased in the rock cycle. Regardless, there still has to be an explanation of going from little to no continental material in the Hadean, and up to 70% of the volume of the modern continents by the end of the Archean . Overall, Archean rocks are still very rare, and they make up only a small portion of the continental rocks that exist on the surface today. Once present and persistent at the earth’s surface, continents are subject to tectonic activity (now related to plate tectonics), burial by sediments, and erosion. Tectonic processes probably destroyed the earliest continental rocks that formed, and only a few rocks survived. Tectonics could also explain the origin of lighter materials through magmatic differentiation. Or, perhaps plate tectonics had not fully started because oceanic crust was too hot and buoyant to subduct, and something similar to a hot spot with rising material from within the Earth could form continents .
These early continental masses would lead to a craton, the strong, seemingly stable nucleus of each continental mass or fragment (see figure). Cratons have two main parts: the shields which are crystalline basement near the surface, and platforms, which have sedimentary rocks covering the shield. Cratons are mostly Archean in age, and have remained relatively unchanged since then, with most (if not all) tectonic activity occurring around them, instead of within them. Whether plate tectonics or another process started the continents, Archean continents initiated the origin of the Proterozoic continents that now dominate our planet.
The general guideline as to what constitutes a continent, a subcontinent, and differentiates oceanic from continental crust is under some debate. At passive margins, the continental crust grades into oceanic crust, making a distinction difficult . Even island arc and hot spot material can seem more closely related to continental crust than oceanic. Continents and subcontinents are generally cored by one or more cratons, as well as contain felsic igneous rocks. There is evidence of submerged masses like Zealandia, that includes New Zealand, that could be considered continents . A piece of continental crust that does not contain a craton, like Madagascar, would be called a continental fragment .
8.4.3 First Life on Earth
Life mostly likely started early in the Archean eon or the end of the Hadean eon. The earliest evidence, even if not as robust as obvious fossils, is carbon derived from life found within zircon grains from 4.1 billion years ago . The oldest well-preserved fossils are 3.77 to 4.28 billion year old filaments from a hydrothermal vent deposit in Quebec or 3.5 billion year old photosynthetic microbial mats (stromatolites) from Australia . Therefore, although evidence for life older than 3.5 billion years ago is not as strong, there is significant evidence for life starting at least around 3.5 billion years ago. The mechanism for which life started is unknown, though many ideas have been proposed.
One of the ideas is that life could have arisen from the chemical environment of the atmosphere and oceans that was very different than today. The atmosphere was reducing (had no free oxygen) with abundant methane, carbon dioxide, and other sulfur and nitrogen compounds like other bodies in the solar system. The famous Miller-Urey experiment simulated this early Earth’s atmosphere in a vessel and ignited sparks within it to simulate lightning. Amino acids were found to form in the vessel . Amino acids are the fundamental building blocks of proteins. There are many other possibilities that could have led to life on Earth, including in hydrothermal vents like the black smokers on mid-ocean ridges today. In 1977, when scientists discovered an isolated ecosystem around such vents on a mid-ocean ridge , it opened the door to consider other ways for life to exist and have started, and eventually expanded the search for life to more unconventional places, like Jupiter’s icy moon Europa . The oldest possible fossils, which could be as old as 4.28 billion years old, add credence to this idea .
Another possibility for the origin of life on earth is from comets or other objects from space. Some of the chemical components of life (e.g. amino acids) have been shown to exist within comets and meteorites . All of these are intriguing since they imply a high likelihood of life existing elsewhere in the cosmos.
The Proterozoic, meaning “earlier life,” is the eon of time after the Archean eon and ranges from 2.5 billion years old to 541 million years old. During this time, most of the central parts of the continents had formed and the plate tectonic process had started. Photosynthesis (in organisms like stromatolites) had already been adding oxygen slowly to the atmosphere, but it was quickly absorbed in minerals. Evolutionary advancements in multicellular cyanobacteria completely transformed the atmosphere by adding free oxygen gas (O2) and causing the decimation of the anaerobic (non-oxygen) bacteria that existed at the time . This is known as the Great Oxygenation Event. In an oxygenated world, organisms could thrive in ways they could not earlier. Oxygen also changed the chemistry of the planet in significant ways. For example, iron can be carried in solution in a non-oxygenated environment. However, when iron combines with free oxygen, it creates a solid precipitate to make minerals like hematite (iron oxide). This is the reason large deposits of iron known as banded iron formations are common during this time, ending around 2 billion years ago .
The formation of the banded iron lasted a long time and prevented oxygen level from increasing significantly in the oceans since the rocks literally took the oxygen out of the water and formed alternating layers of iron-oxide minerals and red chert. Eventually, as oxygen continued to be made, absorption of oxygen in mineral precipitation leveled off, and dissolved oxygen gas started filling the oceans and eventually bubbling out into the atmosphere. Oxygenation of the atmosphere is the single biggest event which distinguishes the Archean Earth and the Proterozoic Earth . In addition to changing mineral and ocean chemistry, this event is also tabbed as the likely cause of Earth’s first glaciation, the Huron Glaciation that occurred around 2.1 billion years ago . Free oxygen reacted with methane in the atmosphere, turning it into carbon dioxide. Methane is a more effective greenhouse gas than carbon dioxide, and as CO2 increased in the atmosphere, the greenhouse effect actually decreased, thus cooling the planet.
By the Proterozoic eon, lithospheric plates had formed and started moving according to plate tectonic motions similar to today. As these plates formed and started moving, eventually a supercontinent formed from collisions as the ocean basins closed. The exact number of supercontinents during the Proterozoic (or earlier) is unknown, but Rodinia is the best understood. It formed about 1 billion years ago and broke up at the end of the Proterozoic, about 750-600 million years ago . Laurentia, the name for the continental mass that became North America, most likely was in the center of Rodinia. The reconstruction of Rodinia has been accomplished matching and aligning ancient mountain chains to assemble the pieces like a jigsaw puzzle, and paleomagnetics to orient them with respect to magnetic north.
As examples of the complexity of the issue and disagreements among geologists over the reconstructions, there are at least six different models of what broke away from Laurentia in the Panthallasa Ocean (early Pacific), including: Australia , Antarctica , parts of China , the Tarim craton north of the Himalaya , Siberia , or the Kalahari craton of eastern Africa . Regardless of the exact details, it was this breakup that created lots of biologically-favorable shallow water environments that fostered the evolutionary breakthroughs that mark the start of the next eon, the Phanerozoic.
8.5.2 Life Evolves
Early life in the Archean and earlier is poorly documented in the fossil record, but chemical evidence and evolutionary theory states that this life would have been single-celled photosynthetic organisms such as cyanobacteria in stromatolites. Fossil cyanobacteria in these stromatolites produced free oxygen in the atmosphere through photosynthesis. Cyanobacteria are prokaryotes, i.e. single celled organisms (archaea and bacteria) with simple cells that lack a cell nucleus and other organelles.
However, during the Proterozoic a large evolutionary step occurred with the appearance of eukaryotes . Evolving around 2.1-1.6 billion years ago, eukaryotic cells are more complex with cell organelles and a nucleus with more complex DNA replication and regulation, mitochondria for additional energy, and chloroplasts to perform photosynthesis and produce energy. Certain organelles even have their own DNA, like mitochondria. Eukaryotes are the branch of the tree of life that gave rise to fungi, plants, and animals. About 1.2 billion years ago, another important event in Earth’s biological history occurred when some eukaryotes invented sex . By sharing genetic material between reproducing individuals (male and female), evolutionary change was enhanced by increasing genetic variability. This allowed more complexity among individual organisms, and eventually, ecosystems.
It is important to realize that the Proterozoic land surfaces were barren, at least of plants like grasses, trees, and animals. Geologic processes were active just like today, but application of the Uniformity Principle requires realization of differences in the environments in which the processes operate. For example, rain and rivers were present but erosion on barren land surfaces would have operated at different rates than on modern land surfaces protected by plants.
The Ediacaran fauna (635.5-541 million years ago, ) offers the first glimpse at these evolving ecosystems toward the end of the Proterozoic. These organisms were among the first multicellular life forms, and may have been similar to soft jellyfish or worm-like organisms . Since the Ediacaran fauna did not have hard parts like shells, they are not well preserved in Proterozoic rocks. However, studies suggest that they were widespread around the earth . Scientists still debate how many of these are extinct evolutionary dead-ends or the ancestors to modern biological groups . The transition of life from the soft-bodied Ediacaran forms to the explosion of forms with hard parts at the end of the Proterozoic and beginning of the Phanerozoic made a dramatic difference in our ability to understand earth history and the history of life.
The Phanerozoic eon is the most recent eon and represents time in which fossils are common, 541 million years ago to today . The word Phanerozoic means “visible life.” Older rocks, collectively known as the Precambrian (sometimes referred to as the Cryptozoic, meaning “invisible life”), are less common and have only rare fossils, and the fossils that exist represent soft-bodied life forms. The invention of hard parts like claws, scales, shells, and bones made fossils more easily preserved, and thus, easier to find. Since the younger rocks of the Phanerozoic are more common and contain the majority of fossils, study of this eon yields much greater detail. It is further subdivided into three eras: Paleozoic (“ancient life”), Mesozoic (“middle life”), and Cenozoic (“recent life”).
The Paleozoic era was dominated by marine organisms, but by the middle of the era, plants and animals had evolved to live and reproduce on land, including amphibians and reptiles. Fish evolved jaws and fins evolved into limbs. Lungs evolved and life emerged from the sea onto land to become the first four-legged tetrapods, amphibians. Amphibians eventually evolved into reptiles once they developed hard-shelled eggs. From reptiles evolved an early ancestor to mammals . The Carboniferous Period near the end of the Paleozoic, had some of the most productive forests in the history of Earth, and produced the coal that powered the industrial revolution in Europe and the United States. Tectonically, during the early Paleozoic, North America was separated from the other continents until the supercontinent Pangea formed towards the end of the era.
8.6.1 Paleozoic Tectonics and Paleogeography
After the breakup of Rodinia toward the end of the Proterozoic, sea level remained high relative to land in the early Paleozoic. This resulted in much of Laurentia (considered mainly synonymous with North America) being inundated with water over the stable platforms surrounding the craton . While sea level fluctuated during transgressions and regressions after the Ordovician, many of the Paleozoic rocks found in the interior of the United States are marine in origin, due to overall relative high sea level throughout the Paleozoic.
In eastern North America, the assembly of Pangea (sometimes spelled Pangaea) started as early as the Cambrian with a series of events including subduction with island arcs and continental collisions and eventually ocean-basin closures known as the Taconic, Acadian, Caledonian, and Alleghanian (also known as Appalachian) orogenies . The name Pangea, originally coined by Alfred Wegener, means “all land.” Colliding lithospheric plates formed the supercontinent, creating a series of mountain ranges and a broad fold-thrust belt, leaving a large global ocean basin known as the Panthalassa Ocean, with the Tethys Sea being the name of the large “bay” that formed between Laurasia (the northern continents of Laurentia and Eurasia) and Gondwana (the southern continents of India, Australia, Antarctica, and Africa). The eroded remains of the collisional mountains formed on Pangea are still in existence today as the Appalachian, Alleghanian, Scandinavian, Marathon, and Ouachita Mountain ranges . Stress from the Alleghanian orogeny reactivated faulting, produced uplifts, and deformation/folding as far west as the Pennsylvanian-aged Ancestral Rocky Mountains of Colorado .
Animation of plate movement the last 3.3 billion years. Pangea occurs at the 4:40 mark.
Tectonics in western North America during the early part of the Paleozoic was mostly mild, as a long-lived passive margin developed. After the start of the Devonian, the Antler orogeny finally caused faulting and basin development, mostly seen across Nevada today. The Antler belt is most likely a result of an island arc crashing into western North America .
8.6.2 Paleozoic Evolution
The earliest Paleozoic had a significant biological explosion and contains evidence of a wide variety of evolutionary paths, including the evolutionary invention of hard parts like shells, spikes, teeth, and scales. Paleontologists refer to this event as the Cambrian Explosion, named after the first period in the Paleozoic. Scientists debate whether this was a manifestation of a true evolutionary pattern of diversification, better preservation from easier to fossilize creatures, or simply an artifact of a more complete recent rock record. Ediacaran fauna, which lacked easily-fossilized hard parts, may have already been diverse and set the state for the Cambrian Explosion . Regardless, during the Cambrian period, 541-485 million years ago, a large majority of the phyla of modern marine animals appeared . These new organisms had simple cone- or tube-shaped shells that quickly became more complex. Some of these life forms have survived to today, and some were “experimental” whose lineage did not continue past the Cambrian period . Fossil evidence of this time was first discovered by Charles Walcott in a rock layer called the Burgess Shale in western Canada in 1909.
The Burgess Shale is a lagerstätte, or fossil site of exceptional preservation, including impressions of soft body parts. This allowed scientists to learn immense details of the animals that existed at the time, in addition to their tough shells, spikes, and claws. Other lagerstätte sites of similar age in China and Utah have allowed the forming of a fairly detailed picture of what the biodiversity was like in the Cambrian. The biggest mystery is the animals that do not fit existing lineages and are unique to that time. This includes famous fossil creatures like the first compound-eyed trilobites, and many other strange ones, including Wiwaxia, a spiked shell creature; Hallucigenia, a walking worm with spikes; Opabinia, a 5-eyed lobed arthropod with a trunk and a grappling claw at the end; and the related Anomalocaris, the alpha predator of the time, complete with grabbing arms and a deadly circular mouth full of teeth . Most notably at this time, an important ancestor to humans evolved. Pikaia, a segmented worm, is thought to be the earliest ancestor of the Chordata phylum (including vertebrates; animals with backbones ). These astonishing creatures offer a glimpse at evolutionary creativity. At the end of the Cambrian, mollusks, brachiopods, nautiloids, gastropods, graptolites, echinoderms and trilobites had evolved and shared the seafloor.
After the Cambrian Explosion, a similar event occurred which abandoned some of the Cambrian evolutionary animal lines and proliferated others. Known as the Ordovician Radiation or Great Ordovician Biodiversification Event, many common forms and ecosystems recognizable today became common. This includes invertebrates such as mollusks (clams and their relatives), corals, arthropods (insects and their relatives), and vertebrates became more diverse and complex, and dominated the oceans .
The most important of these advancements may have been reef-building organisms. Mostly colonial coral, they took advantage of better ocean chemistry for calcite and built large structures , resembling modern reefs like the Great Barrier Reef off of Australia. Many of the organisms of this time swam around, hid in, or crawled over the reefs. Reefs are so important because of their preservation potential, size (some reef fossils are the size of mountains), and the ability to create an in-place ecosystem in and around them. Few other fossil assemblages in the geologic record can offer more diversity and complexity than reefs. Warm temperatures and high sea level in the Ordovician most likely helped spur this diversification.
A small ice age based on evidence of glacial deposits and associated falling sea level led to a dramatic mass extinction by the end of the Ordovician, the first one documented in the fossil record. A mass extinction is when an unusually large number of species abruptly vanish and go extinct, and this can be observed in the fossil record (see video below). Life bounced back in the Silurian . The major evolutionary event was the development of the forward pair of gill arches into jaws, allowing fish new feeding strategies and opening up new ecological niches.
3-minute video describing mass extinctions and how they are defined.
The Silurian provides the first evidence of land plants and animals . This includes the first ever vascular plant, Cooksonia, with woody tissues, veins for transporting water and food, seeds, and roots . The first bony fish and shark are also Silurian, which includes the first primitive jaws . This also saw the start of armored fish, known as the placoderms. Insects, spiders, scorpions, and crustaceans began to inhabit dry-land and freshwater habitats .
The Devonian, called the age of fishes, saw a rise in plated fish and jawed fish , along with the lobe-finned fish. The lobe-finned fish (relatives of the modern lungfish and coelacanth) are important for their eventual evolution into tetrapods, the four-limbed creatures that went on to dominate land. The first evidence of walking fish, named Tiktaalik (about 375 million years ago), gave rise to amphibians. . Most amphibians live on land, but lay soft eggs in water. They would later evolve into reptiles that lay hard-shelled eggs on land. 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 timing of the event or events. Reef building organisms were the hardest hit, leading to dramatic changes in marine ecosystems .
The next time period, called the Carboniferous (North American geologists have subdivided this into the Mississippian and Pennsylvanian periods), 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 non-aquatic environments for reproduction. This widened the terrestrial reach of reptiles compared to amphibians. This 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, with Pangea assembled, the 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 3 million cubic kilometers that is approximately 4,000,000 times larger than the famous 1980 Mt. St. Helens eruption in Washington. The unusually large volcanic eruption would have contributed a large amount of toxic gases, aerosols, and greenhouse gasses 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 .
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.
8.7.1 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 sea-floor spreading (Chapter 3), 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. Last (happening after the Mesozoic ended) was the northernmost Atlantic, with Greenland and Scandinavia parting ways. 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 as 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 . High sea levels due to shallow subduction, and increasing rates of seafloor spreading and subduction, high temperatures, and melted ice also contributed to the high sea levels . These factors allowed a shallow epicontinental seaway that extended from the Gulf of Mexico to the Arctic Ocean to divide 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 .
8.7.2 Mesozoic Evolution
The Mesozoic era is dominated by reptiles, and 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), in the air (pterosaurs), aquatic (crocodilians) and even fully marine habitats (marine crocodiles). The pterosaurs, the first vertebrate group to take flight, like the dinosaurs and mammals, start small in the Triassic.
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 possibly 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 dinosaurs had was changes in the hip and ankle bones, tucking 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 the giant herbivorous (plant-eating) long-necked sauropods weighing up to 100 tons and bipedal carnivorous theropods, with the possible exception of the Therizinosaurs . 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 Archeopteryx was found in the Solnhofen Lagerstätte of Germany, a seeming dinosaur-bird hybrid, 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 from Jurassic wasp-like ancestors 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 .
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, that only is found at impact sites, was 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 a forest fire , enough debris thrown into the air to significantly cool temperatures afterward , and a 2-km 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 be 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 as 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 .
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, as well, 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.
8.8.1 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 Himalaya, 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 and the density of the plate was therefore 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 5000 cubic kilometer Fish Canyon Tuff .
8.8.2 Cenozoic Evolution
There are five groups of early mammals in the fossil record, based primarily on fossil teeth, the hardest elements in vertebrate skeletons . For the purpose of this text, the most important group are the Eupantotheres, that diverge into the two main groups of mammals, the marsupials (Sinodelphys) and placentals or eutherians (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, ended up larger and having just one toe . Cetaceans (marine mammals like whales and dolphins) started on land from small bear-like (mesonychids) creatures in the early Cenozoic and gradually took to water . However, no study of evolution has been more studied than human evolution. Hominids, the name for human-like 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 non-bipedal evolutionary cousins. There is also a growing body of evidence, including the famous “Lucy” fossil of an Australopithecine, 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 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.
Now that the hands are freed up, the next big step is a large brain. There have been arguments from a switch to more meat eating , cooking with fire , tool use , and even the construct of society itself to 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 its own course.
Anthropocene and Extinction
Humans have had an influence on the Earth, its ecosystems and climate. Yet, human activity can not 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 ch. 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 is over 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 carnivores could not find food, and human arrival times in locations has 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, 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.
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, 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.