The Resilient Earth: Science, Global Warming and the Fate of Humanity

by Simmons, Allen

The ozone layer consists of a particular molecular form of oxygen, O3, that is created when ultraviolet (UV) light strikes free oxygen in the atmosphere. By absorbing UV light, the ozone layer protects life from cellular damage. Photosynthesis, the process that emits oxygen making the ozone layer possible, depends on CO2. Labeling carbon dioxide a pollutant is an error—CO2 is essential to life on Earth. So, stealing a phrase from the movie Men In Black, we owe our existence today to “barely evolved pond scum.”

  These developments are the first examples of some common themes that reoccur throughout the history of life on Earth. Life evolves and diversifies into new forms, often with significant impact on the environment. Then, some dramatic event occurs, killing off a significant number of living species. After such extinction events, life struggles back, taking new forms. These new forms expand into ecological niches previously occupied by the unfortunate species that died out; a process called adaptive radiation by biologists.

  With the dramatic changes in Earth's atmosphere, brought about by the ancient prokaryotes, the evolution of life quickened. About 1.6 billion years ago, during the Mesoproterozoic era, new, larger and more complex forms of life began to appear. These newcomers, called eukaryotes, also started as single-cell organisms.

  Eukaryotes differ from their more primitive prokaryotic cousins by having a nucleus, a sort of cell within a cell, to contain their genetic material. As mentioned earlier, Karyose comes from a Greek word, which means “kernel” or “nucleus.” The prefix eu means “true” so, as prokaryotic means “before a nucleus,” eukaryotic means “having a true nucleus”. These differences are shown in Illustration 18.

  Illustration 18 Prokaryotic and Eukaryotic cells.

  The new eukaryotic organisms were much larger than their more primitive cousins, so large that it is believed they surrounded and absorbed some species of the smaller prokaryotes.102 These absorbed cells became part of the larger organism, often maintaining their own genetic code separate from the host cell's DNA. In this way, it is believed that early plants developed by capturing cyanobacteria, enabling them to reap the benefits of photosynthesis.

  Animal cells are also a type of eukaryotic cell, though much evolved from single-cell organisms. Human mitochondria are examples of organelles, miniature organs providing specialized services to the cells that contain them while maintaining their own DNA.

  The Phanerozoic Eon

  The next major milestone for life on Earth occurred at the beginning of the Phanerozoic Eon with the Cambrian Explosion. This event, with new multicellular organisms popping up in great profusion, resulted in an explosion of life. It marked the end of the Proterozoic Eon and the beginning of the Phanerozoic, Greek for “visible life.” This eon signals the rise of truly complex life, where individual organisms are large enough to be recognized without a microscope.

  From this time forward, the pace of development really speeds up, at least when time is measured using geologic time scales. Though time spans at this point are still hundreds of millions of years long, it is prudent to examine subsequent events with a finer grained time scale. To do this we need to change from observing Eons to observing Eras and Periods of geologic time.

  Table 5 Geological Eras and Periods of the Phanerozoic Eon. Source ICS103 .

  Table 5 shows the three Eras that make up the Phanerozoic Eon; the Paleozoic, the Mesozoic, and the Cenozoic. The root word zoic comes from the Greek word zoion and means “animals.” The prefixes paleo, meso, and ceno mean “old, middle, and new,” respectively. That makes the three eras of the Phanerozoic the times of “old animals, middle animals, and new animals.”

  Different geologic time periods are marked by significant changes in the types of creatures living on Earth. The rock deposited during the Phanerozoic Eon contains evidence of fossilized hard body parts from living things and it is this fossil record that is used to date rock layers from the three eras. By reading the fossil record, scientists have constructed an outline of the development of life during the time following the Cambrian Explosion.

  The Paleozoic Era

  The Paleozoic spanned from roughly 542 mya to 251 mya, and is subdivided into six geologic Periods. From oldest to youngest these are: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The Paleozoic covers the time from the first appearance of abundant, hard-shelled fossils to the time when large reptiles and modern plants began their domination of life on land. From the great explosion of life in the Cambrian, until the catastrophic, worldwide extinction at the end of the Permian, life struggled to evolve. Starting in the oceans, life at the beginning of the era was confined to bacteria, algae, sponges and a few other types of early multicellular animals. With the colonization of the land, many new life-forms emerged, some with relatives that survive to this day. Many others went extinct, disappearing from Earth forever.

  The poster child for this early part of the Paleozoic is the Trilobite, a primitive type of arthropod that thrived during the Cambrian and Ordovician. Arthropods are members of the largest Phylum of animals, a classification that contains modern insects, spiders and crustaceans. Trilobites were an extremely successful family of animals, with an estimated 10 to 15 thousand different types living over a period of 300 million years.104 It is thought that their numbers began to decrease with the arrival of the first sharks and other early fish in the Silurian and Devonian periods.

  Illustration 19 An ancient Trilobite.

  Trilobites went into sharp decline when, during the Late Devonian Extinction, all trilobite orders except one died out. The last of the trilobites disappeared in the Permian-Triassic Extinction, 251 million years ago. The closest living relatives of trilobites might be horseshoe crabs,105 that are themselves throwbacks to a more primitive time.

  There is some evidence that simple life-forms had already invaded the land by the start of the Paleozoic, but large plants and animals did not gain a firm hold on land until the Silurian, 444–416 mya. Although primitive vertebrates (animals with backbones) are known to have existed near the start of the Paleozoic, animal forms were dominated by invertebrates (animals without backbones) until well into the middle of the era. Life on land did not really start to thrive until the Devonian, 416–359 mya.

  The Devonian is named after Devon, a county in southwestern England, where rocks from this Period were first identified and studied. During the Devonian, fish populations exploded and life continued to colonize the land. Early bony fish grew legs and learned to walk. Insects spread across the continents along with seed-bearing plants.

  In 2005, paleontologists working in upstate New York discovered the fossilized remains of the oldest known trees. These plants would look strange, almost alien, next to a modern oak or spruce tree. They had 19-30 ft (6-9 m) tall trunks with pronounced fibrous strands running from bottom to top, like giant stalks of celery. At the top, there were no limbs with leaves or needles. Instead, there were a number of bifurcating branches ending in structures resembling bottle brushes. The branches were about 3 ft (1 m) long, which were shed as the tree grew taller. This forest is thought to date from 385 mya.106

  Illustration 20 An artist's view of the Devonian.

  The continents of the time would not have been recognizable to us today; in the south was the supercontinent of Gondwana, to its north, Euramerica, with Siberia forming a smaller continent of its own even farther north. All of these land masses were in the process of a slow-motion collision that would form a single super continent, Pangaea, by the end of the era. The movement of the continents will be presented in greater detail in Chapter 8, Moving Continents & Ocean Currents.

  In the late Paleozoic Era, during the Carboniferous Period, great forests of primitive plants thrived on land, forming extensive peat-swamps. These huge masses of plant matter were buried with sediment, eventually forming the great coal deposits found in North America, Europe and around the world. A global drop in sea level at the end of the Devonian reversed early in the Carboniferous, creating larg
e shallow seas and huge deposits of carbonate minerals. These deposits trapped large quantities of atmospheric carbon that would later form vast beds of limestone.107

  Illustration 21 A Carboniferous coal forest.

  During the later part of the Carboniferous, the amount of oxygen in Earth's atmosphere was about 35%, nearly double what it is today. At the same time, global CO2 went below 300 parts per million—a level which is now associated with glacial periods.108 The abundance of O2 led to the existence of the largest insects ever seen on Earth. Hawk-sized dragonflies, with 29 inch (75 cm) wing spans, spiders the size of house plants, 5 foot (1.5 m) long centipedes and soup bowl-sized crawling bugs.109 It was truly a time when insects ruled the planet. Perhaps it's a good thing the atmospheric oxygen level is only 21% today.

  Carboniferous plants resembled the plants that live in tropical and mildly temperate areas today. From fossils, we know that many of them lacked growth rings, suggesting a uniform climate. But the climate was changing. By the middle of the Carboniferous, Earth was sliding into an Ice Age, the Permo-Carboniferous. The growth of large ice sheets at the southern pole locked up large amounts of water as ice. Because so much water was taken out of the environment, sea levels dropped, leading to a mass extinction of shallow marine invertebrates, the gradual decline of the swamps, and an increase in dry land.110

  Many times, these conditions were reversed when the glaciers receded. Glacial melt water was released back into the oceans, and again flooded the swamps and low-lying plains.111 Carboniferous rock formations often occur as a pattern of stripes, with alternating shale and coal seams indicating the cyclic flooding and drying of the land. Even under these stressful conditions, or perhaps because of them, life continued to develop. By the end of the era, the first large reptiles and the first modern plants, ancestors of today's conifers, had appeared.112

  At the end of the Permian, life had just weathered the extreme cold temperatures and glaciation of the Permo-Carboniferous Ice Age. Most of the world's dry land was still concentrated in a single super continent, Pangaea, meaning “all lands” in Greek. But Pangaea was starting to break up, eventually forming continents we are familiar with today. Events seemed to be progressing when disaster struck. Approximately 251 mya a sudden event, the nature of which is still a topic of hot debate among scientists, caused 95% of all life on Earth to become extinct.113

  We still don't know the cause of the Permian-Triassic Extinction, or “Great Dying,” but we do know that it was of cataclysmic proportions.114 Was it a meteor? A sudden release of methane from the oceans, or the outbreak of an intense period of volcanic eruptions? The possible causes of this worst of all mass extinctions will be discussed in Chapter 6. Continuing with the history of Earth, the impact this extinction had on the development of life will be examined next.

  The Mesozoic Era

  The Mesozoic Era spans the time from 251 mya to 65 mya and has three geologic Periods: the Triassic, the Jurassic, and the Cretaceous. It is the era that most people think of when someone mentions ancient life on Earth, the time of the dinosaurs. But things didn't start out with giant beasts roaming the plains in scenes reminiscent of Steven Spielberg's “Jurassic Park” movies.

  Life, at the beginning of the Mesozoic Era, must have been very hard. The great Permian-Triassic Extinction had set life back to a state not seen since the early Cambrian. Stromatolites were once again the main builders of reefs in the oceans and life on land was greatly diminished. At least nine of every ten species in the oceans went extinct. This extinction, now thought to consist of two separate events occurring about 10 million years apart, was as disastrous as the next two largest extinctions combined.115

  Illustration 22 Life in the Jurassic.

  Given the unusual severity of the Permian-Triassic Extinction, it might seem reasonable that recovery would be slow—it was. The fossil record from this time shows a period of five to ten million years when life was greatly impoverished, with diversity levels lower than any time since the Cambrian. But slowly life rebounded with the appearance of new and more dynamic life-forms. During this time, the first primitive mammals made their debut along with a new form of reptilian life destined to rule Earth for the next 185 million years. By the end of the Triassic, the first dinosaurs had evolved and the stage was set for the coming Jurassic Period.

  The Triassic was also a time of great change in the terrestrial vegetation. The forests were dominated by giant ferns, cycads, ginkgophytes, and other unusual plants. Modern trees, such as conifers, first appeared in their current recognizable forms in the early Triassic. The Triassic Period closed with an extinction event that primarily affected marine life, including most marine reptiles.

  By the Jurassic, 200-146 mya, the Age of Reptiles was in full swing. Dinosaurs proliferated, dominating life on land and in the seas. Sauropods, immense plant-eating dinosaurs, were ubiquitous and were preyed on by large flesh-eaters, including Allosaurs (the wolves of the Jurassic) and Megalosaurs (great lizard). Pterosaurs (winged lizard), also known as Pterodactyls (wing finger), flew in the Jurassic skies. During the later Jurassic, the first true birds appeared, as did more modern mammals.

  Illustration 23 Archaeopteryx, an early bird ancestor. Source Berlin Museum.

  The entire Mesozoic Era is thought of as being warm and arid, but there are some conflicting signs. Coral reefs did not have an extended range beyond what they have today, an indication that the climate was more temperate. On the other hand, ferns, which do not thrive in cold climates, are found at higher latitudes than today, a sign of warmer temperatures. One thing is certain, however, there is no evidence of glacial conditions during most of the era. Earth had no polar ice caps.

  Geological evidence indicates that the warmth of the Triassic and Jurassic Periods continued into the Cretaceous, 146–65 mya. Global sea levels rose during the Jurassic and Cretaceous, possibly because of increased sea-floor spreading as Pangaea began breaking up.116 The elevated sea levels created an equatorial seaway, called the Tethys Sea, that circled the globe bringing warmth and moisture to low latitude regions. There is also evidence of warmer temperatures in higher latitudes during the mid-Cretaceous.117 Earth was a warmer planet by 11°F to 18°F (6°C to 10°C). CO2 levels were between two and eight times higher than preindustrial values.118

  The Cretaceous Period brings the height of the dinosaurs' rule on Earth, the first flowering plants, and the oldest known ants. The land was covered with primitive coniferous forests and surrounded by shallow seas. Seasonal variation increased causing mass animal migrations. Several of the more exciting dinosaur species, including the ferocious Velociraptors and Tyrannosaurus Rex portrayed in the movie “Jurassic Park,” were actually residents of the Cretaceous. But somehow “Cretaceous Park” doesn't have the same Hollywood zing.

  Illustration 24: The Chicxulub asteroid hitting Earth 65 mya. Source NASA.

  Finally, the Cretaceous Period, and the entire Mesozoic Era, came to a sudden end with the second-most extensive mass extinction in the history of Earth. Scientists had long speculated about what caused the dinosaurs to die out. Then, in 1980, a research team led by Nobel-prize-winning physicist Luis Alvarez, geologist Walter Alvarez (Luis' son) and chemists Frank Asaro and Helen Michels discovered that sediment layers found worldwide at the Cretaceous-Tertiary (KT) boundary contained high concentrations of the rare metal iridium. This concentration, hundreds of times greater than normally found on Earth, led them to conclude that an asteroid, colliding with Earth, caused the end-Cretaceous extinction.119

  These researchers also found many small droplets of basalt rock, called spherules, in the KT boundary layer, evidence that rock from Earth's crust had been melted and flung into the air by a violent impact. Add the presence of shocked quartz, tiny grains of quartz that form in high pressure impacts, to the materials found in the boundary layer, and the evidence seemed clear—an asteroid killed the dinosaurs.120 Recent research suggests that the impact site may have been in the Yucatan Peninsula of Mex
ico.121 This led scientists to name the collision the Chicxulub event, after an ancient Mayan village found close by.

  There is still some controversy surrounding the Alvarez Theory. Competing theories put forth other explanations for the demise of the dinosaurs, but no matter what the cause, the Age of Reptiles had come to an end. The Age of Mammals was about to begin.

  The Cenozoic Era

  The Cenozoic Era began 65 million years ago with the extinction of the dinosaurs and continues into the present. The extinction of the dinosaurs at the end of the Mesozoic Era allowed early mammals and birds to adapt, occupying new habitats and ecological niches. It was as though life on Earth had started over again—for the third time.

  Before examining the Cenozoic Era, our time-scale needs to be expanded again. We will be breaking Periods into smaller time spans called Epochs. The Cenozoic is broken into two Periods, the Paleogene and the Neogene, which have three and four Epochs, respectively. The names of these divisions and their dates are shown in Table 6. Note how, as the time periods approach the present day, the time spanned by each Epoch becomes smaller.

  The Period names come from the Greek; gene means “born,” and the prefixes paleo and neo, meaning “ancient” and “new.” So Paleogene and Neogene mean “ancient-born” and “new-born.” The Cenozoic Era is sometimes partitioned into different sub-periods, the Tertiary and Quaternary Periods, using an older naming scheme. The Tertiary spanned all of the Cenozoic, except the time of the Holocene Epoch, which was covered by the Quaternary.

  Table 6 Periods and Epochs of the Cenozoic Era. Source ICS.122

  This naming scheme has recently been abandoned by the International Commission on Stratigraphy (ICS), but the older Period names still appear in scientific papers. We mention this because the end-Cretaceous extinction event is often referred to as the KT extinction. The “K” is for Cretaceous, used to avoid confusion with the earlier Cambrian and Carboniferous Eras, and the “T” is for Tertiary.