Life thrives in a thin strip of real estate situated between a molten furnace and a frozen void. We call it “Earth.” Life clings tenaciously to the surface of our earth. Life’s permanence has no guarantees. At seven points in the long history of our planet, life’s grasp on the earth’s surface faltered. Mass extinction events threaten the survival of the biosphere.
The diameter of our earth is 12,742 kilometers (about 8000 miles). At 4000 miles below earth’s surface lies a planetary core where temperatures reach 6,000 degrees Celsius. These are temperatures that are hotter than the surface of the sun. Above us lies space. The pervasive temperature in deep space is 2.7 Kelvin or -270.45 degrees Celsius. This is the temperature of the cosmic, microwave, background radiation, remnant radiation from the beginning of time and the universe. At zero Kelvin atoms stop moving.
Our piece of real estate seems substantial to us, but in reality, it is thin and fragile. Life exists from the deepest ocean at 10,994 meters (36,070 feet) below sea level in the Mariana Trench, to the highest mountain at 8,848 meters (29,029 feet). Our layer of life reaches a maximum thickness of about 20 kilometers or 0.156 percent of the earth’s diameter. For comparison, an apple’s skin is about 0.4 percent of its diameter.
This thin 20-kilometer thick strip at the surface of the earth is where the geosphere, the atmosphere, and the hydrosphere interact and create conditions for the biosphere of life to either survive or disappear.
The story begins under mysterious circumstances in the dim annals of geologic history some four billion years ago. The time was the Hadean Eon, named after the Greek god of the underworld, Hades. The earth was newly formed from accumulated cosmic debris, and the crust had cooled. A critical component for the development of life pooled in the low areas of the earth’s surface. Studies of zircons from some of the planet’s oldest rocks provide evidence of water on earth 4.4 billion years ago. This was not long after the formation of the planet. Without water, there would be no life.
The when, the how, and the why regarding the origins of life’s fundamental building blocks (DNA) remain unknown, lost in the primordial haze. Perhaps inception was on a deep ocean floor in the extreme pressures and hellish temperatures around a hydrothermal vent. But maybe it occurred in a warm puddle of slime as electric current pulsed through the water from lightning strikes. Some have even argued it fell to earth from outer space (the panspermia theory).
Regardless of its origins, deoxyribonucleic acid (DNA) is the virile seed of life for nearly all living organisms in our biosphere. The genius of DNA is that it embodies the “sine qua non” of existence: a quality that life cannot do without. Its mantra is, “if one of me is good, then two of me is even better.” The trick of self-replication defines life itself.
The magic of life
An eternity passed before DNA could work the magic of life in its simplest form. In rocks formed during the Archean Eon 3.5 billion years ago, are traces of the oldest uncontroversial evidence of life. Fossil evidence from Australia contains the remains of large bioherms that were once home to colonies of cyanobacteria (blue-green algae). During the first billion years of the planet’s history, the seeds of life had managed to evolve into algae. Life was on the move, traveling along the first leg of its evolutionary journey.
Cyanobacteria could not only survive; they could thrive. The world into which these bacteria entered contained a multitude of single-celled organisms living in clusters and mats on the floors of ancient seas. Before cyanobacteria, single
The atmosphere at the beginning of the Archean Eon was anoxic, lacking any significant component of free oxygen. The cyanobacteria thrived under the eye of an ancient sun using their photosynthesis trick to convert free solar energy to chemical energy. Carbon dioxide (CO2) and water (H2O), spewed
For a billion years algae dominated the biosphere. Ever so slowly, molecule by molecule, the algae took CO2 + H2O + sunlight and produced the sugar needed for life. All actions have unintentional consequences, and a byproduct of photosynthesis was the free oxygen (O2) that the algae discarded. This slow leakage of oxygen, first into the oceans and later into the atmosphere, set the stage for life to continue on the next leg of its journey.
The Archean gave way to the Proterozoic Era about 2.5 billion years ago, and during this next era life learned its first lesson: success always comes at a price. Cyanobacteria were the rulers of the earth, and the early Proterozoic oceans were a virtual garden of Eden for them. Stromatolites (cyanobacterial bioherms) were at the top of the evolutionary chain, and they dominated the biosphere. But in the seeds of their success, they crafted their own demise.
The Proterozoic Era lasted for almost two billion years, and at the end of the era, life was forever changed. When oxygen levels in the atmosphere reached a critical mass, life
Acritarchs strode on the stage about 1.8 billion years ago. These were eukaryotes with a more complex cellular structure than cyanobacteria, and an ability to process oxygen for energy production. They stole the photosynthesis trick by incorporating cyanobacteria into their cells. This is the process of endosymbiosis, but in simple terms, the Acritarchs ate the blue-green algae but didn’t fully digest them.
These eukaryotes would eventually evolve into the plants and animals that thrive today. But first, they would have to survive the ice world of the Cryogenian period.
The Oxygen Catastrophe
Success came at a price and for the algae world of the Proterozoic, and that price was oxygen. The first victims were the chemosynthetic bacteria that had dominated the oceans for a billion years. Oxygen gives us life, but oxygen brought them only death. Oxygen was a toxic poison that sent them into the void of extinction.
The second consequence of free oxygen in the atmosphere was to dilute the effects of greenhouse gases that had kept the earth warm for the better part of three billion years. If you change the atmosphere, then you change the hydrosphere. If you change them both, then you threaten life.
The stunning success of a simple bacteria species triggered devastating climate change and ushered in the Huronian glaciation. For over 100 million years the earth existed in a deep freeze where being warm meant -20 degrees Celsius. Much of life disappeared in the first of the great mass extinction events. The lesson is stark. Species can be victims of their own success.
Life is fragile but tenacious. While much of the biosphere ceased to exist, life clung to those small enclaves where survival was a possibility. Perhaps they survived beside volcanic vents on the seafloor, or at hot springs over magma intrusions into the crust. The answer is unknown, but life did survive.
When the ice retreated at the end of the Cryogenian period, and warmth returned to the surface of the earth, life accelerated down unknown pathways. In the waning 80 million years of the Proterozoic (Ediacaran Period), the ocean floors became home to a multitude of soft-bodied metazoans, multi-celled animals with specialized cell types.
The Cambrian Period followed the Proterozoic, and the biosphere exploded into a diversity of complex life forms never seen before. The Cambrian explosion happened approximately 541 million years ago. For almost four billion years the best the biosphere could do was bacteria. Then suddenly at 541 Ma most of the major animal phyla we know of today crashed the party. Why then? Where did they come from? The answers
The rapidity with which complex life evolved during this period is spooky. In mere millions of years (not billions or even hundreds of millions) life embarked on the third leg of its journey. The hyper-evolution that occurred during this time represents the single most important event in the history of the biosphere. It also establishes a pattern. Life has a gift for evolving to fill ecological niches. Extinction of a species leaves an ecological void that evolution will seek to fill. Is species death a requirement for continued evolution? This is a question pertinent to modern humanity.
Ordovician Silurian Mass Extinction
A 100 million years after the Cambrian Explosion, life came under threat again. At the end of the Ordovician Period, 85 percent of all existing species disappeared. It would not be until the ensuing Silurian age that life would colonize the land, so when catastrophe descended, it was life in the oceans that took a beating.
About 445 million years ago earth’s temperature started to drop. Large continental land masses were concentrated in the southern hemisphere, with the proto South American and African continents being near the south pole. The earth again entered into an ice age and glaciation on the southern continents locked up massive amounts of water causing sea levels to drop. Dropping temperatures and falling sea levels took their toll.
In the Ordovician, many of the biosphere’s species existed in vast shallow seas within and at the edge of the continents, and many of these were endemic species (confined to a particular location and not spread worldwide). The cold climate and falling sea levels destroyed the habitats of these endemic species as the shallow seas dried up.
Despite the loss of so many species, this extinction event did not result in
Devonian Oxygen Crisis
The Devonian mass extinction event is really a series of up to ten events over a 25 million year period. The understanding of how this crisis developed lies in the Silurian Period that preceded it. The Silurian Period saw a rise in global temperatures along with rising sea levels. Life thrived in the warm oceans, and the Silurian also marked the “age of plants.” Evolution took a huge step forward as plants took root on the land, covering the continents with green life.
The algae eating eukaryotes, the Acritarchs, that had appeared a billion years before had evolved, but they had retained the magic of photosynthesis and passed it on to the plants. Sunlight, water, carbon dioxide, and nutrients drove the plants to adapt, survive and populate the world of dry land.
The oceans were thriving with life too. Rising sea levels had allowed vast, shallow seas to return, providing marine habitats for a wide variety of species. The success of the plants laid the groundwork for the Devonian Oxygen Crisis.
Free Oxygen again became the catalyst for change. Not because there was too much, but because there was too little. The geologic record recognizes the widespread occurrence of black shales throughout Devonian strata. Black shales are one of the more prolific sources of hydrocarbons. These shales are rich in organic content preserved in the rock. Normally aerobic bacteria will devour the nutrients and food sources that are shed from the land and deposited on the ocean floors. If this organic matter is not being devoured, it implies that the bacteria are not present, but the bacteria are always there unless there is no oxygen.
There are places today like the Black Sea where aquatic life only occurs in the thin upper layers of the water column. As organic material sinks below the top oxygenated layer, it passes into anoxic waters where no bacteria are present to consume it. This material accumulates on the seafloor in layers of rich, organic ooze.
The success of plants in colonizing the dry land had consequences. One of the consequences was an increasing supply of organic matter and nutrients being shed into the surrounding waterways. For shallow seas with restricted circulation, this means death. Massive algal blooms were created by the nutrient-rich waters entering these seas, and the sheer amount of algae overwhelmed the system as oxygen was used up faster than it could be produced. Without oxygen, sea life died off, and the seafloor became a barren wasteland.
Eventually, the loss of carbon from the atmosphere added insult to injury and brought on an ice age at the end of this mass extinction event. This was another nail in the coffin for the biosphere. About 80 percent of all species disappeared during this crisis. It’s a shockingly high number, but only a harbinger of what was to come.
Permian Extinction: The Great Dying
Ninety-five percent of the existing marine species and seventy percent of the terrestrial species on earth vanished from existence at the end of the Permian, 250 million years ago. This was without question the most severe of all mass extinction events to threaten
Located in the northern regions of Siberia are the remnants of a massive outpouring of flood basalts called the Siberian Traps. The volcanic events that created these flood basalts are different than the more well known volcanic mountain building we see today. Flood basalts occur when the crust of the earth splits open along massive fissures and the molten core of an underlying mantle plume (hot spot), disgorges onto the surface of the earth. Literal ‘rivers of lava’ flow like water across the earth’s surface, filling in valleys with lakes of molting rock and traversing the topography like a flood from hell. Individual flows can travel hundreds of miles before they freeze into solid rock.
These flood basalts covered an area estimated at seven million square kilometers (2.7 million square miles). For two million years liquid rock poured from the earth in flood after flood. When the devastation finally stopped, one million cubic miles of rock had been pushed onto the surface of the earth. For life on earth though, it was not the basalt that posed a threat, it was the degassing of the mantle plume that led the assault.
Sulfur dioxides and nitrogen oxides from the volcanic outpouring reacted with water, oxygen and other chemicals in the atmosphere to produce acid rains. Sulfuric and nitric acids fell from the sky for a million years, burning away life on earth.
Extinction by suffocation
If that wasn’t enough, there is mounting evidence that the Siberian volcanism coincided with an oceanic catastrophe. This was the time of Pangea where all of the landmasses had amalgamated into a single, super landmass. Geological evidence indicates that deep ocean currents were disrupted, and the world’s oceans became a giant version of the Black Sea. The deep oceans turned anoxic, killing off life below the surface waters.
Eventually, climatic conditions from the volcanism caused a massive aquatic overturn where the deep waters of the oceans rose to the surface. Anoxic waters flooded into the shallow, continental seas. This was a sentence of death for the remaining aquatic life. Modern analogs of large overturned bodies of water demonstrate that the event would have released massive clouds of carbon dioxide into the atmosphere. As these clouds of carbon dioxide flowed over the land, they would have suffocated all oxygen-breathing life in their path.
The assault was brutal and the patches of life that survived the cold, acid, anoxia, and carbon dioxide fog, then dealt with rapid heating once the aerosols had cleared from the sky. Greenhouse gases from the eruptions lingered in the atmosphere, heating the earth, and rapidly pushing life from one extreme to another.
Again, life survived and limped into a new period, and again life learned the principle that near death from mass extinction events leads to dramatic change. The time for amphibians and dinosaurs to dominate the top of the food chain had come.
End Triassic Extinction
Compared to the intervening time between previous mass extinctions, life was quickly under attack again at the end of the Triassic, only 50 million years after the Great Dying. This time the biosphere lost 76 percent of its marine and terrestrial species and 20 percent of the existing taxonomic families.
There is a general consensus that climate change was responsible, but the root causes of that change are still under debate. At the end of the Triassic, the Pangea supercontinent was fracturing. Africa was separating from South America. The birth of the Atlantic Ocean was in progress and with that came the formation of the Central Atlantic Magmatic Province (CAMP).
As the earth tore apart, an area of 11 million square kilometers (4.2 million square miles) was affected. Flood basalts inundated the earth’s surface, and massive, intrusive deposits of magma formed. The intrusive magmas reached the near to surface but froze in place before they could
Similar to the Great Dying, the gases from this volcanic activity wreaked havoc on the biosphere. Rising temperature, rising sea levels, and acidification of the oceans took their toll on the biosphere. Evidence indicates that atmospheric heating, related to the volcanic degassing, was a dominant factor in the fifth of the great mass extinction events.
A twist to this event, not yet explored in the other extinction stories, was a cause and effect amplification of the greenhouse problem. Carbon dioxide released into the atmosphere from the CAMP volcanism caused a temperature rise, and the increased temperatures triggered the melting of methane gas hydrates. These hydrates reside on land in arctic permafrost, and they are also found trapped on the ocean floors.
Just as carbon dioxide can be turned into dry ice under the right conditions, methane and crystalline water turn into an icy solid under the correct pressure and temperature conditions. If temperature or pressure conditions change, then the hydrates change from a solid to a gaseous state, a process called sublimation. The released methane then enters the atmosphere. Methane is a more potent greenhouse gas than carbon dioxide so this type of feedback could have driven a run-away temperature rise.
But life survived and rebounded. The agents of destruction took a break and let the biosphere thrive for another 135 million years before the next test came.
End of the Dinosaurs: The K-T Extinction
Society’s romance with dinosaurs makes this end-of-the-Cretaceous event the most well-known of all extinction events. The term K-T denotes the geological boundary between the Cretaceous Period and the subsequent Tertiary Period. School children and adults alike are enthralled with the story of a colossal asteroid striking the earth and bringing the biosphere to its knees. The mighty dinosaurs disappeared, relegated to the fossil record, and the age of the mammals ushered in. The carnage was substantial with 75 percent of all plant and animal species disappearing. However, the
The asteroid struck at the ancient location of the Mexican, Yucatán peninsula. The crater left by the strike is still measurable today at 150 kilometers (93 miles) in width and 20 kilometers (12 miles) in depth. Fifteen billion tons of fine-grained rock, soot and aerosols were ejected into the atmosphere. Sunlight was blocked out, and the earth plunged into darkness for years. The impact triggered earthquakes, volcanism, and tsunamis. Today, in the geological record, a clay layer rich in the metal iridium exists around the world at the Cretaceous-Tertiary boundary. Iridium is a metal that is rare in the earth’s crust but prominent in asteroids.
A less known contributor to this event was the formation of the Deccan Traps in India. Flood basalts from the Deccan Traps started flowing 66 million years ago and continued erupting for 30,000 years. These eruptions resulted in the extrusion of about 600,000 cubic miles of liquid rock onto the earth’s surface. It was, of course, the volcanic gases that took their toll on the environment.
The short-term effects of the asteroid combined with the longer-term degassing of the Deccan Traps again sought to overwhelm life on earth. But, life survived. The dinosaurs departed and new species arose to fill the vacant ecological niches.
The Anthropocene Extinction: An Unfolding Catastrophe?
Six major mass extinction events have been addressed. Avid researchers will note that only five mass extinctions are usually noted in the literature. The Cryogenian Oxygen Catastrophe is generally not included since it occurred before the Cambrian Explosion of life. However, this first big extinction event defined two underlying principles of evolution. The unchallenged success of a species can create the seeds for its own demise. Secondly, extinction events always produce an ecological void. A void that the biosphere will seek to fill with new and evolving forms of life.
This lays the groundwork for the present day where a single species has achieved unbridled dominance over the biosphere. The Anthropocene is the age of human-kind. The rapidity with which Homo sapiens have ascended to dominance is unparalleled in geological history. The change from being a mere part of the animal kingdom to total dominance has been in thousands of years, not millions of years.
The modern climate change debate is a political football and denial runs high. However, if bacteria can send the world into crisis, then creatures with intelligence can certainly do it much faster. Past extinction events have been created via biological activity, geologic activity, and cosmic activity. These events play out in three ways. They can deprive life of essential resources like oxygen or sunlight, produce substances like acid rain that are toxic to life, or they evoke rapid climate change that overwhelms the ability of life to adapt.
Human activity is currently fulfilling two of these conditions. Greenhouse gases produced by human activity are building a rapid climatic warming trend. As Arctic terrains warm, methane hydrates in the permafrost will become unstable and release even more methane into the air. This will exacerbate the warming trend. The second condition we are fulfilling is the acidification of the oceans. Carbon dioxide, sulfur dioxide, and nitrogen oxide, thrown off as byproducts of our
Life is tenacious but not invincible. The rate of change is important. Historically species have disappeared at a rate of about 0.02 percent every 100 years. Five hundred species have gone extinct in the past 100 years. This rate is 50 times greater than the background level.
Twenty-five percent of the carbon dioxide released into the atmosphere ends up being dissolved in the oceans. This lowers the pH of the water and is thus referred to as acidification. Since the beginning of the industrial revolution, there has been a 30 percent increase in ocean acidity.
This is why the rate of change is important. One of the key factors in many extinctions was not simply change, but rapid change. If a system changes too fast, then it outstrips the ability of the evolutionary process to adapt. It is only with great hubris that humans can assume this four-billion-year-old principle somehow does not apply to the species Homo sapiens.
The Archean Eon and the Hadean http://www.ucmp.berkeley.edu/precambrian/archean_hadean.php Also:
Proterozoic Era http://www.ucmp.berkeley.edu/precambrian/proterolife.html Also:
Proterozoic Earth – The First Animals https://earthlyuniverse.com/proterozoic-earth-first-animals/ Also:
Earth was a frozen Snowball when animals first evolved http://www.bbc.com/earth/story/20150112-did-snowball-earth-make-animals Also:
First Land Plants and Fungi Changed Earth’s Climate, Paving the Way for Explosive Evolution of Land Animals, New Gene Study Suggests https://science.psu.edu/news-and-events/2001-news/Hedges8-2001.htm Also:
Ordovician-Silurian Mass Extinction: Causes, Evidence & Species https://study.com/academy/lesson/ordovician-silurian-mass-extinction-causes-evidence-species.html Also:
Late Devonian Mass Extinctions http://www.devoniantimes.org/opportunity/massExtinction.html Also:
The Siberian Traps https://www.le.ac.uk/gl/ads/SiberianTraps/GeolSett.html Also:
Extinction http://www.ucmp.berkeley.edu/education/events/cowen2a.html Also:
Amphibians and dinosaurs were the new large predators after the mass extinction https://www.sciencedaily.com/releases/2014/03/140320100822.htm Also:
Evolution: The beneficiaries of mass extinction https://phys.org/news/2017-10-evolution-beneficiaries-mass-extinction.html Also:
End-Triassic extinction https://www.britannica.com/science/end-Triassic-extinction Also:
Humans: Cause Of Extinction Of Nearly 500 Species Since 1900https://www.techtimes.com/articles/64542/20150630/humans-cause-of-extinction-of-nearly-500-species-since-1900.htm Also:
Feature Image: Hadean (Tim Bertelink) – This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license. – Modified Version