Mass Extinctions: _{Major turning points in biodiversity}

Defining mass extinction

They seem to me to prove the existence of a world previous to ours, destroyed by some kind of catastrophe.

“Georges” Cuvier French Natural Scientist, 1796

For as long as there has been life on Earth, species have constantly been going extinct. Therefore, a “mass extinction” is defined as a sudden rate of species loss that is much higher than the usual constant rate. Generally, paleontologists define mass extinctions as events where at least three-quarters of all living species go extinct within a short period of time.

The unit that is often used for extinct rates is extinctions per million species per year (E/MSY). For reference, the typical background rate of extinctions is between 0.1 and 1 extinction per million species per year. During Mass extinction events, the global extinction rate can be ten or even one hundred times higher than that.

When looking at extinction rates in the deep past, scientists tend to refer to the extinction of families of species, rather than individual species, because our knowledge of each species that lived millions of years ago is very incomplete, while our knowledge of families is more reliable (see our Taxonomy I module) . Figure 2 graphs the rate of extinctions of families over the last 500 million years. You can see that extinctions are always occurring, punctuated by some particularly large extinction events.

To understand these numbers better, we can return to our analogy of books in a library. The Library of Congress has approximately 51 million cataloged books. If this library had the same “extinction rate” as species on earth, it would lose between 0.1 and 1 book per million per year, so this would be between 5 and 51 books lost each year. After all, books do wear out, fall apart, get lost, etc.

That doesn’t sound too bad, since the library regularly acquires new books. On the other hand, some books may be extremely rare and could be lost forever, a true extinction of a book.

In contrast, during a catastrophic event, such as a roof leak, the Library of Congress might permanently lose 500 of its books in a year, or maybe 5,000, or even 50,000 if an entire wing of the library burns down. This is a more significant loss and there is a much greater chance that a rare or unique book will be lost forever. An event like this is like a mass extinction of species, where the collection is suddenly and permanently changed. During mass extinctions in earth’s past, entire groups of organisms have gone extinct at once.

What causes mass extinctions of species on Earth? This remains one of the most provocative questions in the study of life.

Tracking mass extinctions

When was the first mass extinction?

“ The initial increase of O2 in the atmosphere, its delayed build-up in the ocean, its increase to near-modern levels in the sea and air two billion years later, and its cause-and-effect relationship with life are among the most compelling stories in Earth's history.”

(Timothy Lyons et al., 2014)

The mass extinctions shown in Figure 2 were discovered by looking at the fossil record. However, the further back we go in time, the fewer fossil representatives we have. Because of the constant movement of the tectonic plates of our Earth’s crust (see our Origins of Plate Tectonic Theory module), landscapes change, lakes dry up, mountains rise, erosion continues, and sediment piles up. The result is that the record we have of the past slowly gets erased.

In addition, for the first 2.5 billion years since life first emerged, the only life was microscopic and aquatic, with the vast majority leaving no fossil traces whatsoever. The history and diversity of life during that period are shrouded in almost complete mystery, but there is still a great deal that we can deduce from the geological record - the rocks and mineral deposits around the world, which includes detailed evidence of the chemistry of Earth throughout its history. By studying how the chemistry of Earth changed over time, we can learn something about the living things that were active at various times.

For example, we can deduce that there was a massive extinction when the first living cells were just getting started. The very early atmosphere was made of carbon dioxide, methane, and water vapor, a combination of gases that would kill most of the life that exists on Earth today. But, about 2.33 billion years ago, molecular oxygen gas (O2) began to appear in the atmosphere and carbon dioxide began to diminish (see our History of Earth's Atmosphere II: The Rise of Atmospheric Oxygen module). Oxygen gas cannot appear on a planet spontaneously, so this tells us that Photosynthesizing bacteria had evolved that captured carbon dioxide and released oxygen, making possible the existence of plants, animals, fungi, and all the other oxygen-dependent life on Earth.

However, at the time that it occurred, the rise of oxygen would have caused mass extinctions of the microscopic species at that time. This is because oxygen is a highly reactive molecule that is extremely toxic to cells that are not adapted to tolerate it. Today, organisms that are poisoned by oxygen are rare but still exist, restricted to oxygen-depleted environments like deep sea vents and the digestive tracts of animals. Therefore, even though the evidence is indirect, scientists are relatively certain that the great oxygenation event caused one of the first large mass extinctions on Earth, affecting most microorganisms alive at that time. The few organisms that survived were those that could tolerate oxygen. These oxygen-tolerant pioneers became the ancestors of almost everything alive today, including us.

The first major extinction of complex life

Thanks to that Great Oxygenation Event, more complex cells evolved, but all life was still just microscopic single cells. Eventually, by the Ediacaran Period (~ 635 – 542 million years ago), some of the earliest multicellular organisms – the “Ediacarans” – lived in the seas. These organisms became the ancestors of plants, animals, fungi, and protists. Do you recognize the organisms in the Ediacaran depiction (see figure 3)?

You probably don’t recognize the species shaped like discs and fronds in Figure 3 because few of those species are still alive today. The fossil record shows that something momentous happened about 540 million years ago causing nearly all these species to go extinct.

“Behavioral innovations associated with the advent of predation and ecosystem-wide changes triggered by filter-feeding sponges, grazing and burrowing bilaterians, and the replacement of firm, microbially-bound substrates by aerated mixed grounds, converged and resulted in the first large-scale extinction of macroscopic life.”

(Laflamme et al., 2013)

Diagnosing the causes of a major extinction requires some deep detective work. Based on fossil and geochemical evidence, scientists have developed two competing hypotheses for the cause of the end-Ediacaran event.

The Catastrophic Change Hypothesis proposes that the massive die-off occurred due to dramatic chemical changes in the oceans, making it no longer hospitable for the Ediacaran organisms. In contrast, the Biotic Replacement Hypothesis proposes that newly evolving organisms gradually replaced the Ediacaran groups by out-competing them in the increasingly crowded oceans.

In evaluating the evidence, American geobiologist Simon Darroch and colleagues (Darroch et al. 2015) deemed that a catastrophic change should leave signs in the rock record, such as a drop in oxygen, reduced salinity (salt concentration), or other stressful conditions. In contrast, a biotic replacement should show a gradual turnover of species in the fossil record without major disruptions in ocean chemistry. Their geochemical analyses of Ediacaran rocks from Namibia showed an oxygenated and otherwise favorable ocean environment, evidence against the Catastrophic Change hypothesis.

Furthermore, the Namibian rocks contained fossils of both Ediacaran organisms and organisms that evolved later. Their overlap is the strongest evidence for a gradual replacement of species rather than a sudden environmental change. The researchers conclude that Ediacaran organisms likely were gradually marginalized as more complex and robust organisms evolved that changed the seafloor by burrowing and grazing. The Ediacarans could not adapt.

Both the great oxygenation event and the demise of the Ediacarans occurred so long ago and involved lifeforms that have neither bony skeletons nor hard exoskeletons, so the evidence we have is sparse and indirect. However, beginning around 500 million years ago, animals with harder, more durable body parts began to emerge. These animals left a much more extensive fossil record, so scientists are able to study the more recent mass extinctions with greater detail.

The five big mass extinctions

Look again at Figure 2. When was the biggest extinction event of the last 500 million years?

End-Ordovician mass extinction

The tallest point on the graph marks 444 million years ago, which ended the geological period called the Ordovician. Indeed, scientists mark the starts and ends of geologic periods with major changes such as mass extinctions. Based on the fossil record, scientists estimate that this End-Ordovician mass extinction event killed off 86 percent of the species in Earth’s oceans and scientists are still trying to understand the cause.

The End-Ordovician extinction happened at a time when Earth was in a cooling phase, heading into “icehouse” conditions. One hypothesis is that the rapid cooling trend caused glaciers to spread and make the ocean unsuitable for most life. For example, New Zealand paleontologist James Crampton and colleagues examined tiny, fossilized ocean animals called graptoloids (Crampton et al., 2016). As part of the plankton at the base of the ocean food web, graptoloids are sensitive indicators of big environmental change (see our Trophic Ecology module). Crampton’s study showed that 77% of the graptoloid species went extinct during the late Ordovician. The survivors were mainly the cold-adapted species.

But a 2019 study by Danish paleoecologist Christian M. Ø. Rasmussen and colleagues proposed another cause of the End-Ordovician extinction event: volcanic activity (Rasmussen et al., 2019). They analyzed all Ordovician fossil evidence recorded in a paleontology database and found that even after temperatures reached icehouse levels, ocean biodiversity continued to increase. This is evidence against the hypothesis that cold temperatures were the culprit. Instead, they point to geological evidence that the oceans acidified and became lower in oxygen and propose that increased volcanic activity was responsible.

Do you think the End-Ordovician event can be attributed to both the cooling and the changing ocean chemistry?

American geologist Nevin P. Kozic does. He and colleagues, knowing what an important role oxygen played in the past rise of complex life, wondered if it was a culprit in this huge extinction event (Kozik et al., 2022). Figure 4 shows two pulses of extinction, called LOME-1 and LOME-2 (Late Ordivician Mass Extinction)., Kozik then tracked geochemical changes including oxygen, along with climate and sea level, during the Late Ordovician. Do you see changes in the directions of any curves at the first or second extinction pulse? (Hint: don’t get caught up in the details).

The graph shows changes in oxygen, other geochemical parameters, temperature, and climate during the Ordovician. Ignoring the details, it’s clear that multiple environmental variables shifted together to make conditions less suitable for life. The study authors concluded that the first extinction pulse resulted from a combination of cooling oceans, falling sea levels, and changes in ocean chemistry; the second pulse from warming temperatures, rising sea levels, and further changes in ocean chemistry. Because each pulse reduced species diversity, ecosystem complexity and resilience also dropped, leading to a cascade of impacts that, together, added up to a catastrophic mass extinction event (see our Trophic Ecology module).

What happened to life on land?

From about 4 billion years ago until just 750 million years ago, life was pretty much restricted to the seas. However, as simple plants such as mosses slowly started to colonize the shorelines,animals quickly followed. By about 500 million years ago, more advanced land plants that were not restricted to shorelines or wet lands began to evolve and the full colonization of dry land began. This caused catastrophic changes in environmental conditions. In fact, it was the transformation of the dry landscape by the arrival of plants and animals that may have caused the second great mass extinction shown in figure 2, the Late-Devonian Mass Extinction, roughly 360 million years ago.

American geologists Thomas Algeo and Stephen Scheckler (Algeo and Scheckler, 1998) hypothesize that the early terrestrial plants set off a chain of events such as the emergence of soil (dry land was entirely rocky prior to this). Once soil appeared, it began to erode, leading to runoff of minerals into the oceans. This results in algal blooms, which had great impacts on the climate conditions of earth and disrupted ecosystems around the globe.

End-Permian mass extinction, aka “The Great Dying”

The third great mass extinction took place about 250 million years ago, at the end of the Permian period. In terms of the number of species lost, the End-Permian mass extinction is the largest to date, with an estimated 96 percent of species going extinct (see figure 2).

“The mass extinction at the Permian–Triassic boundary eliminated > 90% of marine and terrestrial species and is widely considered the most severe biotic crisis in Earth's history.”

(Marco Romano et al., 2020.)

In this case, there’s an obvious culprit. In Siberia, there are vast deposits of basalt rock – which is formed from volcanic lava – as thick as 6500 meters in some places and that dates to about 250 million years ago. This, combined with other evidence of changes to ocean and atmospheric chemistry, is evidence that this was the largest and most explosive volcanic period since complex life evolved. Since this coincides with the fossil evidence of the mass extinction, scientists are relatively certain that the two events are related.

A recent study by Chinese geologist Meghan Li and colleagues (Li et al., 2021) probed the specific “kill mechanism” of these volcanic eruptions. They found geologic evidence that nickel-rich particles carried by volcanic gasses rained down into Earth’s oceans, causing a bloom of bacteria that produce methane. The spike in methane would have made the oceans unliveable for most organisms. The rock record also shows that mercury in the volcanic ash accumulated to toxic levels. Coupled with a rising global climate from volcanic “greenhouse gasses” such as CO₂ and CH₄ (see our Factors that Control Earth’s Temperature module), these toxins spurred a catastrophic cascade of impacts to life on Earth.

What other changes do you see in figure 5?

Why don’t mass extinctions end all life on Earth?

With ongoing extinctions punctuated by mass extinctions, one could wonder how there are any species left on Earth. The short answer is recovery. Despite the magnitude of some past extinctions, they have been followed by recoveries. Some organisms survived to reproduce, spread, and evolve into new species. Every mass extinction event changed the trajectory of life on Earth by closing the door on some lineages and opening the door to others.

For example, in the End-Triassic mass extinction 201 million years ago (see figure 2), caused by another set of massive volcanic eruptions and the associated environmental changes, crocodile-like animals called phytosaurs were wiped out. Their extinction is thought to have made room for dinosaurs to diversify into the large ones that would later dominate landscapes for over a hundred million years. But environmental changes would ultimately eliminate them as well.

Indeed, the most famous mass extinction – the End-Cretaceous extinction of the large dinosaurs – is thought to have helped pave the way for mammals to dominate in our current Age of Mammals. A mass extinction reduces competition for resources, giving surviving species the opportunity to thrive and new species to evolve. Just like in a library, the loss of some books opens space on the shelves, which ultimately get populated with new books.

End-Cretaceous mass extinction

Since paleontologists began unearthing and studying dinosaur bones in the early 19th century, people have been fascinated with what happened to these unusual creatures. With the exception of the lineage that led to modern birds, dinosaurs were wiped out along with 65% of species on Earth during a cataclysmic event 66.5 million years ago (see figure 2). But figuring out what caused this End-Cretaceous mass extinction has required assembling a series of clues, like scientific detective work. Hypotheses for what eliminated the dinosaurs have included falling sea levels or huge volcanoes like those implicated in earlier extinctions. But in 1980, a new and dramatic hypothesis emerged.

A father-son team of American scientists (Alvarez et al., 1980) presented evidence that a thin layer of iridium was present in deep layers of rock all around the world - from Italy and Denmark to New Zealand. In all these places, the layer dates to about 66 million years ago. Iridium is a metal that is rare on earth, but much more common in asteroids, raising the possibility that an enormous asteroid crashed into earth around 66 million years ago, and the resulting cloud of dust and rock spread the iridium all over the planet.

Asteroid impacts leave craters, so an enormous asteroid would leave an enormous crater. But no such crater dating to the right time period had been found. While it was possible that geological activity could have erased it, the scientific community began to hunt for a large previously undiscovered impact crater. In 1991, the discovery of the Chicxulub crater, off the coast of the Yucatán peninsula, was announced (Hlidebrande, et al., 1991). This enormous crater - 180km in diameter - is rich in iridium and dates to about 66 million years ago. Since then, more evidence that Chicxulub is an asteroid impact crater has been found, including materials that form from shock, such as little glass balls (tektites), “shocked” quartz that has undergone extreme pressure, and a pattern of far-flung impact materials in a layer that gets thinner with increasing distance from Chicxulub.

What evidence might provide clues that an asteroid had collided with Earth?

“Our hypothesis suggests that an asteroid struck the earth, formed an impact crater, and some of the dust-sized material ejected from the crater reached the stratosphere and spread around the globe… we would like to find the crater produced by the impacting object.”

(Alvarez at al., 1980)

The weight of evidence supports the hypothesis that an asteroid collision with Earth and all the associated impacts caused the End-Cretaceous extinction. But how could a single asteroid collision have caused extinctions all over the world?

In 2010, an international group of 41 scientists collaborated in a review of all the evidence (Schulte et al., 2010), and described a scenario like this:

• An asteroid about 10 km in diameter (~6 miles) collided with Earth 65.5 million years ago;
• It landed in the Gulf of Mexico-Caribbean region, carving out the ~200-km-diameter Chicxulub Crater;
• The impact created shockwaves, earthquakes, pulses of heat, and tidal wave tsunamis around the world;
• Chemically sensitive gases including carbonates and sulfates released from the rocks caused acid rain;
• The volume of dust and debris caused months-long blackout conditions, followed by years of very dim light conditions, leading to global cooling;
• Without sufficient sunlight, plants, seaweeds, and phytoplankton were unable to sustain photosynthesis. Because these organisms are at the base of all trophic relationships, entire food webs collapsed all over the world, including the ones that sustained large dinosaurs (see our Trophic Ecology module).

It is also worth noting that among the animals most likely to survive a catastrophe like this are scavengers and those that feast on dead plants and animals. It is believed that early mammals were just these types of animals, arguing that they were well poised to survive the collapse and then diversify to fill in the niches that were now vacant by the extinction of the dinosaurs.

While there is no longer serious doubt that an asteroid struck earth with devastating impact around 65 million years ago, evidence has also emerged that ecosystems may have already been stressed when the asteroid hit. For example, American earth scientist Benjamin Linzmeier and colleagues (Linzmeier et al., 2020) have argued that the oceans were acidifying from carbon spewed out by volcanoes in the couple million years leading up to the extinction. But Italian paleontologist Alfio Alessandro Chiarenza and colleagues (Chiarenza et al., 2020) countered that the volcanic eruptions may have buffered rather than contributed to the extinction event. Although it is unclear whether the asteroid was the initiating event or the final blow to ecosystems that were already under great stress, nearly all scientists agree that the asteroid that left the Chicxulub Crater caused dramatic changes to the biodiversity on earth.

Mass extinctions and climate change

Considering the mass extinctions of the past, we notice that all of them entailed major environmental changes, whether precipitated by volcanism, asteroids, or the organisms themselves as they evolved new ways of living. One consistent variable in periods of high extinction is climate change. In figure 7, notice how the high rates of temperature changes over the past 500 million years correspond to the spikes in extinction rates. This relationship begs the question of how current climate warming will affect extinctions.

Indeed, current rates of species extinction are estimated to be 100-1,000 times the usual background rate, with some estimates as high as 10,000 times higher. Therefore, many experts believe we are in the midst of another mass extinction. Because mass extinctions are the most common boundaries between major geological eras, some earth scientists are calling our current age The Anthropocene, or the age of humans. Only time will tell how much our climate will change and high extinctions rates will spike, but one thing is already abundantly clear. Humans have transformed the global landscape, seascape, and climate. If the past is a reliable predictor of the future, global climate and biodiversity are entering a tumultuous age.