Has Life on Earth Survived More Than Five Mass Extinctions?

Life’s first major catastrophe crept across the planet with the spread of ice. Early life had been thriving in a warm greenhouse world, as creatures such as trilobites, corals, brachiopods and more evolved to fill in the coastal shallows. But around 445 million years ago, for reasons still not fully understood, Earth’s climate tumbled into a frigid state. Vast glaciers locked seawater into ice and drained the life-filled shallows so many species inhabited, leading many of them to go extinct. When the climate again shifted and warmed over the next million years, the stricken habitats were inundated by seawater once more, and the rapid change caused species that had survived the first change to again come under pressure. The one two-punch would lead 85 percent of marine species to disappear—and come to be known to paleontologists as our world’s first mass extinction.

Extinction is a fact of life. Of all the species that have ever evolved on our planet, the overwhelming majority no longer exist. Even as life continues to evolve and respond to Earth’s ever-changing conditions, every species we perceive will eventually perish. Some will leave descendent species as part of life’s billions-year-old story, others will be the last of their lineage, but all will eventually meet the same fate. And against this steady background of evolution and extinction, there have been multiple periods when species vanished faster than new species appeared.

“To understand the history of life, we need to understand extinction,” says University of Oklahoma paleontologist Selina Cole. The rapid disappearance of species, especially, can fundamentally reshape entire ecosystems and what we think of as the “history of life.” Some of these connections are obvious, such as when our protomammal relatives were nearly wiped out 252 million years ago and reptiles, including early dinosaurs, proliferated in the aftermath. Others are as important if not as dramatic. “One of my favorite examples is that the Late Devonian extinction of massive reef systems removed the physical reef structures from shallow environments along coasts,” Cole says, “changing water circulation that led to an ‘Age of Crinoids,’” or spiky, stalked relatives of starfish and sea urchins. When various forms of life suddenly disappear, the possibilities for life on Earth shift.

Cultivating such an understanding has taken the duration of paleontology’s existence as a science. Paleontologists recognized the influence of global catastrophes on evolution by the early 19th century. But only in the late 20th century, however, did fossil databases and increased accessibility to computers allow experts to quantify the strange times in the fossil record when abundant forms of life suddenly disappeared and were replaced by different organisms—as when the non-avian dinosaurs disappear in the rock record and are followed by a profusion of new mammals. In 1982, paleontologists Jack Sepkoski and David Raup singled out five events over the past 550 million years when extinction rates spiked. The question facing paleontologists is whether there were more, or possibly fewer, mass extinctions than originally proposed. Whether we are in a sixth mass extinction now is debated, even though there is no question that humans have continued to devastate ecosystems from deserts to the deep sea. But paleontologists suspect there may have been even more mass extinctions before our time, the exact count relying both on what we know of the fossil record and how we define what separates a mass extinction from other biological crises.

The first of the Big Five mass extinctions transpired about 445 million years ago, marking the boundary between the Ordovician and Silurian periods back when fish and land plants were still relatively new. Experts hypothesize volcanic eruptions rapidly shifted the global climate and altered oxygen levels. The second event is more of a cluster that took place between 359 million and 372 million years ago in the Devonian. Two extinction pulses scoured reefs and vastly affected animals such as trilobites, although the causes of these shifts are not fully understood.

The Permian-Triassic extinction event, around 252 million years ago, is considered worst of all. Incredible volcanic outpourings at what geologists call the Siberian Traps led to rapid global warming, ocean acidification, oxygen crashes and other effects that wiped out about more than 80 percent of species in the seas and more than 70 percent of land-dwelling vertebrate families. This event laid waste to our relatives among the protomammals and allowed an “Age of Reptiles” to begin in the Triassic. Of course, similar eruptions in Pangaea around 201 million years ago hit reptiles hard at the end of the Triassic, allowing dinosaurs to become more prominent during the Jurassic—the fourth of the commonly recognized mass extinctions. It would be 135 million years before another such catastrophe, when a six-mile-wide asteroid struck the Earth and devastated our planet’s biodiversity in a matter of hours to years. Not only did the impact destroy the dinosaurs and wipe out entire groups, such as the flying pterosaurs, but it also caused severe losses to mammals, birds, lizards and other groups we think of as “survivors” of the event.

All of the Big Five have stood the test of time as mass extinctions because of their scale and severity. Paleontologists have been able to detect qualitative shifts—entire groups of organisms vanishing—as well as calculate the details of the shifts, such as how many species within a surviving group became extinct. “Experts usually define mass extinction as events that are geologically rapid, result in widespread species loss of 70 percent or more, and affect organisms globally across multiple ecosystems,” Cole says.

Even so, no standard definition exists for what a mass extinction is. “It’s been a bit of a nagging problem over the last four-plus decades,” says University of California, Los Angeles, paleontologist Pedro Monarrez. The Big Five still stand out because they were far-reaching, global events that clearly changed the makeup of biodiversity across the planet, but how to categorize other biodiversity shake-ups is still in contention.

The mass extinction count will likely shift as our understanding of prehistory does. Not only is the fossil record incomplete, but so is our knowledge of it. Some organisms appear to go extinct only to reappear in the record millions of years later, what experts call a Lazarus taxon. These unexpected reappearances, like the scientific discovery of a living coelacanth in 1938, then leave ghost lineages, or a line of ancestry that must have been present but has not yet been discovered. And as experts refine time frames and data sets, as well, what were previously considered mass extinctions might get split into biodiversity crises or other disasters that don’t quite meet the level of global disaster implied by the term “mass extinction.” “The Late Devonian mass extinction resulted in part from decreases in the number of new species evolving, rather than increased extinction alone,” Cole says, and in that way has led some researchers to no longer count the events in the Big Five.

The task is detecting the signals of mass extinctions through what’s available to us in our present moment. “In my opinion, there are certainly more than five mass extinction events throughout the fossil record,” Monarrez says. Yet what might appear as a mass extinction based upon the fossil record in one part of the world might be a localized event that isn’t seen elsewhere at the same time. “It could be that these lesser extinction events were concentrated in certain regions but did not necessarily occur globally,” he says, which is taken as a hallmark of a true mass extinction.

Varying interpretations of what counts as a mass extinction certainly play into such considerations, too. One study published earlier this year suggested that there have been as many as 16 mass extinctions over the past 541 million years, correlated with volcanic activity and sometimes asteroid strikes. The number is significantly higher because of a different marker for what counts as a mass extinction, drawing the line at moments where 40 percent of more of known fossil genera went extinct in a short period of time. But even within the paper, Cole notes, the traditional Big Five stand out, with the rest being smaller extinction events whose character can depend on how scientists group them together. “Interpretations of timing, duration and cause of each extinction event play a role in whether we classify it as a mass extinction or not,” Cole says.

How mass extinctions are defined is at the heart of scientific debates over whether we are now in a sixth mass extinction. Though mastodons and other megafauna lost toward the close of the last ice age are often presented as icons of an extinction that has been unfolding for thousands of years, the true indicators of whether or not we are in such a dire crisis are found among invertebrates, plants and less charismatic organisms that often escape our notice. A 2022 review of the subject found that the number of invertebrate species disappearing certainly fits the profile of a mass extinction. Among mollusks, for example, more than 7.5 percent of known species have gone extinct over the last 500 years. Plants, too, have faced similar extinction pressures, as they are often not priorities for conservation despite being the foundation of many ecosystems. Whether the figures indicate a mass extinction or not will surely continue to be debated, but, the researchers note, “the biodiversity that makes our world so fascinating, beautiful and functional is vanishing unnoticed at an unprecedented rate.”

The evolution of life on Earth is not a story of steady progress or linear adjustment. Extinctions both small and cataclysmic have affected what we think of as the Tree of Life. “As morbid as this may sound, mass extinctions have played a crucial role in shaping the history of life on our planet,” Monarrez says, often allowing new forms of life to arise and flourish. In fact, Monarrez notes, the baseline rate at which species normally go extinct has decreased over time. It may be that life has survived so much since its origins that new species have become more resilient. “Mass extinctions have likely increased the resiliency of many animal groups, making them harder to naturally go extinct,” Monarrez says, which, darkly, speaks to how much pressure we’re putting on Earth’s biodiversity at this very moment. Today’s crisis is all the more reason to understand how extinction has shaped life in the past. Having more knowledge about what happened in prehistory might help us make decisions to avert being the cause for a sixth global disaster, or at least to change our role from extinction triggers to ecological caretakers.

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.