Summaries of the five mass extinctions of multicellular animals and plants – make it six extinctions
The setting of the mass extinction events. These events are traceable in the fossil record because multicellular life existed at the time. Earlier extinctions of unicellular life (or life with cells that associated but did not differentiate into types) likely occurred but we can’t trace them. The extinctions eliminated most species of organisms. The continuity of life required the evolution of new species from the survivors, since a de novo evolution of life from abiotic chemicals is not possible once vigorous life forms use up all the resources. That life hung on after each mass extinction was not a given. The Earth’s tectonic upheavals were and are fortunately sufficiently limited. Life caused its own problems, as evidenced by cyanobacteria causing Snowball Earth starting 2.2 billion years ago. Life’s origination of problems (for other life forms) is also evident in the role of trees in episodes such as the end-Devonian extinction. I find the story of extinctions and of evolution overall to be the greatest book. We can read it with the aid of concepts (e.g., stratigraphy), tools (up to mass spectrometry of the isotopes of chemical elements), and a vast amount of work by people all over the world – be they paleontologists or oil prospectors or amateur natural historians. The story also ties into another grand story, that of plate tectonics with its rearrangements of continents (important in several mass extinctions), its illumination of earthquakes and volcanism, its extension to learning about the deep interior of the Earth. Finally, the study of mass extinctions informs us about climate change as it looms over us from our own actions.
End-Ordovician, 447 – 444 million years ago. The Ordovician Period saw a further great radiation of species of all kinds, 50 million years after the Cambrian Explosion of species diversity. Sea levels were extremely high, with little water tied up in glaciers. The period ended with two wild swings in climate, first to cold conditions and then to hot conditions. The causes of the mass extinction of species are still debated. There is good evidence that both climate swings appear to have been related to volcanism, the first one being of silicates that take up CO2 from the air as they weather, the second linked with massive injection of CO2. Sea life, which formed most of all life, collapsed to low levels in the second biggest mass extinction, surpassed only by the End-Permian Extinction about 190 million years later. Note also that the Appalachian Mountains formed about 35 million years before the extinction event. They rose to perhaps 5000 meters in elevation as a result of a collision of tectonic plates. Their erosion also helped remove CO2 from the air.
Late Devonian, 374 Mya and 359 Mya (67), a two-stage extinction, or, well, maybe eight or more, a rolling extinction. Marine life had a very hard time, while life on land did well. Whole families of marine organisms disappeared, particularly at the two peaks. There is evidence of rapid and repeated changes in sea level, with glaciation and deglaciation. Seas seem to have become anoxic; the seas gained excess nutrients, that is, they eutrophied. Dying algal blooms as they decompose use up dissolved oxygen, just as they do now in the Mississippi River Delta as nitrogen- and phosphorus-rich runoff from overfertilized farmland runs into the river. Even the big predatory fish such as Dunkleosteus died off at the end, after resisting anoxic times. Some fish evolved into tetrapods on land such as Hymenopteron, and some of them eventually to us humans. Marine life only began flourishing 25 million years later. The causes are still hard to discern, likely because there are multiple causes. Volcanism occurred on a large scale several times. That helped drive the greenhouse effect both up (from CO2 injection) and down (from weathering of deposited silicate rock). There is also strong evidence for the role of land plants in “deflating” the greenhouse effect by decreasing CO2 in the atmosphere. They had evolved earlier in the Devonian from short nonvascular plants to plants with water-conducting xylem tissue. This allowed them to grow to great heights, 30 meters, much as in modern forests. They also spread into periodically drier environments by evolving drying-resistant seeds. As a result, CO2 was depleted from the atmosphere, both from carbon being tied up in dead plant matter and from strong weathering of rock induced by plants. Re the dead plant matter: vast growth of forests led to accumulation of poorly decomposing wood, leaving us coal deposits. Fungi at that era could not break down lignin as they can now (which means our planet is no longer making coal). Re the strong weathering: vascular plants evolved the exudation of organic acids from the roots to solubilize iron and phosphorus from soil minerals. The rate of rock decomposition greatly increased. Part of the weathering process is the uptake of CO2 as carbonic acid. Putting it all together as well as is possible, it appears that tectonics acted together with land-plant physiological evolution to create havoc in the seas. I find it amazing that we can find such extensive and detailed evidence of who lived and who died as well as of many geophysical processes. Part of our knowledge is due to the fossil-fuel industry prospecting for coal, oil, and natural gas. The industry developed a great knowledge base, especially of marine fossils. So, the industry helps us understand extinctions even though it’s helping the Sixth Extinction from climate change (or, we are in cahoots, in demanding fossil fuels).
End-Permian extinction, 252 million years ago. This was the biggie, where the Earth lost 95% of all species of organisms, which entails the loss of a much greater fraction of individuals. A species can survive the death of the vast majority of its individuals, so that the total loss of individuals may have been well above 99%. Even those intriguing trilobites what had survived from 300 million years died off. It is the only mass extinction that really hit insects. Land vertebrates had already evolved, and enough made it through to radiate evolutionarily to, among other species, us. The extinction may have been very rapid in geological terms, as short as 60,000 years, not millions. The recovery appears to have taken 2 to 10 million years. Several causes are implicated. One is a rapid rise in atmospheric CO2 and thus in temperature. The huge Siberian Traps are the remnants of CO2-injecting volcanoes. They erupted in coal beds, setting them on fire to add even more CO2. Evidence of huge additions of CO2 include the rise in the proportion of light 12C carbon over heavier 13C carbon. The light carbon had been preferentially incorporated into plant tissue for quantum-mechanical reasons (as it is today) and thus into coal. Adding to the excursion to high temperatures for land life for much of the land mass was the coalescence of continents into Pangaea. For oceanic life the high temperatures were abetted by simple acidification of water by CO2. While corals, for one, have survived acidification events, they do so as free-swimming, non-reef-forming forms, and rapid acidification can frustrate physiological acclimation (no genetic change) and genetic adaptation (replacement of genotypes). Piggybacking on climate change might have been hyper-hurricanes fed by high sea surface temperatures. Of itself the wind damage is local and transient. However, recent studies indicate that extreme thunderstorms, not to note hurricanes, can loft water vapor into the stratosphere, where it can destroy ozone. Volcanoes can also emit halogens, chlorides and bromides, that are effective zone-destroyers (as we spray-can and refrigerator-users of old learned from the modern damage to the ozone layer). Evidence of the loss of the ozone layer lies in misshapen spores of plants (e.g., ferns). So, multiple killers appear to have been at work, tied primarily to massive and rapid volcanism. There seems to be a repeated pattern of tectonics turning from its general life-promoting mode. Earth’s habitability is not based on initial conditions. The extinctions are the geological generalization of Will Durant’s epithet, that “civilization exists by geological consent, subject to change without notice.” How much more tenuous might habitability be on a generic exoplanet that otherwise meets minimal conditions for habitability at some given time. We may also reflect on the possibility that we humans could generate an enormous climate overshoot. I have formed the view that ants, with their minuscule brains, exhibit collective intelligence while we humans exhibit collective stupidity. That’s not entirely facetious. We evolved to react effectively to immediate threats in short stretches of space and time. Climate change is the antithesis of this. What will it take for us all to face it effectively?
End-Triassic, 201 million years ago. This is getting be a distressing pattern! Life ha bloomed after the end-Permian extinction for some tens of millions of years, only to be hit again. “Bloomed” is used advisedly. Trees did not recover for about 10 million years into the Triassic! Pangaea remained blocky and arid in its interior. Seas remained very hot and significantly anoxic. A visitor from an exoplanet early into the Triassic might have counted Earth off the list of habitable planets. Still, dinosaurs finally evolved about 243 Mya. Cold-blooded (euthermic) animals with low metabolic rates handle high temperatures much better than do warm-blooded (euthermic) animals, though many of the latter thrived. New orders of amphibians arose. Life survived the impact of a massive asteroid that hit in what is now Manicouagan, Quebec, 14 million years before the mass extinction. Water returned big-time to the continents in the Carnian Pluvial event (which also caused some extinctions of marine life). The end of the Triassic was catastrophic again. Another one of those enormous volcanic events seems to be the killer, the continental flood basalts of the Central Atlantic Magmatic Province. This is the biggest of the big volcanic provinces. Eruptions covered what are now the northeastern United States, northwestern France, parts of North and West Africa, and Brazil, an area of over 11 million square kilometers. The amount of CO2 injected into the atmosphere was again colossal. So, too was the rise in global temperatures, estimated at 6 to 9°C. Corals again lost out from both high temperatures and ocean acidification (which shifts the chemical equilibrium of carbonates that coral use to make their skeletons). The extinction seems to be much more clearly defined that that of others to this point. Our “friend,” tectonics, took on a dark role again.
End-Cretaceous, 66 Mya. This is a most engaging story, if a horrifying one, for many people. The image of a great asteroid hitting the Earth at phenomenal speeds to raise untold volumes of dust and debris holds a morbid fascination for some, an intellectual exploration of physics, chemistry, biology, and geology for many others. The story of this extinction gained great interest, public and scientific, with the discovery of the global geological stratum rich in iridium by Luis and Walter Alvarez. The thinness of the layer indicated a very short time interval, evidence against necessarily slower volcanism. The discovery of the Chicxulub impact crater in the Yucatan Peninsula of Mexico was convincing evidence. Findings of sands deposited by an unimaginable tsunami also fit the picture. Detailed simulations of the impact’s effect yielded a scenario of thick and globe-encircling dust that cut sunlight penetration and, consequently, cut photosynthesis that supports essentially all life. All the dinosaurs died out, other than some birds. Intriguingly it was largely birds nesting in holes in riverbanks and cliffs that survived best, as if shielded from the initial blast. A global earthquake of an estimated magnitude 9 caused some additional damage. Many mammals made it through, even if marginally. The month of the impact is even inferred from the lack of late-season seeds of lotus plants. Still, there was another likely cause of extinction at work, massive volcanism at the Deccan Traps in India. The Traps began erupting just before the asteroid hit and continued for a while after the impact as the Wai Group, with a noted change in the nature of the lava. The debate on causation of the extinctions seems to be converging on joint action of the asteroid and the Traps, with the impact triggering extra volcanism. Other contributions, as from global fires, can be dismissed as there being no plausible global heating of the atmosphere. Tectonics played out again along with an extraterrestrial visitor.
The current Sixth Extinction, 50 kya to date. Throw in an Ice Age but the actors are mostly humans. Our impact is cumulative. The first major impacts occurred in preindustrial times and they continue. Our ancestors made significant changes in land use by clearing land, with the most dramatic being the use of fire by Australian aborigines for hunting. Grain cultivation was a start, still expanding currently and abetted by oil palm plantations; palm oil is in most products one can name in industrial and post-industrial nations. The land clearance has direct and ominous effects on our food supply, for one. Plant breeders have to keep up with evolving or expanding plant diseases and pests. They need to get genes from wild relatives of crops just as those wild relatives become very rare or extinct. Genetic engineering is a Band-Aid© since it is virtually impotent to suggest new genes or gene complexes that are needed. A little-appreciated effect of expanding land use is our increasing contact with wildland reservoirs of zoonotic diseases. Expanding populations went for bush meat in Africa than resulted in the spread of HIV. We’re expanding into areas with mosquitoes that are more effective in spreading malaria. We’re expanding into the areas of bats that carry 500 different coronaviruses that have jumped and will jump to infecting us.
Our ancestors also hunted wild animals and domesticated animals. A result is that the megafauna – giant sloths, mastodons, North American camels, and many more – became extinct, and the 100:1 ratio of wild animal biomass to domestic animal biomass has reversed, irreversibly. They, and we, introduced exotic species, something no prior natural mass extinction did. These included maize around the globe, mosquitoes from Asia to the US that carry human diseases (dengue, West Nile virusm malaria) and animal diseases (bird malaria to Hawaii, devastating and even extinguishing native species), and Phytophthora species that clobbered French wines and native vegetation of Australia. (I find it amusing that in Montpellier, France, there is a statue honoring the US for saving the French wine industry with Phytophthora-resistant rootstock when the disease came from the US in the first place.) I need mention a current bugaboo, genetically modified organisms as crops. There is no significant evidence of human-health effects and minimal evidence of impacts on adjacent wild plants… but the corporate control of the GMO seed industry is very problematic. Another impact of us humans is the spread of toxics, of a diversity so far exceeding what past natural extinction events did, such as H2S in anoxic ocean events. We have introduced into soil, air, and water heavy metals from distant locations, pesticides and herbicides similarly, estrogen mimics such as BPA into our own water and food supplies, persistent halogenated compounds (CFCs, polyfluorinated alkyls, the old and still in place polychlorinated biphenyls), particulate plastics.
Our biggest negative legacy is surely climate change for its sheer size, great difficulty in reversal, and environmental injustice of industrial nations imposing great costs on nations minimally using fossil fuels. We humans started climate change in a modest way with rice paddy farming that generated methane releases. Cattle raising now adds equally large amounts of methane. Of course, use of fossil fuels is the runaway train with its emission of the greenhouse gases CO2, waste methane, and N2O. (The soluble oxides SOx and NOx are mostly a problem of acid rain though also of aerosols that change Earth’s radiative balance on a lesser scale that the greenhouse gases.) Science informs us about the many cascading effects, of which global warming is only a start. The thermal effects are, to be sure, critical. We get increased numbers and duration of the extremes of temperature that exceed tolerances of plants, animals, and humans (and, who knows, fungi, protists, …). Less appreciated is the fact that our global food supply is attuned to the current climate. Sure, the wine-growing areas can shift and are shifting from France to England (worrisome, gustatorially) and maize can move from the US into Canada but where the soils that evolved over millions of years aren’t suitable. Rice pollen gets sterilized in heat waves, affecting China and Southeast Asia, hinting at grave political repercussions, particularly as China is stretched for its food supply with only 8% arable land (hence, it is buying up cropland around the world, even in poor nations). Also of great concern is the expansion of the geographic ranges of disease-carrying insect vectors. The US alone has great worries. We only eliminated endemic malaria in 1950, with the last case being in Farmington, New Mexico, and that was possible only because night-biting native mosquitoes could be kept out of homes with window screens. Now we have day-biting mosquitoes both imported and spreading north from southern nations.
The radiative forcing and resultant warming is uneven, with polar regions most affected. In a cascading effect, precipitation patterns change worldwide. While higher temperatures increase global atmospheric water content and total precipitation, there are regions that lose precipitation from the convergence of large-scale circulation to the wet areas. A very bad omen is the northward shift of the Intertropical Convergence Zone. Its high rainfall supports triple-cropping of rice in Indonesia. When 200 million people in that region alone eventually get deprived of a third or so of their crop, where to they go? Extremes of drought and flooding increase. In response to changes in climate regionally, mobile organisms are migrating – marmots upslope on mountains, butterflies northward in the San Joaquin Valley – but it’s often not possible for migration to be fast enough (think, trees) or not to be blocked by other biotic communities. The effects of extreme events have long been a focus of my own research. This is so true for other scientists, engineers, urban planners, and others. It is becoming increasingly accurate to attribute extreme events to climate change, not wholly for an event but as an increased risk or risk ratio relative to a climate unamended by our fossil-fuel use and land-use change. The finger is pointing at us.
An effect of higher CO2 on crops that’s almost unknown, even to most scientists other than physiological ecologists is the decrease in nutrient content of plants. The empirical fact has been demonstrated in diverse ecosystems, especially in the recently completed Free-Air CO2 Enrichment studies around the world. The mechanistic basis is under study. With my wife and other colleagues I have delved into that with a look at the functional balance between plant nitrogen acquisition and plant deployment of nitrogen for photosynthesis. Colleagues and I looked at the specific response of plants to elevated CO2. A recent study implicates the drop in plant nutrient content to the global decrease in insect abundance – perhaps poorer nutrition contributes to this. Another concern for us scientists has been the water balance of plants. Higher temperatures increase the water loss rates of plants, all else equal. The gain of biomass growth per unit water used, or water-use efficiency, WUE, decreases. However, all else is not equal. At higher CO2 levels plants reduce the openings of their stomata, the pores that allow CO2 in and water out. This increases WUE, generally nullifying the WUE drop from higher temperatures. The compensation is not uniform among plants, however. We don’t yet know the winners and the losers.
How do we get off the runaway climate train? Taking fossil fuels as the main factor in climate change, it becomes a question of replacing their use. We’ve changed before, from vegetable oils for lighting to whale oil for lighting and some heating to coal and partly to oil. Now increasingly we are moving to natural gas mined directly or liberated from coal. Methane has a lower net climate impact when burned to CO2 at a higher heat yield per unit of carbon, though methane that leaks is 72 times as potent a greenhouse gas in the short run but is fortunately shorter-lived in the atmosphere. Very encouraging is the switch to wind and solar power as low-carbon energy sources, with carbon entering only in the manufacture of the turbines and photovoltaic panels. The change is still largely driven only by price rather than directed action in the face of a clear crisis. An analysis by geographer Vaclav Smil is sobering. He found that changes of fuel source have taken about 60 years each. We don’t have that much time. That point is amplified by the knowledge that current changes in climate are the “transient” phase. In the longer term various feedbacks about double the impact, such as global ice loss that increases the absorption of solar radiation. Let’s hope that an international consensus arises in timely fashion. I see hope in the revision of the economic system. Capitalism is built on indefinite growth that has already collided with the reality of resource depletion, including the depletion of the airshed to take up CO2 without major effects. Herman Daly, economist for the World Bank, once said that our current economic accounting system treats the Earth as a business in liquidation. Reality will set in. Let’s work toward making that timely. It’s an effort from the ground up. We can’t just fault fossil-fuel companies as we demand their product.
The saving of habitability in the sense of habitability for humans is up to us. We may do ourselves in but life will continue on Earth, assuming that we don’t trigger a runaway greenhouse effect, which is nearly impossible. In any event, large vertebrate species tend to last about 2 million years. Whether or not we solve the current climate crisis, something beyond us is waiting.