[Note: Many viewers sent in questions about the possibility of current and future mass extinctions, including the following:]
Q: Do you expect a mass extinction in the future? Is one happening right now? Or is evolution continuous and will positively impact biodiversity in years to come? Narrabundah College - year 11, Kingston, Australia
Q: Is there any evidence or do you have an idea of what would cause the next extinction, if you had to guess? Kevin Carnahan, United States
Q: Is there any substantial or physical evidence of mass extinction happening again on Earth if it has happened in the past? Jessica, State College, Pennsylvania
Doug Erwin: Hi Jessica, Kevin, and Narrabundah students,
There is no reason to think that mass extinctions will not happen in the future, and, in fact, many biologists believe that we are in the midst of a human-caused mass extinction right now. Our ancestors began killing off many large vertebrates some 12,000 years ago (mammoths, saber-toothed cats, and dire wolves, among others), and we continue by depleting the oceans and destroying habitats for plants and animals. There are lots of other potential causes of mass extinction, including the collision of extra-terrestrial objects into Earth, massive volcanism, and glaciation, but no way of predicting which of them might affect us or when (contrary to some bad Hollywood movies). None of these is very likely to happen soon, so we would be better off worrying about our own effect on the planet.
Q: Could species become extinct merely due to evolution rather than a mass extinction? Student at Pennsylvania State University, State College, Pennsylvania
Erwin: Yes—in fact, some species become extinct all the time, but mass extinctions are recognized by a large number of extinctions, far above this so-called "background extinction rate," in a relatively short amount of time. We think that about 98 percent of all species extinction occurred during the intervals between mass extinctions, leaving just 2 percent to die off during the great mass extinctions. But mass extinctions are also a part of evolution, as the late Stephen Jay Gould of Harvard has argued. All of these extinctions have an impact on the evolution of life.
Q: I liked the show. Is there any hard evidence about climate change during the Permian or other extinction? Or is this still theory? Jim Parsons, Rapid City, South Dakota
Erwin: Dear Jim,
We have good evidence that the nature of rivers changed at the [Permo-Triassic] boundary, evidently because there were few plants to hold back sediment, and we also have a variety of geochemical markers that suggest global warming in the earliest Triassic. But we do not have an unambiguous "paleothermometer" that we can use to determine changes in climate, so the argument for climate change is an inference based on data. It is always possible that some bright young graduate student might come along with a better way of interpreting the data that does not involve climate change.
[Note: Many viewers sent in questions about the types of life that survived the Permian extinction, including the following:]
Q: Since almost all life was wiped out [during the Permian extinction], what kind of life did survive, and how was it able to flourish? Nicole at Penn State University, State College, Pennsylvania
Erwin: Hi Nicole,
In the oceans, some species of many groups survived, but the number of surviving species differed considerably between different groups. For example, lots of snails and clams survived, but only one or two species of sea urchin. Other survivors include fish, ammonoids (in very small numbers), brachiopods, and bryozoans. One view of the survivors, advanced by Andy Knoll of Harvard and Richard Bambach [professor emeritus of paleontology at Virginia Tech], is that groups with higher metabolic rates (including molluscs, for example) did better than those with lower metabolic rates. We also know that groups that lived attached to the seafloor and fed by filtering material from the water suffered greater extinction than more mobile species.
[Note: Several viewers sent in questions about how life on Earth recovered following the Permian extinction, including the following:]
Q: If, as the segment seemed to suggest, the Permian (and other mass extinctions) happened as a result of global climate change, what has research revealed about how that process reversed or changed to allow for the next development of life on Earth? Michael Whitney, Skokie, Illinois
Q: I understand how the geological conditions of the Earth and oceans may have caused mass extinction during the Permian era, but how did the Earth "heal" and allow plant and animal life to reappear? Vicki Webb, Glendale, Arizona
Erwin: Dear Michael and Vicki,
Understanding how the Earth and its life recovered is probably the most challenging part of this area of scientific research (and it is actually what originally got me interested in the extinction). We know from fossils in many different parts of the world that it took many millions of years for life to flourish again, both on land and in the oceans, far longer than after any of the other mass extinctions. Recently Jon Payne (now at Stanford) has shown that there were very large swings in carbon isotopes during the Early Triassic. We don't yet know what caused these, but whatever it was probably contributed to the delayed recovery. The "healing" of the Earth involved an end to what caused the extinction, an amelioration of the harshest of the environmental conditions (high CO2, for example, increases weathering rates, which eventually draws down CO2 levels in the atmosphere) and the appearance of new species and new ecosystems. One of the most exciting and challenging problems now is trying to understand how the ecosystems rebuilt themselves.
Q: When the flood basalts covered Siberia, would they have depressed the land both due to collapsed magma chambers and isostatic redistribution? Would this depression of the land yield an apparent rise in sea level? Are flood basalts accompanied by large outpourings of greenhouse gases? Or did the basalts burning the plants and peat cause the release of stored CO2 [carbon dioxide] and CH4 [methane], building to large-scale global warming due to greenhouse gases? What would the method be for determining whether the CO2 and/or CH4 in the Permian atmosphere came from inorganinc (volcanic) or organic (burning organic matter) sources? Do we have enough material to provide reasonable data to explore these "suspects"? Mary Jane, Middlebury, Connecticut
Erwin: Hi Mary Jane,
Excellent questions, and most have puzzled many geologists. Subsidence does normally accompany such a massive outpouring of lava, but normally uplift occurs before the volcanic eruptions, as the plume of magma intrudes below. One of the many odd things about the Siberian volcanism is that the subsidence evidently began before the eruptions. The global rise in sea level was probably not related to the Siberian volcanism, in large part because most of the volcanism was on land. Finally, the volcanism would have introduced greenhouse gases both directly, as carbon dioxide to the atmosphere, as well as through burning the massive coal beds that underlie the Siberian volcanics. Both methane (from burning coal) and sulfur dioxide (from volcanism) would have been produced as well. It is very difficult to figure out how large a volume of any of these gases would have been produced. The carbon isotope values differ substantially for all of these sources, and that is the primary way that we know that a large volume of organic carbon was introduced at this time. You might ask if this was simply from all the organisms that died, and it turns out that that wouldn't be enough to cause the change we see, so methane release, burning coal, or some other source must have been involved as well.
Q: If recent evidence from Greenland suggests that marine life may have gone extinct before life on land, how does that impact the idea that volcanoes may have been the reason behind the mass extinction? You would think volcanoes would hurt terrestrial life before marine life. Brendan Gavigan, University Park, Pennsylvania
Erwin: Dear Brendan,
That's a good point. In part it depends on how the volcanism in Siberia actually caused extinctions. It could be that, for example, acid rain caused by sulfur from the volcanism may have killed off shallow life in the oceans before global warming killed off life on land. It is also possible the data from Greenland are misleading. We do know from very detailed work done by our colleagues in Nanjing at sections across South China that if there was any delay in terrestrial extinction, it was a very brief interval of time.
Q: It is possible that the Permian extinction was the result of a series of events. You stated [in the essay The Permian Puzzle] that some of these events are difficult to distinguish as causes or effects of other events. At what point is one possible reason for extinction seen as the cause and not as an effect of related happenings? Is it at all possible to distinguish causes from effects? David King, Junior at Penn State, DuBois, Pennsylvania
Erwin: Dear David,
Excellent question, and in fact this has long been a problem for research on this event in particular. Many things that have been seen as causes are now seen as effects. Distinguishing cause and effect is very difficult without being able to date, very precisely, the various events, and then trace through cause and effect, distinguishing a downstream cause from the initial trigger. For example, methane may have been released at the PT [Permo-Triassic] boundary, but if it came from heating of coal during the eruption of the Siberian volcanics, then the volcanism is the real cause. The shift in carbon isotopes is a reflection of some other cause as well.
Q: All of our textbooks teach us that Pangea broke apart (and use this to teach continental drift and plate tectonics). None talks about Pangea forming—we're usually led to believe that it was always there—but we know that it formed sometime in the Permian. What is the evidence for Pangea forming, and what caused it to break apart again? Russia Local School 8th grade, Russia, Ohio
Erwin: Dear Russia Local School 8th grade,
The evidence for the presence of Pangea has been around for many years, and, in fact, geologists in places like Australia and South Africa were convinced of the existence of continental drift decades before their colleagues in Europe and North America simply because many fossils and geological formations, for example, matched up across the Atlantic, and between Africa and India. Today we have much more evidence in the form of records of paleomagnetism, including past positions of the continents through time. This record shows that almost all the continents collided through plate tectonics to form Pangea in the Early Permian.
Q: Have there been any recent advances in technology that have helped or can help scientists to explore the possible causes of mass extinction further? Katie, Mont Alto, Pennsylvania
Erwin: Dear Katie,
Yes! In fact, most of our new understanding about this [the Permian] mass extinction is because of new scientific techniques that allow us to probe this event in more detail. One of the best examples is the work by Sam Bowring (featured in the NOVA episode), who has revolutionized using radiometric minerals to very precisely date the age of the mass extinction (by dating minerals produced in volcanic eruptions). In this way, Sam and I and our colleagues have been able to establish that the extinction was a very rapid event, occurring in just a few hundred thousand years at the most, and possibly much more rapidly.
Q: Since there seems to be little evidence for an extraterrestrial impact, why do you think so many scientists still insist on this option? Do they have any actual evidence? And do you agree with Sam Bowring [in Extinction Happens] that the quick assumption of an asteroid impact causing each mass extinction lacks imagination? Kristen, State College, Pennsylvania
Erwin: Good question, Kristen.
I wish I knew the answer. I think that there are a number of scientists who want to find some sort of general theory of mass extinctions. But the evidence for any such theory is pretty sparse, and I think it is probably better to start with evidence rather than theory. And yes, I do agree with Sam, and I have also argued that impact would actually be the least interesting answer, because we wouldn't learn anything new about how the Earth responds to crises like this. From work on the KT [the mass extinction at the end of the Cretaceous], we already know something about what happens after an impact.
Q: Why are Earth scientists so confident about what caused the KT extinction that took place 65 million years ago, which wiped out the dinosaurs, but yet are so unsure about what caused the massive Permian extinction that took place 250 million years ago? Andrew Chin, Penn State University
Erwin: Dear Andrew,
Good question. Much of the reason is that we simply have many more areas that preserve a record of the KT extinction (and many are in western North America where there are lots of geologists). More importantly, these sections contain a "smoking gun" in the many different sorts of debris from the [asteroid or comet] impact that hit the Yucatan peninsula. But we haven't found any such simple solution to the Permo-Triassic mass extinction, at least not yet.
Q: I heard that accelerated continental drift occurred in the Cambrian and Permian Periods. In the Cambrian, it could have been responsible for the "Cambrian Explosion," and in the Permian it could have been responsible for the evolution of mammal-like reptiles, the ancestors of mammals. Have you heard this, and could it figure into an extinction scenario? Fred Musante, helton, Connecticut
Erwin: Dear Fred,
The theory is actually a bit more than accelerated continental drift, but the hypothesis that it may have been associated with the initial diversification of animals in the Cambrian now looks increasingly implausible. I do not know of any recent suggestions for such rapid change near the Permo-Triassic, but we have pretty much been able to rule out any such hypotheses by establishing that the extinction took only a few hundred thousand years at most. While this may sound like a long time, it is actually to short a period to allow such a tectonic cause.