Public use of masks to control the coronavirus pandemic

Nicholas R. Longrich, PhD, Samuel K. Sheppard, PhD

From the Department of Biology and Biochemistry, University of Bath, Bath, BA2 7AY, United Kingdom

author for correspondence:

[NOTE: this is a preprint version of a paper submitted for review on March 29, 2020. If you have suggestions for references to add or constructive criticism, please feel free.]

The US and UK governments advise against the use of masks by the public to fight the ongoing Coronavirus Disease 19 (COVID-19) pandemic, and so does the World Health Organization 1. But could they be wrong? The governments of China, South Korea, Hong Kong, and Taiwan 1 all recommend that the public wear masks to slow the spread of the coronavirus. China has ramped up production of facemasks, converting Foxconn factories that once made iPhones to make masks. Taiwan has also ramped up the production of facemasks, and prohibited their export. Both approaches can’t be right. Increasingly, advice against the use of face masks has been questioned 2, including by the head of China’s CDC3.

Common sense, scientific studies, and most of all the success of Asian countries in fighting the coronavirus suggest that masks may make a major difference. There are fewer scientific studies available to guide decision making than we would like, and the evidence is not always clear-cut. However, decision-making in a crisis requires that decisions are made in the absence of perfect clarity. What is clear is that the exponential mathematics of epidemics suggest that even if masks are of limited benefit in reducing infection rates, they could make a large difference over time, potentially slowing the pace of the epidemic, limiting its spread, saving lives, and ultimately allowing countries to restart the economies that their citizens  depend on for their livelihoods.


Masks protect you from others, others from you

It would seem sensible to assume that any barrier between two people’s airways would reduce the probability of an airborne virus being transmitted between them. Masks worn by infected people catch some fraction of the virus-laden respiratory droplets that are exhaled during breathing and coughing. Perhaps just as importantly, breathing through a mask slows and deflects air as it is exhaled, potentially reducing the distance viral droplets are carried as aerosols. As an experiment, one can hold some flour in one’s hand, then give it a puff of air, or cough: it flies everywhere. When the same is done with a cotton T-shirt over the mouth, no matter how hard you try, it’s difficult to move more than a tiny fraction of the flour. In slowing the air released from your mouth, a mask limits the ability of your breath to disperse particles.

            Meanwhile, masks worn by infected people catch some fraction of the virus they would otherwise inhale. If both infected and uninfected people wear masks, these effects multiply. For example, hypothetically, if an infected person’s mask reduces the amount of virus spread by 75%, and the uninfected person’s mask reduces it by another 75%, then the total reduction of the virus spread would be 94%.

            It remains possible that this reduction will not be enough to prevent infection. However, masks could still help protect people who become infected, because dosage matters. Lower dosing of virus means the infection takes longer to build up, giving the immune system more time to mount a response. Higher viral dosage gives the virus a head-start in its race against the immune system, leading to a more dangerous, rapid course of infection. This is exemplified in animal models. For example, mice exposed to lower doses of inluenza virus get less ill 4 than those exposed to high doses – which became more ill and suffer more lung damage. Similarly, in chickens exposed to avian influenza, the higher the initial dosing, the faster the birds become sick and die 5.

The immune system fights viruses, like a farmer trying to remove weeds from his field. How difficult those weeds will be to remove depends a lot on how many weed seeds there are. 1000 seeds might not pose a major challenge 1,000,000 will make it far more difficult to control the weeds. Similarly, even where masks fail to prevent infection, by lowering the initial dose of virus they could make the difference between mild symptoms and a severe illness that requires hospitalization in already over-stretched health care systems.



Masks reduce viral spreading in laboratory and real world settings

Experimental laboratory studies suggest that surgical masks can reduce the inhalation of viral particles an average of tenfold 6, with some masks providing less protection, and some providing more. In the real world, several studies suggest that masks provide protection against the spread of airborne infections, specifically influenza. A study of a hospital in Germany found that after staff began wearing masks continuously, the number of patients contracting influenza in the hospital declined by almost half 7. 7. Furthermore, in a study of German households, use of masks and hand-washing helped prevent the spread of influenza between family members 8. 8. Strikingly, a study of university students found that masks plus hand sanitizer were able to reduce the spread of influenza by up to 75% 9.

Studies of Severe Acute Respiratory Syndrome (SARS) are likely to be particularly informative for understanding COVID-19 because both have a common origin as bat-derived coronaviruses 10, 11, and share broadly similar epidemiology. Within a hospital setting, consistent and proper mask use has been shown to be crucial in preventing transmission of SARS coronavirus to hospital personnel 12. Critically, two separate studies found that frequent mask use in public spaces during the SARS epidemic was associated with a lower risk of infection by the public 13, with one concluding that masks were strongly protective. The authors concluded that “…mask use lowered the risk for disease…” supported community use as a control strategy 14. This study found that intermittent use of masks was associated with a 60% reduction in risk and always using a mask was associated with a 70% reduction in risk of acquiring SARS 14. A systematic review of all retrospective studies of the SARS epidemic concluded that masks were effective in preventing the acquisition of SARS15.

            It is hard to overstate the implications of these findings in the context of the current COVID-19 pandemic. The reproductive number of the virus determines the course of an epidemic. When the virus’ reproductive number falls below 1, i.e. each infected person passes the virus on to an average of less than one other person, the epidemic slows, then dies out. A number of estimates have been published for COVID-19s basic reproductive rate, R0, with a mean of 3.28 and a median of 2.79 16. If it’s spread could be reduced by 70% by the universal use of masks, its reproductive rate would become 0.837-0.984. The effective reproductive rate would drop below zero, and the epidemic would cease. This is a simplistic scenario, but could actually underestimate the effectiveness of masks, because it only assumes reduction in the rate at which people catch the virus, not the rate of transmission.


1 Figure 1 Small Reductions in R.png

Figure 1. A simple model showing exponential growth in an uncontained outbreak over time (generation time = 7 days, R0 = 2.5) and with small reductions in the reproductive rate R. Interventions with small effects on transmission, especially if applied early, can have a large effect on total number of cases.



Models suggest masks could work to control pandemics

It remains possible that masks may have a more limited benefit, perhaps because they are not as effective as suggested by some studies, because people fail to use them effectively, or because of shortages of effective masks such as surgical masks and N-95s. To understand the potential effectiveness of masks it is important to consider them in the context of the impact of surprisingly small reductions in viral transmission rates. For example, consider how epidemics grow exponentially. Allowed to spread, one case of Covid-19 becomes 2.5 (assuming for this model and R0 of 2.5), each case causing 2.5 more, and so on. Over the course of 15 reproductive cycles, each taking 7 days, or about 3 months in total, one case can become 2.5 x 2.5 x 25… or 2.5^15 =   931,323 cases (Fig. 1).


Suppose that the use of masks cut the growth rate by just 10%. Each person infects 2.25 others, who infect 2.25 others, etc. Over 15 cycles, 2.25^15 = 191,751 cases. An 80% reduction. Understanding this exponential growth explains how the virus caught the world by surprise even as the pandemic was monitored in real time. But another aspect of exponential growth is that small decreases in the exponent greatly slow growth. A 10% increase in the exponent can have a massive effect, but even a limited intervention, with a 10% decrease over time, will pay large dividends (Fig. 1).


These are simple models, but more sophisticated modeling studies show large scale use of masks could mitigate, even suppress pandemics. A 2010 study found that above a certain threshold, widespread use of effective masks in the population was able to reduce the reproductive number (R) of an influenza virus below 1, at which point the pandemic would stop 17. If masks were highly effective (well-designed, used properly and consistently), then widespread use of masks could stop a flu pandemic if used by 50% of people. If masks were less effective, then they would stop the pandemic only when used by more than half of the population; if masks were highly ineffective, then they would flatten the curve of the epidemic, but not stop it 17.17. All of these scenarios would be extremely beneficial in the current outbreak.


A second study 18 found similar results in modeling pandemic influenza. The model found that the pandemic was highly sensitive to the proportion of people wearing masks. Even if 25% or 50% of the people wore masks, the number of cases is drastically reduced 18 and flattens the pandemic’s curve. Critically, masks are far more effective when implemented early, than implemented late in the course of an epidemic 18. The study also found that it was important for both infected and uninfected individuals to wear masks 18. These studies support the results of simple calculations: use of masks can make a major difference.


Real world experience suggests masks work in pandemics

Perhaps the most compelling evidence of the potential effectiveness of masks in the fight against COVID-19 comes from the real world. Specifically, countries that are controlling coronavirus epidemics- China, South Korea, Hong Kong, Taiwan, Vietnam, Singapore, Kuwait, and Japan- use masks (Fig. 2).


Correlation is not always causation. In theory, masks could be a signal of an effective pandemic response- an aggressive willingness by the government and public to do everything possible to control the outbreak- rather than a direct cause of suppression. Yet the diversity of these countries, and their responses, argues against such an interpretation. China, South Korea, Taiwan, Vietnam and Singapore differ greatly in their political organization, ranging from communism to democracies, and in their level of economic development. And strikingly, these countries also differ in their suppression strategies. China implemented a lockdown of Wuhan, shut down industry nationwide, implemented temperature checks and social distancing, tested extensively— and employed masks. Korea responded with aggressive testing and contact tracing—and masks. Japan has done far less extensive testing than Korea, but shut down schools and large gatherings— and used masks. The pandemic management strategies used by these countries far more diverse than has been appreciated. Arguably one of the few things all of these successes share is the widespread use of masks. And on the other hand, one common factor shared by the pandemic suppression strategies of the US, Canada, the UK and Europe is the decisionnot to use mask on a large scale, and to discourage the use of masks by the public. These patterns suggest that masks may be an important element of the most successful suppression strategies, but we currently lack the evidence to prove this hypothesis.


Figure 2

Figure 2. Western countries (US, Canada, UK, Europe; yellow, red and orange) versus countries and territories using masks as part of official or de facto government policy (China, South Korea, Japan, Hong Kong, Taiwan, Vietnam, Thailand, Kuwait, in blues).



What kind of mask? Surgical masks as good as N-95s, and improvised masks are better than nothing.

If masks are helpful in preventing the spread of respiratory disease, then which masks are effective? Surprisingly, surgical masks appear to be as effective as respirators in preventing infection. One study randomly assigned surgical masks or N95 respirators to nurses 19; both got influenza at about the same rate. A study of a hospital in Singapore during the 2009 Swine Flu outbreak showed the same 20. Another study tried to isolate influenza virus from infected patients wearing a surgical mask or an N95. Both were equally effective in stopping viruses 21. A recent study randomly assigned health care personnel to wear respirators or surgical masks. Those using respirators fell ill at a slightly higher rate than those using surgical masks (8.2% versus 7.2%) but the difference was not significant 22. Similarly, a systematic review found “limited evidence” that respirators were superior to surgical masks during the SARS epidemic 15.

This has important implications given current shortages. Respirators and surgical masks are in short supply, but surgical masks are cheaper and simpler, which should make it easier to accelerate production. Surgical masks are more comfortable and so compliance might be better, improving effectiveness.

If simpler masks can be effective, this raises the possibility of using improvised masks, homemade, or made in factories, to fill the gap. Would improvised cloth masks work? Research into the effectiveness of cloth masks is limited  23. The research that has been done suggests that homemade masks are inferior to surgical masks, but that they are better than nothing. One laboratory study found a homemade mask was half as effective as a surgical mask in filtering particles 24. Another laboratory study found that homemade masks made from a variety of materials stopped virus aerosols, but not as effectively as surgical masks 25. A surgical mask stopped 90% of viral aerosol particles, a dish towel, 72%, linen, 62%, and a cotton T-shirt, 51% 25. Improvised masks made out of cotton fabric would presumably perform similar to a T-shirt or linen, letting through about 3-5 times as many viral aerosol particles. The only real-world study of cloth masks found that they were less effective than medical masks, consistent with would is expected from the laboratory studies  26. Unfortunately, this study failed to have a proper control – a no-mask group – and so it cannot be used to argue that cloth masks don’t work, because the alternative – no masks – wasn’t evaluated.

Finally, another alternative would be the use of non-medical masks. Although not specifically researched in the context of preventing viral transmission, studies of non-medical masks, including dust respirators and cycle masks, showed that all of the dust respirators evaluated were superior to surgical masks in filtering fine particles, and two of the three cycling masks were comparable to a surgical mask in performance 27.

Clearly, improvised or non-medical masks should only be used when access to N-95 respirators or surgical masks is impossible. However, the speed and spread of the current pandemic have created a widespread shortage of respirators and medical masks. This suggests the need to implement value engineering 28 of the sort that dealt with materials shortages in World War II, and specifically to identify ways to produce filtration comparable to that of medical masks with cheaper, more easily sourced materials and production techniques, and also to find new ways to sterilize, reuse, and/or recycle masks  29.


Arguments against masks don’t hold up


The public doesn’t need them because doctors and nurses do.

The argument that the public doesn’t need masks because doctors and nurses do is logically inconsistent 30. Both can’t be true: masks can’t work for doctors and nurses and be vital to protect them, but fail to be useful to the public. A more logical argument is that doctors and nurses need masks more than the public. This may well be true, and it is of the upmost importance to protect frontline public health and care workers. However, this argument assumes that wearing masks is a purely defensive act. In fact, wearing a mask protects not only the wearer against infection, but also reduces the potential for onward spread from an infected individual. A mask stopping a chain of infection or a super-spreading event, could save many lives, members of the public and also doctors, nurses, and heart attack victims and cancer patients who might otherwise die if health care systems are overwhelmed. Particularly if worn by people who are otherwise likely to either catch or spread the virus, public use of masks will reduce the number of people coming into hospitals. The argument that no one else needs masks isn’t backed up by modeling or real-world evidence. Clearly, ensuring access to masks by health care workers must remain a priority, but it makes sense to issue them to others as well.


Masks are only needed to prevent infected people from spreading the virus. Another argument is that masks are only needed by infected people. The CDC officially recommends using masks by infected persons to prevent influenza and has made the same recommendation for coronavirus. The problem with COVID-19 is that it is difficult to know who is infected. Around half the people spreading show no symptoms, and do not know they are infected 31. This is the point of the lockdown strategy; if it was known who was infected, these individuals could be isolated. Because we don’t, we are forced to act as if everyone is. The same logic can and should apply to masks. In an uncontrolled pandemic, it is logical to act as if everyone is infected; everyone wears a mask. It’s an inconvenience, but less of an inconvenience than shutting down shops and pubs, canceling events, staying away from work, and forced quarantine.


We don’t know enough to act. Admittedly, there are many unanswered questions about COVID-19 and masks, and more studies are needed. But it has been argued that we can’t really do anything because we know so little. This isn’t logical. If one wakes up in the middle of the night to find the house burning down, there are a number of unknowns: how did it start? What’s on fire? How fast is it spreading? Where is everyone? Despite these uncertainties, one immediately takes action. The  same applies in a pandemic; public health workers must act with incomplete data, with gaps and inaccuracies in their intelligence, but never the less must make difficult, life-or-death decisions, because failure to make a decision is often fatal. In a crisis situation, one does not wait for studies.

Asymmetrical gambles. Masks provide a potential upside, with limited downside. Widespread use of masks might help a little, or a lot, but they are unlikely to do much damage. Fears that masks would inspire ‘false confidence’ in their protection are exaggerated. Individuals wearing masks now tend to be those most aggressive in protecting themselves, their families, and society. Far from inspiring risk-taking, masks are likely to send a powerful social signal that the threat is real, and that we need to rapidly and collectively change our behavior.

Masks provide a high potential for an upside with no risk, and assuming they can be produced on a large scale, limited cost. Even assuming it is a gamble, it is arguably a rational gamble to take. The situation is much like Pascal’s Wager. Pascal argued that we should believe in God because if He existed, you’d go to heaven, and if not, one had nothing to lose by believing. Similarly, strategies with uncertain upsides and no downside are worth pursuing, particularly if there is a relatively inexpensive opportunity to help reduce the spread of COVID-19. The alternative, to wait for certainty, would seem a poor strategy to addressing an existential pandemic threat. That being said, as we have laid out above, the evidence for the efficacy of masks is far stronger than has been appreciated.



Strong evidence and arguments exist in favor of the widespread use of facemasks by the public. The principle behind facemasks- they reduce the amount of virus exhaled by infected people, and inhaled by uninfected- suggest that they should be a primary tool in combating a respiratory virus. Scientific research, including experimental studies, retrospective studies of the SARS epidemic, and modeling studies, suggests that they can be effective. Most importantly, the experience of countries that have used masks against SARS and the current coronavirus pandemic imply that they are likely to be highly effective when used by the public. The current shortage of medical masks is the only valid argument against their widespread use. More effort needs to be made to increase the production of facemasks, to find alternatives to surgical masks and respirators, and alternative materials and methods to manufacture facemasks on a large scale. Until this is possible, the use of homemade and improvised facemasks should be explored to protect individuals, and the public. We need more information on the effectiveness and proper design of facemasks, and emergency funding for research into this and other strategies to combat the coronavirus pandemic are needed to allow for informed decision making.





Box 1. Recommendations.


  1. Undertake research into the effectiveness, design, and use of masks

Make funds immediately available to scientists, engineers, and doctors. The granting process should follow a model in which funding rates are high, and approval is rapid. Following the DARPA strategy, large numbers of small grants should be made available, with successive rounds of funding contingent on success.


  1. Research should focus on the following
  • effectiveness of masks
  • design of effective masks, including alternative, easily sourced materials
  • sterilization and recycling of masks
  • feasibility of reusable masks
  • modeling studies for rational strategies for optimal mask allocation and usage to maximize lives saved per mask.


  1. Increase production of masks
  • Convert existing domestic factories to mass production of masks.
  • Encourage small-scale manufacture and sale of masks at home and by small businesses, assuming effective designs and materials are available
  • create partnerships in which developed countries invest in the production of masks abroad where costs are lower, in return for guaranteed access to masks


  1. Promote public awareness of masks
  • campaign to promote public awareness of proper use
  • campaign to change public perception of masks, from something viewed as strange or suspicious, to an act of solidarity and community support







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Evolutionary history suggests intelligent life is rare in the universe

378589main_cepb_1920_full.jpgAre we alone in the universe? It all comes down to whether intelligence is a probable outcome of natural selection, or a fluke. Is evolution repetitive and predictable? If evolution repeats itself, independently converging on similar outcomes, our evolution seems likely. But 4.5 billion years of evolutionary history shows that many key events- not just the evolution of intelligence, but events preceding it, like the evolution of animals, complex cells,  photosynthesis, life itself- were unique. The evolution of intelligence may, therefore, be unlikely. Like winning the lottery, only less likely.

           The universe is astonishingly vast. The Milky Way has over 100 billion stars, and there are around 100 billion galaxies in the visible universe. That’s just the tiny fraction of the universe we can actually see. Huge, possibly infinite gulfs of space exist beyond, far enough away that there hasn’t been time for the light from those stars to reach us. Even assuming Earthlike worlds are rare, the sheer number of planets- there as many planets as stars, maybe more- suggests that a lot of life is out there. So where is everyone? This is the Fermi Paradox: our universe is vast, and old, with ample time for intelligent life to evolve, but there’s no evidence of anyone else out there.

 We can’t study alien worlds to know if they’re likely to evolve intelligence. But what we do have is billions of years of Earth’s history, and we can study the fossils and living things produced by that history, to see where life repeats itself, or doesn’t.


The Tasmanian Wolf (Thylacinus cynocephalus). Although it looks like a wolf, it actually had young that lived in a pouch- because it was related to kangaroos, not wolves. The similarities are an example of convergent evolution.

            Striking examples of convergence do exist in evolution. The Tasmanian wolf was a wolf-like creature that filled the predator niche in Australia, but it has a kangaroo-like pouch. It evolved from marsupial stock- as did marsupial moles, marsupial anteaters, and marsupial flying squirrels. In fact, Australia’s entire evolutionary history, with mammals taking over and diversifying after the dinosaur extinction, parallels other continents. Other striking cases of convergence include dolphins and extinct ichthyosaurs, which evolved similar streamlining to slice through the water, and the birds, bats, and pterosaurs, which each evolved wings and flight.

The squid eye looks and works much like a human eye, but built differently- because it evolved its eyes independently from our own.


You also see convergence in individual organs. Eyes evolved not only in vertebrates, but in arthropods, octopi, scallops, worms, and jellyfish. Vertebrates, arthropods, octopi and bristle worms independently evolved jaws. Legs were likewise invented convergently in the vertebrates, arthropods, and mollusks. In fact, several kinds of fish- tetrapods, frogfish, skates, mudskippers- specialized their fins for walking.

            But here’s the catch. All these examples of convergence exist within just one group of organisms- the Bilateria. Bilaterians are complex animals, with left-right symmetry, a mouth, a gut, a nervous system. Once they evolve, we see different animals evolving similar solutions to similar problems. But the evolution of the bilaterian body plan that makes this possible was a unique innovation- complex animals evolved once in life’s history.

The vertebral column is a one-off event.


The argument that evolution tends to repeat itself makes sense- if you only focus on the places where it does repeat itself, like marsupial wolves, eyes, jaws, and soforth. But it’s a case of confirmation bias. If we only study cases of convergence, we’ll arrive at the idea that convergence is everywhere, and evolutionary history is likely to repeat itself. But if you start looking for nonconvergences- places where evolution hasn’t repeated itself, you’ll find them.

Surprisingly, many of the most critical events in evolutionary history are unique. One is the vertebral column that lets large animals exist on land. But only vertebrates evolved this particular kind of skeleton (curiously, the jointed exoskeleton, which underlies the most diverse group of animals, the insect, is also unique). The complex cells all animals and plants are built from- eukaryotic cells, with nuclei and mitochondria- evolved only once. Sex evolved once. Photosynthesis, which increased the energy available to life, and filled the atmosphere with the oxygen needed by complex animals, is an evolutionary one-off. For that matter, so is human-level intelligence. There are marsupial wolves and moles,  but not marsupial humans.

The eukaryotic cell of amoebas is far more complex than that of bacteria- and humans and other complex, multicellular organisms are made out of eukaryotic cells.

            Furthermore, these unlikely events depend on one another. Humans couldn’t evolve without vertebrae to let fish move onto land. Skeletons couldn’t evolve before complex animals existed. Complex animals required complex cells. Complex cells didn’t exist until photosynthesis filled the world with oxygen. And nothing could happen without the evolution of life: a singular event among singular events. All living things come from a single ancestor: as far as we can tell, life only happened once.

Strikingly, these events seem to take an extraordinarily long time. Photosynthesis didn’t evolve until around 1.5 billion years after the Earth formed, complex cells after 2.5 billion years, animals after 3.9 billion years, and human intelligence, 4.5 billion years later. That these innovations are so clearly useful and yet took so long to evolve implies that they are also highly improbable.

Because so many key innovations are one-offs, it seems likely that  these are fluke events, low probability events that act like a series of chokepoints in evolutionary history. The emergence of intelligence isn’t like winning the lottery- it’s like Earth won the lottery not just once again, and again, and again, and again. On other worlds, such critical adaptations might evolved too late for intelligence to emerge before the sun went nova— or not at all.  Let’s say there are seven innovations that are both critical and unlikely- the origin of life, photosynthesis, complex cells, sex, complex animals, skeletons, and intelligence, each with a 10% chance of evolving. The odds of an Earthlike planet evolving intelligence would be one in 10 million. But such adaptations could be even more improbable- photosynthesis required a suite of innovations in proteins, pigments, and membranes. Similarly, bilaterian animals result from multiple innovations. So maybe such critical, complex adaptations evolve just 1% of the time. If so, intelligent life would appear on just one in 100 trillion worlds. And if highly habitable worlds are rare, we might be the only intelligent life in the galaxy— or the visible universe.

If an infinite number of monkeys hit the keys on a typewriter, one of them writes Hamlet. In the same way, given a seemingly infinite (or for all we know, actually infinite) number of planets, if natural selection plays around with DNA long enough, one of those planets is likely to have a string of lucky breaks and evolve intelligence. What might that world that look like? It would look an awful lot like Earth’s evolutionary history- a series of remarkable and unique innovations like photosynthesis and complex cells.

And yet, we’re here. That has to count for something, right? If evolution only gets lucky one in a trillion times, what are the odds we happen to be on the incredibly improbable planet where it actually happened? In fact, the odds of us being on that lucky world are 100%. Because obviously we couldn’t have this conversation on one of those other worlds that didn’t first evolve photosynthesis, complex cells, or animals. This is the anthropic principle: the history of Earth must have been such that intelligent life could evolve, otherwise we wouldn’t be here to wonder about it.


Do Mass Extinctions Increase Diversity, and if so, how?


Over the past half-billion years, the Earth has been hit again and again by mass extinctions, wiping out most of the species on the planet. And yet every time, life recovered, and ultimately went on to increase in diversity. Is life just incredibly resilient, or is something else going on: could extinctions actually have helped life diversify and succeed? And if so, how?

Mass extinction is probably the most striking pattern seen in the fossil record. Vast numbers of species disappear, rapidly, simultaneously, and around the world. Driving extinction on this scale generally requires some kind of global environmental change, both so severe and so rapid that species disappear, rather than adapt to it. Most extinctions seem to be driven by catastrophes. Massive volcanic eruptions drove the extinctions at the end of the Devonian, Permian and Triassic. Severe glaciation drove the Ordivician-Silurian extinctions. One, the end-Cretaceous extinction of the dinosaurs, was driven by an asteroid. These “Big Five” extinctions get the most attention because, well, they’re the biggest, but other minor, but still civilization-ending events occurred as well, like the pulse of extinction that preceded the end-Permian event, and the Cenomanian-Turonian event that wiped out spinosaurs and carcharodontosaurs among the dinosaurs.

Sepkoski curve.jpg

These events were indescribably destructive. The Chicxulub asteroid impact that ended the Cretaceous shut down photosynthesis for years and led to decades of global cooling. Anything that couldn’t shelter from the cold, or find food in the darkness- which was most species- perished. Dinosaurs and pterosaurs went extinct, as well as most birds, mammals, lizards, snakes, plants, even many insects. Extinctions were equally severe in the sea, with plesiosaurs, mosasaurs, and ammonites disappearing; many invertebrates and even many plankton and other single-celled organisms, like foraminifera, were hit hard. Perhaps 90% or more of all species disappeared in just a few years.

But life bounced back, and the recoveries were rapid. Over 90% of mammal species were eliminated by the asteroid, but they recovered, and then some, within 300,00 years, then went on to evolve into horses, whales, bats, and our primate ancestors. Similarly rapid recoveries and radiations were seen in birds, lizards and fish. Much as the first finches to reach the Galapagos Islands diversified to occupy the vacant niches, the survivors of the extinction found a largely empty world, and diversified to fill it.

This pattern of recovery and diversification occurs after every mass extinction. The end-Permian extinction saw mammal-like species take a hit, but reptiles flourished afterward. When the reptiles were hit during the end-Triassic event, the surviving dinosaurs diversified and took over the planet. Although a mass extinction ended the dinosaurs, they only evolved because of mass extinction in the first place.

Despite this seeming chaos, life has slowly diversified over the past 500 million years. In fact, several observations hint that extinction might actually drive diversity. For one, the most rapid periods of diversity increase occur immediately after a mass extinction. But the recovery isn’t just driven by an increase in the number of species. During a recovery, animals innovate, finding new ways of making a living.

They exploit new habitats, new foods, new modes of locomotion. Our fishlike forebears first crawled onto land after the end-Devonian extinction. Giant dinosaurs, larger than anything that had existed before, appeared after the end-Triassic extinction. And when those dinosaurs then disappeared, whales took to the sea, bats to the sky, horses started eating plants, and our primate ancestors became specialized for life in the trees. Extinction doesn’t only drive speciation, it drives innovation. It’s not a coincidence that the biggest pulse of innovation in life’s history- the appearance of complex animals in Cambrian Explosion- occurs in the wake of the extinction of the Ediacaran animals that went before them.

Innovation may increase the number of species that can coexist because it allows species to move into new niches, instead of simply fighting over the old ones. Fish crawling onto land didn’t compete with fish in the seas; bats hunting in the dark with sonar didn’t compete with day-active birds. Innovation means diversity isn’t a zero-sum game; species can diversify without driving others extinct. So why does extinction drive innovation?

It may be that stable ecosystems hold back innovation. A modern wolf is probably a far more dangerous predator than a Velociraptor, but a tiny mammal couldn’t evolve into a wolf in the Cretaceous because there were velociraptors. Any experiments in carnivory would have ended badly, with unspecialized mammals competing with- or simply eaten by- the well-adapted Velociraptor.

But the extinction of that Velociraptor gives the mammal the freedom to experiment with new niches. Initially, mammals were poorly adapted to a predatory lifestyle, but without dinosaurs to compete with or eat them, they didn’t have to be very good to survive. They flourished in an ecological vacuum- ultimately becaming large, fast, intelligent pack hunters. In the lull after an extinction, evolution may be able to experiment with designs that are initially poorly adapted, but which have long-term potential. With the show’s stars gone, evolution’s understudies finally get their chance to prove themselves.

Life will even recover from the current wave of human-induced extinctions, perhaps reach even higher diversity given time. That’s not to justify complacency- it will take millions of years.

Economists talk about creative destruction, the idea that creating a new order drives the destruction of the old one. But evolution hints there’s another kind of creative destruction, where the breakdown of the old system creates a vacuum, and destruction actually precedes and drives the creation of something new, and often better.

This idea has relevance to human history. The extinction of the Pleistocene megafauna must have decimated hunter-gatherer bands, but it may have given farming a chance to develop. The Black Death produced untold human suffering, but the shakeup of political and economic systems may have led to the Renaissance.

That’s perhaps worth bearing in mind given our current political and social upheavals. The loss of something old creates opportunity for  new things to take root. Every cloud has a silver lining, even the debris cloud from an asteroid impact. It’s not just that there are opportunities when things are bad. It’s that when things are at their worst, is precisely when the opportunity is the greatest.

The Dinosaur Microbiome

nick longrich hat and bone
Dinosaur Bone in Dinosaur Provincial Park, with Estwing and Stetson

Life lives pretty much everywhere. In fact one of the most astonishing biological discoveries of the past century has been the discovery that the biosphere extends much further than we realized- into hot springs and hydrothermal vents, below Antarctic ice sheets… and miles underground. We tend to think of the subterranean world as being the realm of geologists, but in fact it’s the largest ecosystem on the planet- extending deep below the surface of the earth and the floor of the ocean.


So what does this have to do with paleontology? In recent years, we’ve seen some startling claims published about organic material preserved inside dinosaur bones- protein and protein-derived materials, DNA, even cells and tissues like blood vessels. Contamination is always an issue with these kinds of studies. Techniques designed to detect small amounts of DNA or protein from 75 million years ago must be incredibly sensitive- that also means that they’re sensitive to picking up biological material from you, the bacteria on your hands, a specimen in the lab, or what you had for lunch. As a result, rigorous protocols have been put in place to deal with contamination. But what if the contamination starts before the bone has even left the ground?

Spotting a bone. Note the bright orange stuff- it’s a species of lichen.

Doing fieldwork in Dinosaur Provincial Park, in Alberta, I was always struck by the fact that a bright orange lichen sometimes grew on the bone- it was common enough you could actually use it to help find the bones in the field, like they were sprayed with dayglo orange paint. The lichen seemed to like the bone for some reason. For one thing, the bone was stable, whereas the muds and sandstones eroded away, so the lichen couldn’t grow there. It also had lots of moisture inside its spongy internal structure. And it was full of minerals, which a growing lichen needs. Living things actually used bone as a microhabitat. What’s more, when you peeled back the rock to expose the bone, you’d sometimes find sagebrush wrapped around it, probably feeding off the minerals. Which raises the question- why would we assume that bone is sterile? Why wouldn’t it be shot through with microbes- just the way the rocks around it are?

Orange lichen growing on a dinosaur bone

In a new study, my colleagues and I (to be honest, almost entirely my colleagues) have been able to isolate DNA and amino acids from a dinosaur bone, dug up from Dinosaur Provincial Park in Alberta, in what must be one of the strangest dinosaur digs ever undertaken- since we weren’t hunting dinosaurs at all, but microbes living inside them. We excavated a bonebed of the dinosaur Centrosaurus, sterilizing tools with bleach, alcohol, and a blowtorch to prevent the introduction of contaminating bacteria, wrapping the bones in sterilized aluminum foil as soon as they were exposed, and then freezing them for later analysis in the lab. What we were able to show is that the organic material in these bones not ancient.
The DNA comes from bacteria. The most common thing is called Euzebya which is (don’t ask me why) found in odd places like Etruscan tombs and the skin of sea cucumbers. What’s more, the bacteria in the bone are at high densities- about ten times that of the surround rock. Bacteria don’t just work their way into the bone, they actively prefer living there.

longrich Centrosaur hook.JPG
The hook off of a Centrosaurus frill. This bone isn’t dead, but home to a community of microbes. 



The amino acids also show biological activity, since they’re mostly left handed. If you remember intro biology, the chemistry of all living things has a handedness. The individual amino acids that make up proteins can be in either a left-handed form, or a mirror-image, right-handed form. Living things exclusively make and use left-handed amino acids, but after they die, the amino acids slowly flip their symmetry back and forth, to convert to a 50:50 mixture of left- and right-handed forms- a process called racemization. Racemization takes thousands, not millions of years. So if we have amino acids from dinosaurs living millions of years ago, they should be fully racemized, a 50:50 ratio of left and right handed amino acids. But since the amino acids we found are mostly of the left-handed variety, that means the biological material is mostly very recent, not from the Cretaceous.

We also carbon-dated the material. Our atmosphere is full of trace amounts of carbon 14, a highly radioactive element with a half-life of around 6,000 years- that is, half of it undergoes radioactive decay and breaks down every 6,000 years. When plants and other organisms use C02 to build things, they (and anything they eat) incorporate that c 14. After death, they stop taking in new carbon 14, and the amount left halves every 6000 years or so. The upshot is that we can figure out how old stuff is by the amount of C 14 remaining- up until a limit of 50,000 years, at which point its basically all gone. Since dinosaur bones are more than 50,000 years old, any carbon 14 from the dinosaur era incorporated into DNA and proteins would have long ago disappeared. But we found a lot of carbon 14- meaning most or all of the material we isolated wasn’t from dinosaurs, but from recently living things.

All this points in the same direction- the bones are in a very real sense alive. They host a thriving bacterial community, a microbiome, and the amino acids and proteins we isolated were primarily recent. Some of the material is a bit older- there are some right-handed amino acids, and the carbon 14 ratios are a bit lower than you’d expect on a modern living thing- so it does seem that there are some older organics in there. But whether they are thousands of years old, or millions, we don’t know.

What does this mean for dinosaur DNA and proteins? Well, it’s too soon to say, but it’s one more reason for skepticism. Currently, the record for the preservation of DNA is a genome extracted from a 750,000 year old horse. DNA has a relatively short half-life, about half the DNA breaks down every 500 years, so it’s thought to become unreadable after about 1.5 million years even under optimal conditions. It’s extremely unlikely we can get Jurassic DNA from 150 million years ago, or even DNA from the much younger T. rex, which went extinct a mere 66 million years ago.
Proteins appear to be more robust than DNA, however, protein also tends to break down over time, with water molecules breaking apart the bonds between amino acids. The oldest confirmed proteins are about five times older than the oldest DNA: protein sequences from ostrich eggshell have been recovered going back almost 4 million years. Which is not to say that organics can’t survive from dinosaurs- we’ve been able to isolate pigments, for example- but we need to be a little skeptical. How and whether things like cells and vessels can survive that long isn’t entirely clear.

Clearly some organics do survive- actually a lot of stuff. Sporopollenin, for example, is a biopolymer that makes up the wall of pollens and it is an incredibly robust molecule. It’s nigh-invulnerable, to the point that you can dissolve rock in concentrated acids and all that remains are little 75 million year old pollen grains from the Cretaceous. So we can get pollen grains from dinosaur age sediments. We can also recover organic-walled fossils like dinoflagellates.

Personally, I’m mostly a morphologist- I don’t have the expertise to critically evaluate finer points of debates about organic chemistry. But I can say that extraordinary claims require extraordinary evidence. The claims of DNA and protein isolated from dinosaurs are far, far older than anything else. Importantly, these finds have been difficult to replicate. And there’s a lot of contamination down there. That some kind of heavily altered protein- basically fossil protein– might remain seems more plausible. I’m not saying it doesn’t exist. I don’t know. But there’s a lot of other stuff down in the subsurface community- not just bacteria, but archaeans, eukaryotes, multicellular organisms like fungi, nematodes. Before we identify ancient traces, we need to be able to rule out contamination, and contamination is going to be everywhere. But by getting a better handle on what that contamination looks like, we can at least do a better job telling the contamination from the original organics.

But  I also think the microbiome is fascinating in its own right, not just an annoying contaminant. It’s interesting to think about what it says about the evolution of life on our planet. It’s bizarre to think, but dinosaurs didn’t stop interacting with the environment when they died, they just did so in a different kind of way. After that Centrosaurus died, it was food for scavengers- tyrannosaurs, dromaeosaurs, pterosaurs, beetles and flies. Then the skeleton was buried underground, and bacteria continued to eat away at the proteins. After thousands of years, it was part of the subsurface microbiome. Then the Western Interior Seaway rolled in, and it was part of a deeper microbiome existing under the bottom of the ocean. Then it rolled back out again after the dinosaurs died out, thousands of feet of sediments were piled up, and the Centrosaurus bone was part of a deep biome, far underground. At last, the glaciers came through, ground the land down, and an ancient glacial lake burst, gouging out the badlands that would become Dinosaur Provincial Park. From the time that Centrosaurus died up until now, it has seen a succession of microbial communities- on a Cretaceous floodplain, then underneath it, then under a sea, then under a glacier, and at last, just below the prairies of Alberta. It’s amazing to think that the life of the past still affects the life of today- as Faulkner put it, the past isn’t dead, it isn’t even past.

nick longrich - more lichen bone
A piece of fossil wood hosts a diverse community of lichens. 

There’s a good writeup of the paper here in the Atlantic by Ed Yong.

The original paper is here: Saitta, E.T., Liang, R., Lau, M.C., Brown, C.M., Longrich, N.R., Kaye, T.G., Novak, B.J., Salzberg, S.L., Norell, M.A., Abbott, G.D. and Dickinson, M.R., 2019. Cretaceous dinosaur bone contains recent organic material and provides an environment conducive to microbial communities. eLife8, p.e46205.