“Existential threats”, radiation genetic damage and biological resilience

This essay will focus on a detailed matter of biology – the domain of this newsletter – but I begin it with a seemingly very different topic, the question of whether the human species faces a near-term “existential threat” to our existence.

Thinking about the end of Homo sapiens is not a pleasant thing. One accepts that, as individuals, we all die at some point. Considering the possible death of our species, however, is much harder. Still, there is a point in doing so: the more people who consider our possible end, the greater the chances that steps will be taken to reduce its chances or at least improve the odds of human survival.

Let us consider the most likely terminating catastrophes. I think that there are three likely candidates (even one is more than we want): nuclear war, extreme global warming, and a giant asteroid from outer space colliding with Earth. (I leave aside the threats from AI and the idea of a global pandemic that wiped out all humans.) Which, of these three possibilities, might terminate our species?

Of course, the answer is unknown because it would depend on the severity of the event. All three catastrophes can be regarded as “risk factors” (see no. 9 in this series) but the degree of risk depends on the magnitude of the destructive element. My belief, as explained below, is that none of these threats is likely to eliminate our species or, even worse, all animals and plants on Earth.

In principle, a sufficiently massive asteroid might end human existence on Earth. The biggest one we know of was the one that ended the Cretaceous and countless animal and plant species. It is estimated to have been “only” six kilometres in length. Nothing that large or larger, however, has collided with Earth in the last 66 million years, nor, as far as we know, earlier (though it is possible since there were other mass extinctions in the 550 year history of complex life on Earth). The end-Cretaceous event itself did not end animal life on Earth; many species survived. Hence, I think we can downgrade the Killer Asteroid to be an unlikely existential threat. (It was, incidentally, the subject of a fictional film last year “Don’t Look Up!” but the actual purpose of the story was to be a cautionary tale about the dangers of denying the threat of climate change.)

What about nuclear war and global warming as existential threats? Again, if either were severe enough, it could be terminal for us. Take global warming first. On the face of it, it would be a lesser catastrophe, or at least a less dramatic one, and would unfold over several decades. Indeed, one can make the case that it has already started to and will only get worse, unless curtailed by appropriate actions now. The fact that steps are being taken, though insufficient at present, already reduces the risk somewhat.

Nevertheless, the eventual death toll from climate change brought on by global warming threatens to be huge. These would result from extreme weather events, forced migrations due to the loss of coastal settlements all over the world, and not least, from great reductions in agriculture world-wide. Many years ago, James Lovelock, the father of the Gaia concept, estimated that severe global warming might reduce the world human population to one billion, probably preferentially clustered in the higher latitudes in the Northern Hemisphere. This would, of course, be a disaster for our species and the life of the planet and its consequences would roll on as a continuing assault on our world for decades, centuries or millennia.¹

If we eliminate Killer Asteroids and catastrophic climate change as unlikely to terminate our species, what about nuclear war as an existential threat? It has had that potential since the first atomic bomb test explosion at the Trinity test site in New Mexico on 19th June, 1945, and was understood to have that potential then, at least by the more thoughtful scientists who were involved in the Manhattan Project. ²

If a large fraction of the thousands of nuclear weapons possessed by the US and Russia were to be detonated in a war between those two countries, the immediate reduction (within days or a few weeks) in human population would be vast, in the hundreds of millions or even several billion. (Remember that about 50% of all people now live in cities, which would be the main targets). This would be followed over months with perhaps comparable numbers as food supplies ran out and people died of radiation poisoning. An all-out nuclear exchange might be the end of humanity or close to it. My guess, however, is that in any nuclear war, the exchange of missiles and resulting deaths would terminate much sooner – the first strikes might grossly interfere with later ones – with only a fraction of the available nuclear weapons used. This would still be an unimaginable catastrophe for humanity, unlike anything before, even if a large fraction of the people survived the initial exchange.

There are two major, and very real, dangers to the survivors of any nuclear war. The first is that of a “nuclear winter”. A nuclear winter involves the cooling and planetary reduction of sunlight and warmth produced by the smoke that would be unleashed by the fires ignited by a nuclear war. The world has been appropriately horrified by the fires that global warming has already triggered in so many parts of the world in the last three to four years: areas in Canada, the United States, Australia, southern Asia, Brazil and Chile, parts of Europe, even Siberia. But the pall of smoke enveloping the world unleashed by a nuclear winter would make those fires seem trivial, in terms of their consequences. Apart from horrendous levels of air pollution a nuclear winter would involve, it would undoubtedly collapse world food production. The result of that, in turn, would be massive starvation over the following months, as crops died and food stores ran out. In turn, the surviving animals would be largely exterminated for food by starving humans.

The possibility of a “nuclear winter” was apparently unrealized for several decades after nuclear weapons had been invented, judging from the lack of published commentary. It was brought to prominence in an influential article in Science magazine, published in 1983, which also introduced the term.³ This article also explained that the mass extinction event that ended the Cretaceous Era was almost certainly caused by the reduction in heat and light that would have ensued from the smoke of the fires following the event. Yet, the survival of so many animal and plant species after the end-Cretaceous asteroid indicates that even a nuclear winter would probably not put a direct end to the existence of complex life on Earth. What would make a true nuclear winter different from the global pall of smoke that almost certainly followed the end-Cretaceous event is the scale and numbers of fires that a nuclear exchange would trigger.⁴

Yet, horrific as a nuclear winter and its consequences would be, the ultimate danger to the long-term survival of the human race would be the radioactive nuclear fall-out that would inevitably follow a nuclear exchange. Again, it is impossible to estimate how bad the effects would be because their magnitude would depend on how many nuclear bombs were exploded, their total megaton yield, the location of the blasts, the directions in which the fall-out spread, which would depend on the weather patterns that would ensue, and other factors. The only certainty is that the amount of radioactive fall-out would be huge.

Nevertheless, there is some information that is relevant and it comes from two events that can be regarded as “experiments”. Neither was a deliberately designed scientific experiment and the second was simply a major and unintended accident but, nevertheless, it was also a kind of experiment. The first was the atomic bombing of Hiroshima and Nagasaki in Japan in 1945; the second was the explosion of the Chernobyl atomic plant in the Ukraine in 1986. In both cases, the information is not only highly interesting but has a distinctly surprising element.

The atomic bombing of Hiroshima and Nagasaki in August, 1945 was of course

not carried out to obtain scientific information about the effects of nuclear war on human beings. It was done to end the war in Japan speedily and to give a spectacular 9999999demonstration of the power of the new weapon, especially to impress the Soviet Union, which was already being sized up as the new enemy (though it was still, as WWII ended, formally an ally). Once the bombs had gone off, with approximately 200,000 killed and many thousands severely injured, however, it was realised fairly quickly by geneticists and medical people that a study of the survivors could yield immensely valuable information about the effects of radiation on human beings.

Of course, there was some information about radiation effects on living organisms at this point. These were the fruit fly, mice and a plant, maize. They were employed in proper experiments carried out in laboratories, with controls, and involved testing for genetic effects of increasing doses of radiation, in particular X-rays. In particular, it had been shown first in Drosophila (1927) and soon after in maize that X-rays caused permanent genetic changes or “mutations” and that these were of two broad classes, small or “point” mutations, and larger ones that involved bigger changes, “chromosomal rearrangements”. The latter could be seen in stained chromosomal preparations under the microscope. Although these early studies did not emphasize the biological harms caused by such genetic changes, the presumption was that such changes would be harmful to the organism.

With the atomic bombing and destruction of Hiroshima and Nagasaki, more information, specifically about the effects of radiation on the survivors of those attacks, could be obtained. Of course, it would have to be collected in a systematic manner, and compared to suitable control populations (those not close to the atomic explosions). Within a year, the groundwork for doing so had been laid in the US. There were two data sets of special interest. First, what were the cancer rates in the survivors? Were they much higher than in non-exposed populations? Second, were there many more mutations in the children of the survivors? Here, the special interest was in children who were conceived after the atomic blasts. Mutations could not be directly measured at that time but if many were induced by radiation in the reproductive cells of women and men survivors, this should show up as reduced life-spans in the children of those survivors.

The earliest data to be collected concerned rates of cancer development in survivors, people directly exposed to the radiation but who had neither been burnt to death (or vaporized) near the centre of the explosion nor died from injuries in the weeks or months immediately following the bombings. It was possible to calculate from the size of the blast, the physics involved, knowledge of the kinds of radiation produced, and the location of individuals relative to the “hypocentre” of the blast when the bombing took place, what radiation dose each survivor had likely received. One could therefore plot the cancer rate amongst survivors as a function of the estimated dose and the time elapsed since the blasts.

As expected from what was known about radiation effects even in the 1950s and ‘60s, there was a measurable increase in cancer rates amongst the survivors. Also, unsurprisingly, it was a function of the dose of radiation received. The surprise was in the relatively small increase in the excess cancers attributed to radiation exposure due to the blasts. For the highest doses, and for solid tumours specifically, it was close to 30% excess, but less for lower but still significant doses. Deaths from leukaemia were higher but for both kinds of cancer, fewer than expected. For some of the kinds of solid tumours, the increase was only a few percent. What about effects on the lifespan of the survivors? Was it dramatically shortened? No, it was not. For the higher doses received by survivors, lifespan was shortened on the order of a year. In short, those who survived and did not have major injuries did not have greatly increased cancer rates or greatly decreased life spans.

Did the children of survivors suffer a greatly increased mutation rate, as indicated by a decreased lifespan? The evidence again indicates, at most, a very small reduction in lifespan for individuals, nor were there other visible signs of effects on them from their parents’ exposure. An urban myth is that many were born with malformations, as a result of the radiation exposure of one or more parents. The actual statistics, however, show that this is not true. There was no significant increase in malformations in the children of the survivors. The data on this is solid. In further genetic studies using DNA technology available before the advent of cheap, massive sequencing methods, there was again no evidence for increased numbers of mutations in the children of atomic bomb survivors.

And here we come to the second “experiment”, the explosion at Chernobyl. Here, due to the advances in sequencing technology, there is actual DNA sequence information on mutation rates in radiation survivors. The study was of the DNA sequences of 130 children born to people, both men and women, who were part of the clean-up teams at Chernobyl, who were exposed to high doses of radiation, which could be estimated afterwards from the locations they had worked in.

The genetic work involved the complete sequencing of the parents’ DNA and the child’s, hence a “trio” of results. Any change in the DNA sequence found in the child’s DNA not present in the mother or the father must have been a new (or de novo) mutation. There were about 50-100 de novo mutations per child, about 75 on average. That may sound like a lot but in fact, it was about background level, what one would expect from spontaneous “point” mutations, which occur at the rate of 10-8 per generation (or about 10-10 per cell replication). Remember that in each human cell, there are two set sets of genomes, each at 3 x 10⁹ base pairs. The observed “excess” new mutations were well within the expected range of spontaneous mutations.⁵

There was another kind of evidence from the Chernobyl accident that indicates that there was no major heritage of genetic damage from the explosion. It is more anecdotal, however, and concerns the dogs that are the descendants of the hundreds of pet dogs abandoned by people in the nearby town of Chernobyl city, when they were forced to evacuate immediately after the accident. This was the town where many of the workers at the atomic power station had lived and the priority was saving the people.

Many of the abandoned dogs probably died from radiation poisoning after the explosion but many survived and stayed in the area, and then bred amongst themselves over the following year or so, and their descendants then did likewise. The result has been a thoroughly mixed breed population of dogs that has developed over time in the 38 years since the explosion. They tend to associate in packs, and show the hierarchical relationships typical of dogs in packs, with leaders and followers. They remain sufficiently domesticated, however, to be friendly and affectionate to the people from animal help organizations who come to help them. The area itself remains contaminated with hot-spots of radioactivity that human visitors must avoid but the dogs of course do not understand such restrictions. Needless to say, there are no statistics on the mortality of those that wander through these highly radioactive areas. The dogs also retain certain higher-than-normal levels of radioactivity in their bodies but there are no systematic studies of this. Visitors who pet them are advised to wash their hands after such visits.

The key point is that a population of dogs has been living in a highly radioactive area for nearly 40 years without dying off. This information does not make up a proper study, with hard conclusions about the animal’s health, their longevity, the levels of radioactivity in their organs, etc., but the fact of a viable, stable population of animals in a highly radioactive area is surprising and must have biological significance. (There are of course, various kinds of wildlife, including various birds, in non-trivial numbers remaining in the zone but we have even less information on their state of health, longevity, etc.)

How might we interpret this high level of, let us call it “radiation resilience” amongst both the atomic bomb survivors, their children, the children of the Chernobyl workers, and the dog population of Chernobyl? It is a puzzle but I would tentatively offer two ideas.

First, there is the phenomenon of DNA repair. This is the ability of living organisms to correct errors in DNA sequence and only began to be understood in the 1960s. There are many kinds of DNA errors and, correspondingly, many modes of DNA repair. The basic forms probably evolved in very early cells on Earth, literally two to three billion years ago, when the atmosphere was very different and much thinner, allowing much more radiation, especially ultraviolet irradiation, to bombard the Earth’s surface. All organisms today have DNA repair mechanisms and long-lived animal species almost certainly have more robust repair systems than shorter lived animals. Hence, humans probably have more DNA repair capacity than mice, for which the original estimates of radiation lethality were the first source of information.⁶

In addition, several of these forms of repair increase in response to damaging agents; they are said to be “induced”. Possibly, both in the atomic bomb survivors in Japan and in the people and dogs of Chernobyl, such DNA repair systems were induced and contributed to the ability to survive higher than normal levels of radioactivity.

The other mechanism that might contribute is elevated abortion rates of damaged embryos. There are, as far as I know, no statistics on miscarriages of the women survivors of Hiroshima and Nagasaki but it is possible that many of their embryos or foetuses suffering radiation damage, might have spontaneously miscarried. The normal miscarriage rate in humans is about 30% and most such events take place without being detected by the women carrying those conceptuses. Perhaps something similar has operated in the female dogs of Chernobyl or indeed in the women workers who were part of the clean-up teams.

None of this fragmentary and partial good news about radiation resilience should promote any degree of complacency about the aftermath of a nuclear war. Any such war would be an unprecedented catastrophe for the human race and the world, even if it were a relatively “limited” one. Nevertheless, the findings from Hiroshima and Nagasaki and Chernobyl provide some hope that human beings, perhaps most mammals and even vertebrates more generally, have a certain biological strength in dealing with radiation poisoning. Scientists should investigate the basis of radiation resilience and learn more. Perhaps one result would be better therapies for radiation poisoning.

Above all, we should think more about how to ensure that nuclear war never takes place. The Bulletin of the Atomic Scientists now places the hands of its “doomsday clock” at 90 seconds to midnight. It has never been that close, which should serve as a real wake-up call to the world to take serious steps to lower the danger of nuclear war. The matter is urgent. And, while we are thinking big about this topic, shall we also try to do more to reduce the chances of catastrophic climate change?

(Note: I thank Bertrand Jordan for reading an earlier version of this article and providing helpful comments.)

---

Supplementary reading:

1) Turco, R.P. et al. (1983). Nuclear winter: global consequences of multiple nuclear explosions. Science 222: 1283-1292.
The article that introduced the term “nuclear winter” and which explained what the end-Cretaceous mass extinction and a nuclear war might have in common.

2) Jordan, B.R. (2016). The Hiroshima/Nagasaki survivor studies: discrepancies between result and general perception. Genetics 203:1505-1512.
An excellent short review of the studies of the atomic bomb survivors in Japan and their surprising conclusions.

3) Yeager, M., et al. (2021). Lack of transgenerational effects of ionizing radiation exposure from the Chernobyl accident.
Science doi: 10.1126/sciencealg2365
A landmark genetic study of the children of the members of the clean-up teams of the Chernobyl accident, who were exposed to high levels of radiation. The conclusions are fully concordant with the studies of the children of survivors of the Hiroshima and Nagasaki bombings.

4) Wu, K.J. (2023). There’s something odd about the dogs living at Chernobyl. The Atlantic Monthly (March, 2023). www.theatllantic.com/science/archive/2023-chernobyl-dogs
A short article about the dog colony that lives half-wild, half-cared for, on the Chernobyl site. There is also an excellent You Tube video about the dogs of Chernobyl.

1. There is one form of global warming, however, that would probably terminate all animals and plants on Earth: a “runaway greenhouse effect”. This would occur if the process of heat-trapping in the atmosphere entered a long positive feedback loop, in which the greenhouse gases (carbon dioxide and methane) accumulated in the atmosphere to the point where the resulting warming evaporated the oceans, creating a blanket of gases and water vapour that trapped the heat. The planet Venus, with its dense cloud cover and atmospheric temperature in the hundreds of degrees, far above the boiling point of water, seems to be an example. How it happened on Venus is unknown.

2. See the film “Oppenheimer”, released last year, or the book “American Prometheus” by Kai Bird and Martin Sherwin (2005) upon which the movie was based.

3. This idea of how the end-Cretaceous mass extinction might have taken place was developed by a physicist, Luis Alvarez, and his son Walter, a geologist. Luis Alvarez had been one of the young physicists who had worked on the Manhattan project to develop the atomic bomb.

4. There is now strong evidence that a sequence of volcanic eruptions in the Deccan traps in Asia happened about the same time as the asteroid and contributed to the pall of smoke that contributed to the mass end-Cretaceous extinction.

5. The sample size of 130 children might sound small but the actual sample was the set of the DNA sequences in these children, hence about 2 x 3 x 109 x 130 total base pairs, hence a huge sample size to measure the mutation rate.

6. There are several pieces of evidence that robust DNA repair systems help ensure a long life. Amongst them is the finding that the genome of the elephant, an animal with a long life-span, has multiple copies of a gene called p53, an essential DNA repair gene. There is also the fact of Werner’s disease, a condition of rapidly accelerated ageing in human beings, which is caused by a defective DNA repair gene.