Yo, Dr. Zoomie! I saw this story in my feed saying there was a star that blew up close to the Earth a few million years ago and the radiation from the star caused evolution to speed up. Something about viruses mutating or something. Can this really happen?
Well…maybe? But I’ll need to take a look at the paper before I can do much more than guess about the likelihood that this might have actually occurred. So let me start with some background information that applies regardless, then I’ll find and read a bit so I can talk about the rest.
So let’s start with what happens when stars explode and why we have reason to believe that Earth has been “close” (from an astronomical standpoint, less than several tens of light-years) to an exploding star in the “recent” (from an astronomical and geologic standpoint) past.
Large stars burn hot and die young and the larger the star the hotter it burns and the shorter its life. Stars that explode are those that are more than a few times the mass of our Sun, and they usually burn out and blow up after only a few to several tens of millions of years. There are two primary mechanisms for causing a star to explode catastrophically, both with similar endpoints:
- Stars that are a few times as massive as the Sun and that are in a binary star system can, at the end of their lives, shrink to form a white dwarf star circling its companion. If the white dwarf is close enough to the companion, or if the companion swells to become a giant or supergiant star, the white dwarf will be able to capture some of the mass of the companion star. Capturing enough mass can trigger a cataclysmic collapse resulting in a massive explosion that blasts the outer layers of the star into space.
- Stars produce energy by fusing hydrogen to helium, with helium “ash” collecting in the core of the star. As the star ages the helium ash collects in the core until there’s enough for it to start fusing to form carbon. This continues with carbon fusing to form silicon and silicon fusing to form iron, but when iron fuses, instead of releasing energy, energy is absorbed. When this happens, the energy from fusion drops precipitously. Since this energy is what helps to hold up the star’s outer layers, when the energy vanishes the outer layers of the star come crashing down from all directions, rebounding into space in a massive explosion that leaves behind a neutron star or black hole while blasting the remnants of the star into the galaxy.
- In both cases, the debris from the supernova includes elements all the way up the periodic table, including tungsten, lead, uranium, and plutonium.
- The energy from these explosions goes into producing neutrinos, light (mostly visible light and ultraviolet), and the kinetic energy of the debris that’s blasted into space.
- And many supernovae produce scads of radioactivity – we’ve found traces of Pu-244 and Fe-60, both produced in supernovae, in deep-sea sediments; in addition to that, we can see gamma radiation from the decay of Ni-56 andCo-56 that lasts for a few years.
- Finally, we currently receive about
- 3 mSv (300 mrem) annually from natural background radiation;
- 2 mSv of this is in the form of radon inhalation, which doesn’t really apply to viruses.
- For humans, background radiation is thought to be responsible for around 1-5% of the DNA damage we undergo on a routine basis;
- it takes a whole-body radiation dose of about 1000 mSv (100 rem) to double the background DNA damage from all sources.
So…now that I’ve time for a quick read, let’s take a look at the paper. I’m not going to get into the calculational details (which, to be honest, aren’t very exciting) or the basic physics – the authors are more capable of getting those right than I am. But when it comes to the radiation biology, I think I can add something. And let me cut to the chase – the radiation exposure – because if we’re talking about radiation-induced genetic damage, the only question that really matters is how much dose an organism receives. They postulate that the event that left a trace of Fe-60 in the sediments of the lake would have produced a dose of about 2-10 mSv annually over a period of 10,000 years, depending on the distance to the supernova (the researchers identified two candidate supernovae at 70 and 140 parsecs (1 pc = 3.26 light years). They also note that studies of people living in areas with high levels of background radiation seem to experience a higher level of double-stranded DNA break above radiation levels of about 5 mSv per year.
Here’s the thing – a few things, actually.
First, there’s a difference between irradiating viruses and humans or other complex organisms. For example, radiation – even ionizing radiation – can’t cause harm unless it deposits energy in an organism; energy deposition is what causes the ionizations that start the sequence of events leading to DNA damage. Humans, mice, lizards, and other complex organisms are larger than viruses – radiation that would interact inside a multicellular organism will pass right through a virus without depositing any energy at all and inflicting no damage at all. And since viruses are so tiny, not much radiation will go through a virus to start with. This is one reason why microbes are so very much more resistant to radiation than we are – they provide such a tiny target that it takes a much higher dose of radiation to deposit enough energy to alter or to kill them than is the case with us.
Second, viruses have different types of genetic material, some of which is easier to damage than the others. Some use DNA (of these, some use DNA with only one strand while others use the same double-stranded DNA that we do) and some use RNA – the authors of this paper don’t tell us what type of genetic material is used by the viruses in the lake.
But most importantly, when I looked at the references they cited, including the one that I assume sparked their interest that led to this paper, it turns out that the viruses that were studied were those infesting cichlid fish in Lake Tanganyika in Africa.
The thing is, these fish are famous (well, in evolutionary biology circles, anyhow) for having experienced a huge amount of diversification, inhabiting just about every ecological niche that this huge and deep lake has to offer. It makes sense that viruses inhabiting a large lake with a large and diverse biota would exhibit a rapid rate of evolution – especially in an area that is, itself, undergoing geologic evolution for the last few tens of millions of years.
The fact is that there are other more likely explanations for the rapid evolution of these viruses that were not considered by the authors of this paper:
- If fish in the lake are still evolving as they compete for and occupy new ecological niches then their pathogens would be expected to diversify as well.
- If new species were introduced to this lake their viruses could evolve to infest the existing fish, and vice versa.
- Environmental factors and stresses (e.g. volcanic eruptions in a geologically active part of the world) can also induce viruses to evolve rapidly.
In some ways this paper reminded me of the saying that “if the only tool you have is a hammer, all problems look like nails.” A group of astronomers looking at what seems to be a curious problem can’t be faulted for using the tools with which they are most familiar. But I really wish they would have given even cursory attention to the genetics, biology, and other factors that are at play. Or that they’d brought in a coauthor with training and experience in those areas. Because of that, this is an intriguing paper that arrives at a believable hypothesis that is simply less plausible than other, simpler explanations.
Header Image Attribution: Artist’s impression of supernova 1993J by NASA, ESA, and G. Bacon (STScI), licensed under CC BY 4.0 via ESA/Hubble.