Mushrooms, Global Warming, and the next Pandemic

How climate change may be driving the emergence of the next great extinction event

One of the more subtle effects of the coronavirus pandemic has been the everyday normalization of getting your temperature taken. Whether it’s at the door to your doctor’s office or your local barber shop, the last year has been an all time high of thermometer-forehead interactions. “98.6, you’re good to go!”, “98.6, have a great day sir!”, "98.6, and if you’ll just follow me…” the number has become a sort of greenlight for transactions, familiarly drilled into your head like the chime of a debit card being approved at the grocery store. But have you ever stopped to wonder why our bodies seemed to settle at that temperature?

The answer lies 66 million years ago, in the aftermath of the meteor that killed the dinosaurs. In a theory proposed by Arturo Casadevall of John Hopkins University. (You can read his paper on the subject here). This is the very moment when mammals took the throne from reptiles and became the dominant group of living creatures on earth. In the aftermath of the impact, dust and particles covered the Earth, plunging the world into a cooling cycle similar to a volcanic or nuclear winter. This is thought to have interfered with the photosynthesis of plants world wide, and caused an astounding amount of them to die. Not only did this mean that the large animals who survived the first few years post impact had a much harder time finding food, it also meant that there was a huge amount of biomass available in the soil for decomposers to capitalize on. This caused an absolute explosion in fungal life that can actually be seen in the fossil record.

The asteroid that killed the dinosaurs is famously known to have deposited a layer of iridium rich rock all around the earth called the K-T barrier. No matter where you are digging on Earth, in rock below this iridium deposit layer, you can find cretaceous era dinosaur fossils, and in the rock above it, we find none. Interestingly, right above the K-T barrier, we find some of the highest concentrations of spores anywhere in the fossil record. For years after impact, the dark, cold conditions of the planet allowed fungal organisms big and small to thrive, eating away at the nutrients of dead plants and even threatening to invade and eat alive any animal whose immune system couldn’t fend them off.

In a world hostile enough to end the age of dinosaurs, wipe out an unthinkable number of species, and slow plant growth to a crawl, how could our mammalian ancestors have survived? 98.6. That’s the answer. 98.6. You see, fungi can be deadly if they invade your body, but they start to denature at 86 degrees, which means that in hot blooded creatures, our latent body heat keeps the fungus out. In reptiles, (the competition) they warm themselves in the sun to induce a fever state and purge the fungi that way. But in the asteroid induced winter of the post K-T era, this isn’t possible. This means that our mammalian ancestors got to fill all these niches left behind by their scaly-adversaries, leading to an event called the mammalian explosion, where mammals quickly supplanted their less fungi-friendly foes and became the dominant group of organisms across the planet.

Okay, so Casadevall’s theory tells us why our bodies are hot blooded, but why 98.6 specifically? Well, Casadevall has an answer for that too. In humans, 98.6 is the optimal tradeoff between our metabolism and protection from fungal infections.

(Paper here)

By creating a model of the tradeoff between metabolic rate and fungal immunity and representing that as the function fitness on the y axis, Casadevall et al obtained the above graph, which shows the peak fitness at 37 degrees Celsius. Amazingly, this relationship is standard across all mammalian organisms, given adjustments for body mass.

Although we can’t stick a thermometer in the mouth of our mammalian ancestors from 66 million years ago, we know that the body temperatures of all mammals follow this relationship, meaning that it hasn’t changed since that common ancestor. In other words, for 66 million years, this relationship has stayed the same for all mammals. Now though, all of that is set to change.

For millions of years there’s been very little pressure on fungal life to make the jump and become more heat hardy. Sure, they could then infect hot blooded organisms, but compared to the vast ecosystems they could compete for instead, the selective pressure on fungi adapting to live in hot-blooded organisms has been relatively low. However, when the temperatures of those ecosystems themselves change, things can get very ugly, very fast. Because of global warming, the amount of space where the number of days that the average temperature is over 86 degrees, has vastly increased. Fungi all over the world are experiencing more hot days in a row every year, meaning that the environment is now selecting specifically for fungi that can survive at hotter and hotter temperatures. As the average temperature increases, the fungi adapt to survive at higher temperatures. This process is very similar to antibiotic resistance, where bacteria can adapt and mutate to survive in higher and higher concentrations of antibiotic very quickly.

Here’s a great video illustrating the concept:

Humans have pushed the climate towards hotter and hotter conditions, essentially making sure that the fungi become more and more resistant to that heat, just like the bacteria in the video become resistant to the concentrations of antibiotic.

This is where things start to get scary. in 2014, a doctor found traces of a strange fungus, Candida auris in the blood of 3 patients in her ward in Karachi, Pakistan. The infection spread through the hospital doubling to six, and then more than tripling to 19, within six months, with 8 of the patients dying. The CDC gets brought in on the case, and references their info to the first known Candida auris infection, in 2009. Back then, it wasn’t deadly at all, and had only caused some weeping pus in a Japanese woman's ear. The CDC hadn’t even had the fungus on their radar until they heard about the Karachi cluster, and they were confused. How had this bug made the jump from living in ears and on the skin, to living in the bloodstream itself, and causing major damage? They start digging through data, and they find that in the years between 2009 and 2014 there’s been reports of clusters of Candida auris infections in South Korea… in India… in South Africa… and even in London, where the infection shut down their ICU for months. The CDC is trying to track the spread of this organism through DNA sequencing and it turns out they can’t. These are all separate populations of Candida auris, they didn’t share a recent common ancestor, and they didn’t spread after mutating. The same deadly mutation of Candida auris was emerging from latent populations at the same time, around the world. They tried to explain it with a theory that farmers had been over using antifungal drugs and training these populations, but they couldn’t make the link.

Eventually, the researchers were able to piece together the grim conclusion that it wasn’t the farmers, it wasn’t transmission, it was the temperature. The world over, heat waves had struck, raising temperatures to the triple digits for days in a row. Every day that it’s harder for a normal fungi to survive, is another layer of concentrated antibiotic training it. The weather was training a killer.

Scared yet? It gets worse. In the same way that harsh environments train strong organisms, cushy environments train easy prey. A recent study (here) showed that humans, especially the ones in more developed countries, are cooling down. In Pakistan and the developing world, temperatures are just what you would expect, 98.6. But in developed countries, like Japan, Western Europe, and North America, the average body temperature has actually dropped since the mid 1800s when 98.6 was set as the standard, to 97.9 degrees. Gulp.

And it’s not just Candida auris lurking in the shadows, there are hundreds of thousands of microorganisms that exist passively in our environment, on our skin, in our food, adapting and changing to their environment, which thanks to mankind is getting more and more well, human.

I’ve been Connor, OfAllTrades

Post Scriptum: Some readers have pointed out that the Earth has gone through temperature cycles before, and reached higher temperatures than today within the last few million years. You can see a graph of those changes here! (adjust the timescale in the top left). While it definitely follows that the fungi would have adapted to these conditions as well, these changes are thought to have become dormant/unselected again once things cooled off. During those times, selective pressure on the fungi goes back to optimizing to not spend energy on making heat-shock resistant proteins. Just like how without fungal pressure, we might get lower body temperatures! The problem is that the speed at which the current changes are happening is much faster than previous cycles, and that we might get way more emergence happening all at once, and get overwhelmed of course, life will always adapt and change, but in this case, evolutionary change means pressure which means human death.

Many of you also asked me to present a solution! Personally, I have to admit that any solution we come up with will only be temporary. The ever changing nature of life necessitates that fact. However, I think that recent breakthroughs in gene editing, and especially mRNA technologies (Thanks Dr. Malone!), may allow us to outpace nature with intelligent design, as ironic as that may be coming from an agnostic. Basically, if you don’t want to die a horrible fungus death, support your local geneticist!

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