Something I’ve thoroughly enjoyed in the first few weeks of taking a phylogenomics course is getting to think about non-microbial life for a change. The class will reorient its focus in a microbial direction here in the upcoming weeks, but thinking about familiar groups of macroscopic organisms to start makes it a lot easier to get a grip on fundamental concepts in phylogenomics and evolution.
Microbes are neat. But would anyone really argue that dinosaurs aren’t at least a bit neat-er?
The course instructor, Prof. Fournier, tends to ask us a lot of questions encouraging us to come up with examples of certain evolutionary relationships (check out this list of phylogenetic terms if you’re interested). But some of his questions are just about sharing fun facts.
I love fun facts.
Last lecture, as we were discussing differences between eukaryotic and microbial evolution, Prof. Fournier pointed out that, in general, microbes evolve by expanding their metabolic capabilities whereas complex multicellular life usually responds to selective pressure by evolving new structures—think hands for grasping, fins for swimming, etc.
Microbes easily adapt their entire cellular toolboxes for survival under extreme conditions—acidity, lack of oxygen, high temperatures, etc. Complex life can’t do that so well, since it’s, well, complex. And complexity is great for things like evolving hands and discovering fire, but it can actually get in the way of survival under extreme conditions. There are just too many interdependent moving parts in a complex organism to allow for the kind of easy metabolic mix-and-match that microbes get away with. Evolving new body plans and parts will only get you so far if you need to start breathing iron or keep your DNA from unzipping at boiling temperatures.
That’s why you’ll find bacteria happily munching away on toxic gas abd living in boiling hot springs and lakes full of arsenic while animals, plants, and (to a lesser degree) fungi tend to need to more clement habitats with plenty of oxygen and food to consume.
But hold on, that’s not even the fun fact. I’m getting there.
Microbes account for the vast majority of life’s metabolic diveristy. And spend enough time in geobiology courses and it’s pretty easy to begin to think that there’s no limit to what bacteria and archaea can overcome. Compared to “higher” lifeforms they can seem downright indestructable—at least on the level of populations. Sure, individual bugs might not survive in some inhospitable environment, but there’s almost always a weird relative or two a few branches over on the tree of life that finds that same boiling hot spring or melt-your-face-off acid pool downright cozy.
So I was a bit puzzled at first when Prof. Fournier followed up his point about structural vs. metabolic evolution by asking “what is the one extreme environmental condition that complex life, especially vertebrates, has more successfully overcome than microbes?”
A puzzle. It took the class a while to figure it out. But we did.
A handful of mammals, birds, and fish have independently evolved ways to survive, maintain activity, and reproduce at sub-zero temperatures, but microbes haven’t.
Prof. Founrier explained that even the most “cryophilic” microbes can’t maintain metabolic activity at just a few degrees below 0 C. In brine pockets with enough salinity to keep water liquid below freezing temperatures, microbes still don’t cope well if it gets too cold. Metabolic reactions required for maintaining cell integrity, growing and reproducing grind to a halt below freezing. Big lifeforms can generate enough heat to warm themselves, but microbes can’t. They’re just too small, their surface-area to voulme ratio just too enormous—that is, relative to their size, microbes have a lot less interior volume that could be used to run heat-producing reactions than external surface area through which any generated heat escapes.
Of course, it’s not as though you won’t find any microbes in the polar air. You will, but they’ll all be hibernating: in order to survive, they form inactive, sturdy spores and bet on warmer conditions arriving before the spore gets too damaged to reactivate—either a seasonal thaw or the warm and toasty gut of a passing animal would do.
So, at least when it comes to cold, I’m an extremophile since I can regulate my own body temperature and survive in the cold.
Technically, if I want to make it below 50 C for more than 24 hours, I’d need a jacket. But, being a complex animal, I’ve got just the body structure to ensure that I can get my hands on one.