Wednesday, March 21, 2012

Science Ruins Science Fiction Again

Last month, Russian researchers struck frozen science gold--an ancient lake, buried deep under the Antarctic ice sheet. Given that Lake Vostok had been isolated for probably millions of years, the Russians were under a lot of pressure (just like the lake! because it's under a really heavy ice sheet, get it?) to protect this unspoilt environment from contamination.

But what about the possibility of contaminating ourselves with stuff from the lake? As my brother pointed out,
While this is undoubtedly an exciting moment for science, all I can think of is a science fiction story in which a bacterium discovered in a place like this causes a worldwide pandemic.
To which I replied: okay, fun concept, but totally unrealistic. Then we got to talking about parasitism and co-evolution and . . . well, let's start at the beginning.

As soon as you move into another organism, you're a symbiont. Symbionts can be beneficial or harmful; the harmful kind are called parasites. So, bacteria that live in people and make them sick are technically a kind of parasite--though people often say "parasites and bacteria" the way they used to say "animals and fish." (Yes, fish are technically and in all other ways animals.)

Now, all symbiotic relationships are products of co-evolution. The parasite evolves to survive inside the host, while the host evolves to reduce the harm done by the parasite. (There are a lot of strategies for that, by the way--from making initial infection more difficult to quarantining, expelling or killing the parasite). As the host environment becomes more hostile, the parasite evolves clever coping mechanisms, and so on.

Because of the specificity of most parasite-host relationships, it's highly improbable that a parasite could survive for millions of years without its host*. And if it did survive, it would probably do so by evolving  into such a different form that it couldn't re-infect its old host.

That's why I'm pretty confident there aren't any nasty little parasitic bacteria in Lake Vostok, waiting to pounce on us.

Okay (said my brother) but why couldn't a non-parasitic Vostokian bacterium initiate a pandemic as soon as it was exposed to people? Every relationship has to start somewhere, right?

Sure, a free-living bacterium that had never encountered humans before could theoretically find its way into an unsuspecting scientist (poor Dr. Lukin!), survive long enough to reproduce, and start a new symbiotic relationship. But the environments of Lake Vostok and the human body are radically different. A bacterium (or any other critter) is much more likely to move inside an organism if that organism's internal decor is similar to the environment it's already adapted to. The 37 °C of the human body would almost certainly kill bacteria adapted to the -3 °C of Lake Vostok.

~Tangential Musing On Evolutionary Timescales~

Even if a brand new bacterium entered a human and survived, we'd probably never know about it. As a general rule, it takes a long time for symbioses to evolve, and it's very hard to study them when they're just getting started.

Imagine a cafe full of college freshman--there's probably a lot of flirting, but none of it may ever turn into a relationship. Tracking all the potential interactions, most of which will be dead ends, would be a huge challenge. Now consider that plenty of pairs of college freshman are likely to hit it off with each other, but most biological interactions that could become symbioses are nipped in the bud when one organism kills the other.

How long would it take to evolve the sort of traits that make for a proper pandemic? I don't know, but I wonder if anyone's done any theoretical modeling of this . . .

~End Tangent~

All the really scary epidemics in human history have come about through jumps between similar environments.

Human to human is the most obvious--Europeans bringing syphilis to the New World, for example. We often use the term "first contact" to refer to the meeting of colonizers with natives, which is a bit misleading, since we also use that term in science fiction to refer to the meeting of humans and aliens. The former is fraught with peril of disease; the latter, not so much.

Humans around the globe belong to the same species and are similar enough to fall prey to the same parasites. But in most speculative cases, humans and aliens belong not only to different species, but to entirely different evolutionary histories, perhaps going back to the origins of life itself. The idea of a parasite, carefully co-evolved with its host, being able to jump across such a gap as that--well, it strains my imaginer.

But what about zoonoses? Aren't those examples of parasites jumping suddenly from one host species to another? Well, yes and no. Many parasites have co-evolved with both human and animal hosts, and require both to survive. Malaria is carried by mosquitoes, but can't complete its life cycle without humans. Other zoonotic parasites, like Toxoplasma, are stuck in an evolutionary dead end if they accidentally infect a human--they can survive but not reproduce.

The zoonoses that truly "jump" from species to species, successfully infecting and propagating through their new host, always move between similar environments. Ebola can only infect primates. Even versatile diseases like West Nile virus are restricted to vertebrates--a tiny fraction of the world's animal diversity. There's no way you're going to "catch" colony collapse disorder from a bee, or bitter crab disease from a crab.

So, let me sum up.

Likely sources of pandemics: "first contact" between groups of humans that have been isolated from each other; places where humans and other vertebrates live in close, unsanitary quarters.

Unlikely sources of pandemics: Lake Vostok, Mars.



* Modern humans (Homo sapiens) weren't even around when Lake Vostok was last connected to the rest of the world, but there were definitely early hominids.

2 comments:

  1. Interesting post. I've always considered symbiosis to be two organisms that had evolved together for mutual benefit, like the symbiosis of algae and fungi that forms lichens. I didn't realise parasites were technically symbionts as well, although thinking about it it does make sense.

    As for lake Vostok I can't wait to see what comes out of there when they start taking samples. I think it's highly likely that new species will be discovered, even if only bacteria it's still exciting.

    One more thing. I'm think you meant to write the 98°F of the human body not 98°C!

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    1. Hah! Thanks for the correction; I changed it to 98°C for consistency in units.

      I too am quite excited to see what's in Vostok! And I'm glad you liked the post.

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