Parasites

DAVEW
I've just started reading Carl Zimmer's "Parasite Rex," and he makes the following very interesting point regarding the discovery of parasites several hundred years ago (page 5):

The mysterious nature of parasites created a strange, new disturbing catechism of its own. Why did God create parasites? To keep us from being too proud, by reminding us that we were merely dust. How did parasites get into us? They must have been put there by God, since there was no apparent way for them to get in by themselves.
Perhaps they were passed down through generations within our bodies to the bodies of our children. Did that mean that Adam, who was created in purest innocence, came into being already loaded with parasites? Maybe the parasites were created inside him after his fall. But wouldn't this be a second creation, an eighth day added to that first week -- "and on the following Monday God created parasites"? Well, then, maybe Adam was created with parasites after all, but in Eden parasites were his helpmates. They ate the food he couldn't fully digest and licked his wounds clean from within. But why should Adam, created not only in innocence but in perfection, need any help at all? Here the catechism seems to have finally fallen apart.

So how about it, creationists? Were there parasites in Eden? If so, how did they survive? And does "microevolution" explain all the variants of parasites that exist today?


POIKILOTHERM
Add to that the existance of highly specialised immune organs in our bodies(thymus, spleen, lymph nodes....), as well as a part of the immune reaction (eosinophils) quite briskly responsive and toxic to parasites....One wonders if Adam had a spleen, thymus, lymph nodes or eosinophils.

Or were the thymus and spleen in all their complexity acquired in all mammals, and the Bursa of Fabricius in birds, in the last couple thousand years? One does indeed wonder.


HELEN
1. God knew in advance what was going to happen to men and had made provision for it. See Revelation 13:8 and understand that our bodies, also, were prepared for what would lie ahead.

2. Parasites are probably degenerations of symbiotic organisms.

3. We do not know the state of the biology before the fall.


SUMAC
Actually, from the observations we can make today, you've got it backwards. Symbiotic organisms usually start as parasites. At least, that's the way it works today. It is impossible to make the reverse extrapolation based on the current evidence.


HELEN
There are a number of parisitologists who disagree with you, Sumac. Mark Armitage, whose articles on parasites are published in the peer-reviewed journal devoted to that yucky subject, is one and he could name many others.

So could you please give me some back-up on your statement? Where is that opinion from?


SUMAC
You could look at some of the papers that I presented in this thread for starters.

[Administrator: Sumac is referring to papers listed on another thread, but Helen evidently understood. We could not find the links.]

Could you please back up your statement? What evidence do you have that any parasite evolved from a symbiont?


HELEN
I'll reference your question to Mark. He is the professional. I'll post his response when it comes in.

[regarding the unknown link] I'd read that. That is based on speculation with the foundational belief that evolution is true to begin with.

What about mistletoe, tape worms, etc.? When I think of parasites, I think I am with the majority here who think of that sort of thing, not of a lot of speculative work regarding the innards of cells.

How do you figure that these sorts of things evolved later into symbionts?

Which leads to another possible problem for evolution -- a parasite cannot exist without a host -- so both came along together? How did the parasite BECOME a parasite before it was something else?


DAVEW
A parasite that kills its host quickly is less effective than one that kills slowly. A parasite that doesn't kill at all is even more effective, and by extension a parasite that prolongs the life of its host would be even more effective.
The bacteria that cause diseases seem to erupt in sporadic outbreaks when conditions are right for them. By contrast, the bacteria in our intestines have thriving and constant populations.
One example of this is syphillis, which apparently is much less debilitating now than it was when it was first noticed.

Selective pressures would tend to make parasites less harmful to their hosts over time, thus tending to encourage them to become symbiotic with their hosts.

That answer is speculative, to be sure, but the question you asked was "how do you figure parasites evolved into symbionts," and that's how I figure it generally happens (when it does happen).

On the other hand, I know of no mechanism by which a symbiont would "microevolve" or "degenerate" into a parasite, particularly as many parasites have multiple hosts, such as tapeworms that get eaten by crickets and then, at a later life stage, by rats, which then deposit their eggs in feces eaten by crickets, etc.

Multple-host parasites are kind of a problem for regular old evolution, but even more of a problem for the hyper-micro-evolution proposed by creationists. The only way to reconcile these parasites with creationism, IMO, is to declare that God created them that way, in the Garden of Eden, before the fall (or that there was a major creation event not mentioned in the Bible).


HELEN
I agree with you that multiple host parasites are a problem for both sides.
That aside, the transformation of a parasite to a symbiont requires the addition of functions which the original parasite cannot carry out on its own (or it wouldn't be a parasite). Going in the other direction requires a loss of function.

We see the latter as quite possible but struggle to account for the former. You want to take it from here?


DAVEW
OK.
The tapeworm Hymenolepis diminuta, the one I mentioned earlier that travels from beetles to rats, lays eggs in the rat droppings that make them highly attractive to beetles. Once inside the beetle, the tapeworm sterilizes the beetle by releasing chemicals that disrupt the reproductive process, thus causing the beetle to divert more energy to survival (which helps the parasite) than to reproduction (which diverts energy and resources away from the parasite).

Once the tapeworm matures in the beetle and is ready to be transferred to a rat, it drugs the beetle, causing it to be much less cautious about seeking cover. It also disables the beetle's defense mechanism of secreting a bad-tasting chemical, thus making the infected beetles more palatable to the rats, which helps them get eaten by (and thus infest) the rats.

(The above is loosely paraphrased from Carl Zimmer's "Parasite Rex," pp. 98-100).

How on earth does this represent a loss of function?

Did the original symbionts somehow benefit their hosts by sterilizing them and making them more vulnerable to predation? If this is "degeneration," then why is it so complex and specific? What could have been the original functions that these behaviors "degenerated" from?

Also, which creature was the tapeworm originally supposed to benefit? The beetle or the rat? Or are you suggesting that tapeworms were once serial symbionts, first benefiting one creature and then another (sort of like Caine in "Kung Fu")?


HELEN
Loss of function would be in its inability to live on its own or symbiotically in the creation model.
The fact that its life cycle is so complex is rather hard to explain evolutionarily, isn't it?

Second, if being a parasite is successful, why change to a symbiont, and how on earth could it happen anyway?


DAVEW
Loss of function would be in its inability to live on its own or symbiotically in the creation model.[/b]

Fine, but it's also gained the ability to attract beetles, and to impair their ability to reproduce and avoid predation.
Where did these new abilities come from?


The fact that its life cycle is so complex is rather hard to explain evolutionarily, isn't it?

Yes. And much harder to explain in a time frame of a few thousand years.


Second, if being a parasite is successful, why change to a symbiont, and how on earth could it happen anyway?

I think I've explained this twice already. A parasite has a vested interest in keeping its host alive. Parasites that are more successful at this task are more likely to survive and reproduce than parasites that are less successful at it.

Meanwhile, I'm still waiting for the logic and evidence supporting your assertion that symbionts have "degenerated" into parasites. Wouldn't the "degenerate" forms tend to be removed through natural selection?


MESK
I agree with you that multiple host parasites are a problem for both sides.

How are they a problem for evolution, Helen? And I don't think is what Dave was saying at all, unless I misinterpreted him.


That aside, the transformation of a parasite to a symbiont requires the addition of functions which the original parasite cannot carry out on its own (or it wouldn't be a parasite). Going in the other direction requires a loss of function.

Eh? Both symbionts and parasites would require "gains of function" to evolve from the other. Parasites need virulence factors that are not required from a symbiont (frequently mechanisms for damaging host cells and evading the host immune response), while symbionts need a variety of mechanisms for productive interactions with the host which are not required for a parasite.

A symbiont cannot become a parasite by simply "losing all its symbiotic bits" - it also needs to gain all sorts of nasty machinery for outwitting host defences and obtaining what nutrients it needs from an unwilling source. For instance, why would the putative symbiotic ancestor of Vibrio cholerae have needed cholera toxin, a complex multi-subunit enzyme which modifies host signalling proteins and leads to explosive diorrhoea? Why would a symbiotic form of Clostridium botulinum have needed botulinum toxin, an enzyme so highly adapted to pathogenesis that a single molecule can completely destroy the function of an entire nerve cell? Why would the symbiotic ancestor of its equally nasty cousin, the gas gangrene pathogen Clostridium perfringens, have needed phospholipase C (which rapidly kills cells by degrading their membranes) and a variety of other degradative enzymes (e.g. collagenase, hyaluronidase, various proteases) whose sole purpose is the destruction of host tissue? Why would a benign version of Mycobacterium tuberculosis have needed such incredibly complex machinery for entering host macrophages and evading the mechanisms of intracellular killing, thus leading to the chronic granulomatous inflammation seen in tuberculosis? And so on...

If anything, you would be better off arguing that symbionts evolved from parasites - since parasites already possess all the machinery necessary for living within the host, all they would need to do is shed most of their virulence factors and evolve a few novel metabolic pathways. Of course this still requires a gain of the genetic information involved in the new metabolic reactions (bad luck for you), but it requires less information gain than the other way around.

Both evolutionary directions require gains of information, but if anything it is easier to go from parasite to symbiont (the change most commonly observed in evolution, as Sumac said) than it is to go the other way. Parasites require specialised mechanisms for pathogenesis that symbionts simply don't need (see examples above, and I can provide dozens more on request), so evolving from a symbiont to a pathogen unarguably requires a gain of information.


RADIOCHEMIST
I suspect that some parasites can exist without a host, contrary to your claim. Also, there is nothing to keep a non-parasite from evolving into a parasite. Life is more flexible than you say. For almost any life style of an animal, there are exceptions, even within the same species. Ticks prefer to latch on to a warm blooded mammal for food. But are you prepared to say that a tick cannot live without sucking blood? I have seen many live ticks on grass. I don't know how long they can live without a host, but they do live for some length of time. Maybe some biologist can tell us. By the way, why on earth would God create a tick?


DAVEW
How are they a problem for evolution, Helen? And I don't think is what Dave was saying at all, unless I misinterpreted him.

Actually, I think you did. I do consider multi-host parasites a problem for evolution, I just consider it an immensely greater problem for the creationist notion of microevolution within a few thousand years.

In other words, if you told me that you are a world-class pole vaulter, I'd have a hard time believing you (even though of course it's possible). But if you told me that you're a world-class pole vaulter with advanced cerebral palsy, I'd have a *really* hard time believing you.

I suspect that Helen understands this, which is why she's trying to explain (or explain away) multi-host parasites in terms of "degeneration" rather than "microevolution."