Intricacies of the tapeworm lifecycle (and its evolution)?

[vger]

Warning, in case anyone has a weak stomach... this isn't exactly pleasant dinner convo.

I was reading about the tapeworm lifecycle recently and found it to be highly unusual. It's a complex system that requires two different hosts to complete a cycle. I'll stick with pigs for the purposes of this since it's the most common one that humans have to deal with.

So in quick summary:

1. pigs eat the eggs
   
2. eggs hatch in pigs and larvae grow
   
3. larvae in pigs get eaten by humans
   
4. larvae mature in humans
   
5. adult worms lay eggs in humans
   
6. humans pass eggs through feces
   
7. rinse and repeat
   

A further complication from this is that if a human were to eat an egg instead of a pig, things turn nasty. The larvae typically finds its way to the brain (which is NOT where it would mature in a pig) and causes a lot of problems and potentially death (for both the human and the worm). The same seems to be true for worms living in a closed system with pigs.

I find this particularly strange because the cycle seems to suggest that these tapeworms would have had to evolve relatively fast. It relies on the relationship between humans and pork, and therefore would only have been able to thrive in the short time that humans have practiced animal farming. Is this a possible scenario in which a species made a quantum leap in evolution? Or are the articles I'm finding on the subject making the tapeworm's life cycle a lot fussier than it actually is?

[Godot]

Well, Tapeworm-Lyfecycles are much shorter than those of humans or pigs -> evolutional changes happen faster.

and due to the huge number of offspring they also get a large population running with the new features

More or less one of the reasons why also bacteriae adapt so fast to new antibiotics.

Lots of recombinations (due to the short generation span and the possibility of "sexual" reproduction) and huge rate of reproduction of those that survive antibiotics

[Camacha]

I find this particularly strange because the cycle seems to suggest that these tapeworms would have had to evolve relatively fast. It relies on the relationship between humans and pork, and therefore would only have been able to thrive in the short time that humans have practiced animal farming.

10.000 years is not a short time in evolutionary terms. People keep on thinking of it as a very slow process, but it turns out to be a lot more dynamic that originally thought. This is probably partly because of genetic triggers - by only changing one gene, larger (probably dormant) traits can be turned on or off. Our thin body hair is one example, another is a creature where (if I recall correctly) changing a single letter in the genetic sequence causes it to change from a creature with one hart chamber and a tail to a creature with two perfectly functioning heart chambers and no tail. Evolution is not just developing new traits, it is also switching on and off older ones and recombining those.

The mechanism turns out to be more and more complex every time we take a close look at it, but that is what makes it so interesting.

[K^2]

I would be willing to bet that the original primary host wasn't human, but some other predator. The secondary host could have been some related wild species as well. Evolving all of that behavior from scratch during the period of domestication would be unlikely, but if the adaptation was simply to switch hosts, it seems quite plausible.