The unfathomable weirdness of (the lower forms of) life

A reader sent me a Wikipedia article on a parasite called the lancet liver fluke, Dicrocoelium dendriticum, which lives inside the liver of cows. But it doesn’t stay there. Here is the story of its amazing life cycle:

D. dendriticum spends its adult life inside the liver of its host. After mating, the eggs are excreted in the feces. The first intermediate host, the terrestrial snail (Cionella lubrica in the United States), eats the feces, and becomes infected by the larval parasites. The larvae (or cercariae) drill through the wall of the gut and settle in its digestive tract, where they develop into a juvenile stage. The snail tries to defend itself by walling the parasites off in cysts, which it then excretes and leaves behind in the grass. The second intermediate host, an ant (Formica fusca in the United States), uses the trail of slime as a source of moisture. The ant then swallows a cyst loaded with hundreds of juvenile lancet flukes. The parasites enter the gut and then drift through its body. Most of the cercariae encyst in the haemocoel of the ant and mature into metacercariae, but one moves to the sub-esophageal ganglion (a cluster of nerve cells underneath the esophagus). There, the fluke takes control of the ant’s actions by manipulating these nerves. As evening approaches and the air cools, the infested ant is drawn away from other members of the colony and upward to the top of a blade of grass. Once there, it clamps its mandibles onto the top of the blade and stays there until dawn. Afterward, it goes back to its normal activity at the ant colony. If the host ant were to be subjected to the heat of the direct sun, it would die along with the parasite. Night after night, the ant goes back to the top of a blade of grass until a grazing animal comes along and eats the blade, upon which the lancet flukes will be back inside their host. They live out their adult lives inside the animal, reproduce, and the cycle continues.

Reader Blake, who sent the article, commented:

How could such a ridiculously complex life cycle possibly evolve through a series of random mutations? The number of small mutations necessary for the development of such a system, which must take into account mammalian, insect, and gastropod physiology, would seem to be astronomically large…and that’s to say nothing of the parasite’s very peculiar ability to alter drastically the behavior of the ant in an extremely specific and unlikely way. How does a creature RANDOMLY MUTATE an ability to tap into an ant’s nervous system (primitive though it may be) and cause it to climb a blade of grass?

As I was reading this part of the article,

… the fluke takes control of the ant’s actions by manipulating these nerves. As evening approaches and the air cools, the infested ant is drawn away from other members of the colony and upward to the top of a blade of grass. Once there, it clamps its mandibles onto the top of the blade and stays there until dawn.

that business about the ant having its nervous system taken over by a foreign entity, then climbing to the tip of a blade of grass, “clamping its mandibles,” and “staying until dawn,” seemed so over the top that I thought the Wiki article had to be a parody. It read like something made up by Hunter Thompson on a bender. I looked further on the Web and found a couple of references that seemed to back up Wikipedia , but something was still telling me that this had to a goof, it couldn’t be real.

But it is real, as shown by this article I found at the Encyclopedic Reference to Parasitology. It’s very similar to the Wiki account.

Dicrocoelium dendriticum
Life Cycle

Fig. 1. Life cycles of the flukes Dicrocoelium dendriticum (A) and Paramphistomum cervi (B) in sheep and cattle (final hosts: see Digenea/Table 1). 1 Adult worms in the bile ducts (a) or rumen (B). 2 Eggs are excreted in feces fully embryonated (A) or not (B). 2.1 In Paramphistomum cervi the finally formed miracidium hatches from the egg and enters a water snail, whereas in Dicrocoelium dendriticum land-living snails swallow the eggs containing the miracidium. 3-4 Intermediate hosts for Paramphistomum cervi are water snails of the genera Bulinus, Planorbis, Stagnicola, and Anisus, whereas in Dicrocoelium dendriticum land snails of the genera Zebrina or Helicella are involved. Development in snails proceeds via two generations of sporocysts in Dicrocoelium dendriticum, whereas in Paramphistomum cervi a sporocyst and two rediae occur. Finally, tailed cercariae are produced, which leave the snail (3) or are excreted by the snails within slime-balls (4.1), but remain immotile (4.2). 5-6 In Dicrocoelium dendriticum ants become second intermediate hosts when eating slime-balls. Most of the cercariae encyst in the hemocoel (6) as metacercariae and can then infect the final host. One or two cercariae enter the subesophageal ganglion, encyst there and cause an alteration of the ant’s behavior. When the temperature drops in the evening hours, the infected ants climb to the tips of grass (and other plants) and grasp them firmly with their mandibles, while uninfected ants return to their nests. The infected ants remain attached until the next morning, when they warm up and resume normal behavior. These attached ants may be swallowed by plant-eating mammals. In Paramphistomum cervi the free-swimming cercariae (with two eye spots) encyst on herbage and other objects (6), thus becoming metacercariae. Upon being swallowed along with forage, excystment of the metacercariae of both species occurs in the duodenum. From there they enter the bile duct (Dicrocoelium dendriticum) or return (via the intestinal wall) into the abomasum (Paramphistomum cervi), and from there go to the rumen, where they attach among the villi.

This is truly one of the strangest things I’ve ever read about.

You wonder, why not just stay where it is, in the cow’s liver, where it’s happy and contented, instead of leaving the cow and going through all these bizarre adventures just to get back to where it started? Perhaps the cow’s liver is only suitable for the adult stage of the fluke’s life-cycle. For its larval and juvenile stages, it needs different kinds of environments. And it finds them.

What this shows us is that life has an unstoppable drive , not just to live and reproduce, but to keep experimenting with new ways of living and reproducing. It seems there are two stages of evolution. There is the creative and experimental stage, when new behaviors, organs, species come into existence, and then there is the stability stage, in which a species, having come into existence, stays in existence, without changing, for long periods.

- end of initial entry -

Thucydides writes:

Great post! The wonderful story of the life cycle of the lancet liver fluke illustrates the sheer wonder of the world as well as anything I have seen, though I believe it is by no means unique.

Darwinian evolution, simple and superficially very plausible, suddenly seems completely inadequate to the explanation of how such a complicated life pattern might have come into existence. Intelligent Design people say that that hypothesis would furnish a better explanation: perhaps, but there is insufficient evidence for that either. I see ID as only modestly less unbelievable, not as “the better explanation.”

People in the life sciences, finding the idea of at least micro-evolution indispensable (for example, change within a species, say the gradual development of lactose tolerance among tribes practicing animal husbandry), are loath to admit that the theory as a general explanation of the origin of species stands on very shaky feet indeed.

To me the interesting question is why is nobody willing to say “we just don’t know—we don’t have any explanation?”

LA replies:

Well, I’ve been saying it all along. If everyone would just say, “We don’t know how species came into being,” then the whole horrid conflict would stop and we could start discussing the issue intelligently.

Posted by Lawrence Auster at April 05, 2008 01:34 PM | Send