- Smithsonian: “The Scariest Zombies in Nature”
“Zombies have a long natural history, stretching back tens of millions of years, and nature is filled with creeping, oozing, blood sucking and otherwise ghastly creatures just as terrifying as anything Hollywood could concoct.” So writes Brian Switek, author of Written in Stone: Evolution, the Fossil Record, and Our Place in Nature. Switek’s pre-Halloween article for Smithsonian Magazine catalogues a number of these ghoulish parasites. He quotes ecologist David Hughes, who has worked with zombie ants: “Now that we know the behavior like this can fossilize, I would not be surprised if we find more. I believe samples tens of millions of years older are likely.”
The “fossilized behavior” Hughes speaks of refers to fossil evidence that fungal infestation of insects produced the same alterations in behavior long ago as those we see today. For instance, today we know the fungus Ophiocordyceps unilaterius parasitizes ants, prompting them to climb to a certain level in the jungle canopy, bite down on the shady underside of leaves where the temperature and humidity are optimal for fungal growth, and remain there until death. Fossilized leaves from Eocene rock (48 million years ago by evolutionary dating) in Germany preserve markings like those left by the ants on modern leaves.
Of course, the claim that these parasites and their zombie-creating effects are millions of years old comes from the radiometric dating of the rock layers in which such fossils are found. Radiometric dating methods are all based on a number of unverifiable assumptions. (See “Radiometric Dating: Back to Basics,” “Radiometric Dating: Problems with the Assumptions,” and “Radiometric Dating: Making Sense of the Patterns.”)
“It’s not yet clear what chemicals signals alter the behavior and appearance of parasitized ants and other victims,” but “Parasites have evolved to be masters of manipulation.”
Switek’s article reviews the “evolutionary success” of a number of parasites infecting ants and snails. He concludes, “It’s not yet clear what chemicals signals alter the behavior and appearance of parasitized ants and other victims,” but “Parasites have evolved to be masters of manipulation.”
All of the zombie-makers in Switek’s hall of creepy fame share a key feature: they affect their hosts so as to enhance their own growth and dispersion. For instance, the snail parasite he describes is a flatworm that infects snail eye stalks. The infected eye stalks are brightly colored and readily attract the attention of hungry birds. The parasites are of course eaten along with the unfortunate snails. The flatworms complete their life cycle inside bird digestive systems, leaving eggs to be deposited in bird droppings where they will be consumed by more snails.
Some scientists and fiction writers tend to personify evolution’s ability to manipulate the behavior of parasite victims. Evolutionist Richard Dawkins coined the term extended phenotype to refer to the fact that, while a gene can only encode for the production of a particular protein, a gene’s “extended phenotype” includes all the effects that gene has on all the organisms it affects.
In trying to discover a mechanism by which a gene can extend its phenotype, researchers working with another zombie-like insect discovered, in the words of David Hughes, “the first empirical evidence that a gene in the body of one organism can have a direct effect on another organism.” A virus affecting gypsy moth caterpillars indirectly prompts the insects to die in treetops by coding for an enzyme that deactivates the hormone that triggers molting. Caterpillar molting behavior is linked to lingering at lower altitudes, so, with no hormonal prompting to molt, the hapless hosts blissfully ascend to die at a location optimal for viral dispersion. This chain of causation illustrates the complexities of natural selection at work in the 6,000 years since the Fall of man and the resultant Curse on creation.
As with the gypsy moth, so with these ant parasites and snail parasites, no new kinds of organisms are evolving. Natural selection is “rewarding” the most effective dispersal strategies. Those parasites that happen to infect their unfortunate hosts in ways that impair neural or hormonal control of behavior or make the hosts particularly vulnerable to predators are naturally selected through successful dispersal.
Genesis says that God called His Creation “very good,” so we are confident that fungi, viruses, bacteria, and even flatworms fulfilled helpful, not harmful, roles in that original world. Since the Fall, a combination of mutations, horizontally transferred genes, environmental changes, and host changes have left us with a number of harmful microorganisms in addition to those that still fulfill vital ecological roles. The zombie-making parasites Switek describes, despite the title of his book, are just reminders of the way natural selection has functioned in this fallen world, not personifications of evolution explaining fossilized behavior. No doubt additional research will elucidate more complicated mechanisms by which natural selection—not evolution—produced extended phenotypes.
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