Phylogenetic humility

Just a welcoming sentiment from Robert M. Sapolsky in his collection of science essays, Monkeyluv. Sapolsky is a professor of biology and neurology at Stanford, and his essays, often humorous, delve into neuroscience, hormones, and human behavior. Oliver Sacks has called him “one of the best scientist-writers of our time,” and while I wouldn’t put Sapolsky on the same pedestal as I do Gould, or Levi, or Thomas when it comes to prose and insight, the man certainly has his moments, which manifest in me dog-earring a corner of a page. This one stood out to me this morning on the 3 train to work while reading an essay titled ” Bugs on the Brain.” As someone who gets excited over pathogenic protozoa and animal behavior, I muttered “Toxoplasma gondii” upon seeing the heading.

Like most people who come across any mention of Toxoplasma (it gets a fair amount of press; e.g., via Carl Zimmer), Sapolsky is interested in the precision of how the protozoa can manipulate behavior. The organism relies on a simple and common two-host system to complete its life cycle: the predator-prey interaction of rodent and cat. Rodents ingest the protozoa, which encysts in the mammal, with particular affinity for denning in the brain. When rodents are consumed by felines, Toxoplasma can reproduce, new organisms are shed through feces, which happen to be a food source for rodents and thus how the life cycle comes full circle. Many pathogens that rely on multiple hosts influence behavior, and there is a bevy of literature that describes just this, particularly with tapeworms. In the case of Toxoplasma, the protozoa interferes with a rodent’s natural aversion to feline pheromones; interferes is a weak term–the organism makes the rodent attracted to feline odor, increasing the probability that it becomes successful prey.

What interests Sapolsky so about this host-parasite interaction is that a rodent infected with Toxoplasma gondii otherwise behaves normally. As he notes, infected rodents maintain their social status within the system’s hierarchy, they continue to mate and thus sense pheromones of the opposite sex, and their recognition of other vertebrate odors isn’t tampered with in the least. The protzoa is simply able to manipulate the recognition of and reaction to the pheromones of one predator, that of the definitive host. To Sapolsky (and most of us interested in the long-term interplay between parasites and their host), this is evidence of how counter-intuitive and beautiful evolutionary process can be. Sapolsky takes the opportunity to highlight Toxoplasma gondii as a correction to teleological interpretations of evolution: its processes aren’t directional, aren’t progressive.  As he notes, “We are certainly not the most evolved species around, nor the least vulnerable. Nor the cleverest.” But the punctum of his message, to borrow a term from Roland Barthes (that which “pierces” the viewer/reader), is the statement, “we need phylogenetic humility.”


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