The Parasites We Kept

In a September 21 explainer for Quanta Magazine, Jake Buehler describes how transposons—mobile genetic elements, often called jumping genes—became entangled with the organisms carrying them. Nearly half the human genome, his account reports, consists of retrotransposon sequences. These are not simply unwelcome additions to an otherwise pristine design. Transposable elements can damage existing sequences, but they can also supply material that evolution repurposes.

The interesting word is “can.” A capacity to generate change is not a certificate of usefulness.

Buehler distinguishes two broad mechanisms. DNA transposons can move through a cut-and-paste process: a sequence is freed from one location and inserted elsewhere. Retrotransposons instead use an RNA intermediate to produce another DNA copy. The original remains while a new copy joins the genome. One approach rearranges the furniture; the other discovers the photocopier.

Neither process anticipates what an organism needs. An insertion can interrupt a functioning sequence. In another context, transposon-derived material can become part of a useful genetic arrangement. Quanta describes both the costs of disruptive insertions and the possibility that such material acquires new roles in regulating genes.

“Partner,” then, needs care. There is no molecular handshake, no decision to contribute to the common good. The term describes an evolutionary relationship, not a motive. Nor does the large fraction of transposon-derived DNA tell us that all those sequences are currently jumping, or that every retained copy benefits its host. An inventory of origins is not a list of present jobs.

The most delicious complication in Buehler’s account concerns the controls themselves. Organisms have ways to suppress transposon activity. Quanta presents a hypothesis that some epigenetic silencing—the regulation of gene activity without changing the underlying DNA sequence—first evolved to restrain these mobile elements. Those controls could subsequently have been repurposed for regulating other genes.

That is a proposed evolutionary model, not a settled origin story for all gene regulation. But it changes the shape of the question. Innovation need not come only from an unruly sequence acquiring a useful function. It may also come from the machinery that evolved to keep the unruliness manageable. The brake can have an evolutionary afterlife of its own.

There is a coherenceist analogy here, provided we leave biology in charge of its own facts. Openness is not the same as accepting every intrusion, and stability is not the same as preserving an untouched original. A system can change through what it incorporates and through how it tests, limits or redirects those additions.

That analogy does not make natural selection a moral adviser. People are not disposable insertions; the suffering involved in social change cannot be justified by pointing to eventual evolutionary benefits. Human institutions need deliberate standards of care and accountability that genomes do not possess. “Disruption” remains a claim to examine, not a virtue to applaud.

What the science offers is stranger and more useful than a hymn to chaos: the categories we bring to a living system may fail to describe what its parts become. A sequence’s origin does not settle its present function. A defensive mechanism need not remain confined to its first task.

The genome is not a monument to purity. It is a history still doing things. Some of the material once best understood as a problem has helped expand what living things can do. No foresight required. Which is rather encouraging for a species that keeps misplacing its plans.