Filthy ideas

gorilladirt.jpgIt’s been a while since I grumbled about inappropriate evolution-speak in science writing in the NY Times. So at the risk of sounding shrill, I’m going to take another stab. Jane E. Brody has written about the immunological benefits of babies eating dirt. The idea that exposure to germs in early childhood can boost immune development and reduce risk of allergies and other auto-immune disorders has been around a while, and Brody’s article doesn’t really cover anything shocking. But this passage was annoying:

Since all instinctive behaviors have an evolutionary advantage or they would not have been retained for millions of years, chances are that [babies putting their grubby little hands in their mouths] has helped us survive as a species.

Do all instincts have an evolutionary advantage? Are all instincts adaptations? That opening line sounds irritatingly like another indiscriminate claim for adaptation. But in biological terms, instincts are hard-wired, not learned, and improve fitness. So I think by definition, instincts are adaptive. But has natural selection really favored babies putting things in their mouths, to increase exposure to germs and boost immunological development? (Is there variation in this? Have babies ever not eaten dirt?) I doubt it.

For most of human history, we didn’t have lysol antibacterial spray, soap, or running water. Babies, like the rest of us, were perpetually grubby and reliably teeming with microbes. So I can’t imagine that they needed an adaptation to increase exposure to germs. Only in our over-sanitized present does an instinct to seek and ingest dirt really make sense. So maybe this “instinct” is an adaptation—a really, really recent one. But… haven’t babies always eaten dirt?

One Response to “Filthy ideas”

  1. Why Does Evolution Favor Babies Putting Things in Their Mouths? | Babys First Year Blog Says:

    [...] turns out that some evolutionary biologists suggest that the value in this particular baby activity comes with the dirt and germs that also end [...]

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