7yr cycle and nostalgia

There’s a common misconception that all the cells in your body are replaced every 7 years. It’s one of those facts that you learn as a young person and is very difficult to unlearn because it seems so specific and so elegant. I’ve since come to understand that the “7 years” figure is directionally correct—most cells are replaced in that amount of time or less—but it papers over a huge range of renewal rates. Intestinal lining lasts a few days; bone lasts a decade; retinal cells are never replaced. Fortunately, I’m in the mood for this kind of vague impression.

I suspect this nugget is catchy because it speaks to a kid’s experience: quite obviously you’re not the same physical stuff at age 2 and at age 22. If you think about it at all, you probably think about “cell renewal” as healing like Wolverine. Then you learn that the things that make you up are dying all the time and being replaced by new versions, but those new versions faithfully reproduce old scars and idiosyncrasies. You have a bit of an existential crisis at this point. Perhaps you think about that Ted Chiang story Exhalation and how “All that we are is a pattern of [energy] flow.” I might be misrepresenting a teenager’s level of introspection about this subject, and I’m certainly misremembering my own engagement with the Chiang story, which was first published when I was—ahem—27 years old. But I’m directionally correct. Change is obvious once you know what to look for. Perhaps it’s more surprising that the retina and cerebral cortex you have as an adult actually are pared-down versions of the same cells you were born with.

But of course the potential for renewal is endlessly appealing; being able to do better or recover from a hurt is probably the most tempting proposition going. The idea that there’s a “self” that persists through the change of all (or nearly all) the constituent parts is awfully appealing. The 7 year cycle also tweaks my numerological fancy; it seems like the sort of thing that ought to be right. And whether or not this is the product of a deep-incepted idea, I do genuinely think that certain large-scale features in my life, from allergic reactions to levels of piety, have recurred at roughly 7yr intervals. I’m thinking of my mid teens, early 20s, late 20s, mid 30s and now early 40s. This isn’t ever the sort of thing that one could prove or disprove; the evidence is incomplete and subjective. But I want it to be true, and to me, it is. The remaining mystery is whether there should be a “therefore” after “and” in that previous sentence.

My son finished elementary school today, which in our district is a seven year process including kindergarten. I don’t have much of a visual memory but I do recall quite clearly a photo we took of him on day 1 of it all, gathered in a group in front of his teacher, body facing her but head turned back to look for us. His expression says, “all right then—what will this be?” Most of that boy has been replaced by new material, but it’s still unmistakably him. He doesn’t look for us as much anymore. I could interpret that as a lack of interest, but I prefer to think of it as confidence that we’re there. And I have faith that, in time, he will again.