Why do we go to our high school reunions?

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Every winter, the pale-bellied brant goose flies from the Canadian Arctic all the way to the British Isles, only to return in the spring. Every five years, salmon in the Atlantic Ocean fight their way up upstream back to the tributaries where they were born. And every 10 years, the graduates of East Lansing High School travel back to an agreed-upon sports bar to eat artichoke dip and ask each other, “How have you been? What are you doing now?”

Why do we do it? Why do we go to our high school reunions? It’s not as though most of the faces behind the name tags — slightly rounder, slightly grayer — had crossed our minds over the past decade. Yet, at the appointed time, like wildebeest streaming across the savannah, we heed reunion committee czar Biza Repko’s call to return.

I grew up in a college town, home of Michigan State University, the nation’s first land-grant agricultural college. It was an idyllic place, full of coffee shops and churches, where the most daring thing you could do as a child was spray-paint a rock outside of a fraternity house. For years, my most acute memory of my home town centered on my yearning to leave it.

Yet I come back for the reunions. Three weeks ago, I traveled to Reno’s Sports Bar to be together with my classmates again, for just one night. We reminisced about how our lives were guided by the circadian rhythm of school bells. The bells told the rebel tribe known as “hillers” that it was time to smoke in the woods behind school. The bells sent preppy kids scurrying to pre-appointed positions in the Commons, an area that served as the axis around which social life swung. A cocky crowd of senior boys staked out prime territory along the railing, where freshmen passed as if running a gauntlet.

We were divided then, into ever-changing clans and hierarchies. But time becomes the great equalizer. Over beer and chips at Reno’s, classmates consoled each other over deaths and indignities, which — it turns out — strike football heroes and rich kids just like everybody else. We smiled at the triumphs of once-awkward boys who grew up to be the cutest men in their corporate offices.

Maybe that’s why we go back to our reunions: to see how far we have come, and revel in the fact that life is long enough for all of us to play many parts.

But there is something more primordial about a high school reunion. The stubborn, almost irrational allegiance to the place where you were born seems coded into nature itself.

According to Sasha Dall, a senior lecturer in mathematical ecology at the University of Exeter in Britain, some species of sea turtles are known to swim hundreds of miles, past perfectly good beaches, just so they can lay their clutch of eggs on the spot where they themselves squirmed out the sand.

A surprising number of animals — from whales to cardinals — return to the place where they were born to have their own kids.

“It’s easier to go back to what you already know,” Dall said, rather than taking the risk of exploring new and untested habitats.

Science can’t really explain why we attend our high school reunions. But it does give us clues about why early experiences tend to trump others later in life.

“All complex life develops from a single cell,” Dall said. “What that means is that the early conditions that an individual finds himself [or herself] in will have a disproportionate impact on where he [or she] ends up in a lifetime.”

Our very bodies seem to know this. Our memories, stored in our hippocampus, are strongest if they have been etched in our brain the longest. That’s why an Alzheimer’s patient can forget her husband’s name, yet remember vivid details about classmates she hasn’t seen in 80 years.

Your hometown is like your family. You didn’t choose it, yet it shapes you indelibly. Most people spend their lives either living up to their hometown’s expectations or defying them. Even if you left and never looked back — never attended a single reunion — the place you grew up is still embedded deep in the master code of who you are.