The antique-store clerk totaled up the prices of the orphaned photos: 50 cents apiece for the loose Kodak snapshots, more if it had a nice frame.

“My sister buys these old pictures whenever she sees them,” the clerk said. “She hangs them in her house. She just feels so bad for them.”

When you walk into a vintage shop and see a basket of pictures, your heart sinks a little: No one in that pile expected their picture to be stuck in with those of a hundred strangers. Someone inherited the pictures and didn’t know who any of the people were. So they sold them off, turning everyone into orphans.

If you come across a pile of old photos, cut loose from albums, mixed up and sold cheap, keep in mind that you’re looking at the development — if you’ll excuse the word — of American photographic history. The pictures represent the eras from which they came:

Early studio portraits. No one ever looks happy. It’s as if the photographer said, “Try to forget your toothache, and don’t move for 30 seconds.” In the 19th century, studios had head clamps that would keep the subject’s head motionless for the required time, which explains why everyone’s always staring straight ahead. Advances in film technology reduced the exposure times, but people were still grim: This was a solemn undertaking, a bid for posterity.

Sometimes people wrote down the names on the backs of the photos, but most didn’t. Why would they? They knew who they were.

Early Kodak snapshots. The Brownie camera put photography in the hands of the masses and produced millions of blurry, off-center, overexposed pictures of Uncle Frank wearing a hat and a suit while standing in the yard with Bingo the dog.

The people are indistinct, their faces mere suggestions. It’s hard to care too much about these pictures — until you realize that this might be the only photo that exists of a front yard in Newton, Iowa, on Sept. 27, 1918. Surely that counts for something.

1920s-1940s studio portraits. Even though people had their own cameras, they still went downtown to the studio for the glamour shot. The pictures have a painterly quality — soft focus, retouched, a hint of rouge, a vignette oval. Most people wore their public smile; there’s an element of pride and playfulness. Men tried to look jaunty; children didn’t care; young women vamped the lens or stared it down.

Let’s say you pick up a photo of a woman, sturdy, unlovely, addressing the camera with a look that could spell resignation or indifference. You turn it over and read: Emily Torgerson (1901-1979). Born in Virginia, Minn., she attended the Teachers College in Moorhead, moved to Chicago and eventually became editor of the Sundial, a literary magazine that published several of her short stories. In 1943 her novel “The Clay of Our Hearts” was called “a promising debut” by Saturday Review.

That’s what you imagine is written, but, of course, there’s nothing there.

Flash Brownie pictures. Introduced in 1940, the Flash Brownie, Kodak’s first synchronized flash camera, revolutionized photography by making it possible to shoot indoors without elaborate lighting setups. So now you had everyone around the Christmas tree with a big white bloom reflected in the mirror. In addition, the lens was better and the prints were bigger than those from earlier Brownies.

Most antique-store photos come from this era, which lasted until about 1955 — soldiers home on leave, bobby-soxer teens goofing around, the farmhands on the old tractor. Mostly it’s just someone standing somewhere, unaware this is the only proof of his existence that will survive. Aside from his headstone.

And then ... nothing. You won’t find many color snapshots in antique stores, perhaps for two reasons:

• People started shooting slides. It was an early form of social media: Company comes over, they have to see the slides. “Here we are at Rushmore!” Now the slides have been cast aside. In the antique stores, they are piled like poker chips from a defunct casino.

• People have a harder time throwing away color shots. A black-and-white snap of your cousin as a baby? Eh. A color picture of your cousin on a trike in your driveway? Precious.

In the next couple of decades, a billion ’70s snapshots will hit the antique stores — matte finish, rounded corners. Kids will inherit their parents’ collections, and while some will care, many will regard the evidence of their forebears as a burden. The lament of the kids going through Mom’s stuff: “Who are these people?”

Fifty years hence, there won’t be any photos in antique stores from this era because no one prints anything out. The digital versions of the pictures might be saved in some still-to-be-developed ultra-high-tech storage facility on Jupiter’s third moon, but few will have made it to a scrapbook left behind when someone passes.

What’s the moral of the story?

For starters, you might have an elderly relation who knows the names of the people in the family photos. Ask them, because when they go, the connective tissue to all those faces dissolves. Write down the information on the back.

Then sort through your own digital photos and give them useful names to replace IMG_934834. You can even add a few notes about the person in the photo’s metadata. Imagine turning over a studio portrait in an antique-store basement and reading, “Harold was funny and loved spaghetti and jazz.” Suddenly you know a bit more about the person in the picture — and you get a sense of the person behind the shutter, too.

Most important, tell your kids to back up their pictures. If you’re lucky, you have a total of 27 pictures of your grandma. Your kids have 27 pictures of yesterday. Save some of them and give them names.

And then print them out. A picture on your phone is swiped through so fast it barely registers on your brain. A picture you can hold is a portal: Fall in, look back. It’s a casual, unplanned memorial: “Look at that guy with his ’40s hair and sharp-creased pants, sitting on a log, God knows where. God knows who.”

Pay 50 cents for the snapshot in the antique store. If anyone asks who it was, you can say you have no idea, but he mattered to someone. There’s the picture. There’s the proof.