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Long ago, I developed shingles in my early 30s. Y’all, let me tell you: Shingles hurts. Worse, it hurts in what I’d call a downright holistic way. My days became a haze of endless pain.

(Folks over 50 in particular, get your vaccine! You do not want this to happen to you!)

At some point in that jolly month-long adventure, I told Mr. Captain that I felt “like a bag of viruses shambling around.”

At the time, I didn’t know how right I was.

Nowadays, just about everybody knows what DNA is. DNA, or deoxyribonucleic acid, contains blueprints for cell reproduction. And every organism on earth that qualifies as alive, and some that reside in a shadowland betwixt and between, like viruses, uses DNA for reproduction.

Early in their schooling, people nowadays also learn that most creatures reproduce by combining those blueprints from their parents’ eggs and sperm, thus mixing genetic material. Genetic mixing achieves better outcomes for survival and more potential for future reproduction. We call a failure to mix adequately inbreeding, and we recognize it as a really bad thing.

But according to this Science Daily article, only a tiny fraction of our DNA is uniquely human.

The messy DNA in our stars

Our DNA contains a grab bag of basically all the stuff that’s ever infected and destroyed our species. Many of us contain entire copies of ancient and hopefully-defunct viruses. Most of us carry around DNA that’s about 8% viruses. And quite a lot of what’s left of our DNA is called “junk.”

Junk DNA doesn’t seem to code for anything. Discover describes it as resembling “gibberish sentences in a book draft.” It still does stuff, but it’s more subtle than a worker, apparently. Indeed, Discover discusses a 2017 paper wherein a scientist estimated that maybe a quarter of our DNA is actually functional. By that term, he means that it actually codes for the creation of proteins.

The rest is just stuff we accumulated over the eons. It serves a purpose, but we don’t always know what that purpose is quite yet. It might just protect the functional DNA from excessive mutations that could easily kill us before we can reproduce.

And of course, reproduction is always the determining factor in what gets kept and what gets tossed. Evolution describes a creature’s chain of ancestors. Every single change in those ancestors’ genes either helped them survive to reproduce, or else it didn’t matter one way or the other. If it had done anything else, then those creatures would have died before they could reproduce – and thus wouldn’t be part of the ancestral chain at all.

Even scientists who know that no gods are behind any of this process often anthropomorphize evolution’s handmaiden, natural selection. They talk of it as a force that “makes deals” and tradeoffs, as I learned years ago. But it’s nowhere near as intelligent as that.

Natural selection is just the raw force of teeming, surging life itself—uncaring, discompassionate, even cruel—and yet it is none of these. It just is.

Genes can’t feel things or want things or even hope for things, obviously. Even if Pixar makes a movie about genes with feelings, which—let’s face it—is probably coming at some point, that wouldn’t reflect reality.

DNA gets around

Sharing is caring, as the saying goes. And our DNA is quite a magpie. A while ago, we learned that human DNA shares code with a lot of other animals. We share 80% of our DNA with cows, 61% with insects, and 60% with bananas.

As animals get smarter, our DNA shares with them get higher. We share 85% of our DNA with mice, 90% with Abyssinian housecats, 93% with rhesus monkeys, and—as the American Museum of Natural History tells us—a whopping 98.8% with chimpanzees. They’re the closest living relatives we have, so that makes sense. Our common ancestor lived only 6-7 million years ago, which isn’t too long at all as scientists reckon it.

So you can imagine that we probably share a lot more DNA with other ancient human species. And we know that our species, Homo sapiens sapiens, coexisted with many other species at one time. A full 300k years ago, when our species came into being, nine human species existed at once – that we know about, anyway. All but one vanished, leaving us triumphant about 10k years ago.

By the time they vanished, though, those other humans had interbred with us. Their DNA mixed with ours, and became a permanent part of us forever after.

When humans aren’t entirely human.

This new paper, which came out just a few days ago in Science Advances, tells us that only a fraction of our DNA—1.5% to 7%—is actually “uniquely human.” The rest is all that I’ve talked about above: old virus code, stuff shared with distant animal ancestors, even gibberish, and yes, some of it is packets we inherited from past liaisons with other human species.

Depending on where a person’s particular ancestors were from, they got more or less DNA from specific species. People whose ancestors largely stayed in Africa got way less Neanderthal DNA. Some modern folks got a lot more DNA from Denisovans than others. And when these researchers studied the DNA of their subjects, they found that we can carry our packets of interbred DNA in different places.

When these researchers whittled out all the junk, interbred, and shared DNA, they looked for how much of our DNA is actually just ours – not shared by any other species, not carried for eons from ancient viruses or other human species, but stuff found only in Homo sapiens sapiens.

And it turns out that only about 1.5% to 7% qualifies as “uniquely human.”

How we got that uniquely human DNA.

That recent paper’s researchers also found out that that small percentage came about in two specific “evolutionary bursts.” One occurred about 600k years ago, which is when we were just starting to separate out as a species. The other happened about 200k years ago.

They think all of this stuff means that seriously, interbreeding was really important to the story of how our particular human species came to be.

And to me, that’s a good thing. Remember above, when I mentioned that inbreeding is bad? That’s because it can concentrate genetic traits that are best kept weak. When we breed with people far outside our own families and close communities, we introduce DNA to the mix that will hopefully give our future children an advantage – and make them more likely, therefore, to reproduce in turn.

Genes can’t feel things or want things or even hope for things, obviously. Even if Pixar makes a movie about genes with feelings, which—let’s face it—is probably coming at some point, that wouldn’t reflect reality.

However, reproduction is very obviously our genes’ only concern. So in a lot of ways, those ancient humans’ DNA succeeded – maybe not in the obvious way, but in a very real way all the same. Those other human species are still here, their DNA quietly passing into the next generation and those to come.

ROLL TO DISBELIEVE "Captain Cassidy" is Cassidy McGillicuddy, a Gen Xer and ex-Pentecostal. (The title is metaphorical.) She writes about the intersection of psychology, belief, popular culture, science,...