— Junk DNA — or, as scientists call it nowadays, noncoding DNA — remains a mystery: No one knows how much of it is essential for life. As one scientist mused, "Is the genome a trash novel, from which you can remove a hundred pages and it doesn't matter, or is it more like a Hemingway, where if you remove a page, the story line is lost?"
However enigmatic, though, noncoding DNA has proved mighty useful for scientists in one way — it's great for tracking evolution, through so-called DNA clocks.
DNA clocks take advantage of the fact that DNA mutates at a constant rate: Every so many years, a new mutation should pop up along a stretch of DNA. So in examining the natural history of two related species — which once had the same DNA sequence — a scientist can count the number of different mutations that have accumulated along a stretch, and estimate from that how many years have passed since the species started drifting apart.
But mutations to genes have bigger consequences: They can disable proteins and kill a creature. As a result, mutations within genes often get weeded out and don't get passed on to future generations.
Geneticists have used noncoding DNA in other types of historical analyses as well. Until the past few years, most scientists doubted that human beings had ever stooped to interbreed with Neanderthals or other archaic hominids.
That view has since been routed: DNA extracted from old Neanderthal bones proves that all people of European and Asian descent have a small percent of Neanderthal DNA inside them today, equivalent to the amount they inherited from each great-great-great-grandparent.
In addition, scientists have discovered that Melanesians, the people who originally settled the islands between New Guinea and Fiji, seduced another archaic human race, the Denisovans, somewhere on the long haul from Africa to the south seas.
The Melanesians still carry Denisovan DNA today. In some sense, then, neither Neanderthals nor Denisovans ever quite went extinct: Their DNA lives on in various non-African ethnic groups.
So what about ancient Africans? Did they ever have hominid paramours? Scientists had a harder time answering that question because the hot climate of Africa — unlike the cold Eurasian climate where Neanderthal and Denisovans lived — tends to destroy ancient DNA. A clever study from 2011 got around this limitation, however, and suggests that Africans did indeed indulge in inter-Homo hanky-panky.
The study's scientists pored over the noncoding DNA of various ethnic groups in Africa, looking for funny patterns. For instance, they looked for random mutations that no other ethnic groups worldwide had.
They were especially keen on finding long stretches of such funny DNA, because these stretches were probably inherited en masse from an archaic hominid. Sure enough, regions on chromosome 18, among other places, fit the bill.
The study concluded that, about 35,000 years ago, some central Africans had children with an unnamed and now-extinct race of hominids. Like Europeans and Asians before them, these people couldn't resist the temptations of nonhuman lovers.
As scientists continue to probe the human genome, they'll likely find even more examples of interbreeding in our past. The DNA memories of those deeds are buried deeper inside us than even our ids, but it seems that all peoples, everywhere, enjoyed cross-species love.
The grand saga of how humans spread across the globe will need some amendments and annotations — rendezvous here, elopements there, and the commingling of genes most everywhere. At least we can say that all humans are united in sharing this shame (if shame it is) and in sharing these scarlet As, Cs, Gs, and Ts.
Sam Kean is the author of "The Disappearing Spoon" and "The Violinist's Thumb."