It begins with a few suspicious snippets of DNA.
They sit quietly inside our chromosomes—genetic heirlooms from ancestors whose names have been forgotten and whose family photographs were never taken because photography would not be invented for another 50,000 years. Europeans and Asians commonly carry about 1–2 percent Neanderthal DNA. Some populations in Oceania possess an additional helping of Denisovan ancestry.
Clearly, something happened.
Officially, scientists call it archaic admixture. But let us dispense with the laboratory euphemisms. Our ancestors were sleeping with the neighbors.
These were the original fusion families—and one suspects they generated considerable gossip around the communal fire.
“Did you hear about Oona?”
“No.”
“She’s seeing a Neanderthal.”
“The one with the brow ridge?”
“They all have brow ridges, Mabel.”
Out of Africa—and Into an Awkward Reception
Early Homo sapiens did not simply leave Africa once in a single triumphant parade. There appear to have been earlier excursions into the Middle East and Eurasia, followed later by the major expansion—roughly 60,000 to 70,000 years ago—that produced most of the ancestry of people living outside Africa today.
So, in broad terms, we were out of Africa more than once. The first expeditions may have faded away or contributed little to later populations. The later travelers were more successful, although “successful” is a generous term for people crossing deserts on foot without maps, luggage, antibiotics or a decent airport lounge.
When Homo sapiens entered Neanderthal territory, they encountered people who had already survived hundreds of thousands of years of European winters. Neanderthals were powerfully built, technologically capable and remarkably well adapted to cold climates.
The newcomers were probably not greeted with gift baskets.
From the Neanderthal perspective, these skinny tropical immigrants must have looked poorly equipped:
“Look, Urg. More sapiens.”
“They’ll never last the winter.”
“They’re wearing shells as jewelry.”
“Pretentious.”
The Neanderthals were the hearty locals—the prehistoric equivalent of people who shovel snow in shorts. Homo sapiens, by comparison, arrived with narrower bodies, complicated social networks and an irritating tendency to keep moving into neighborhoods where nobody had invited them.
The First Culture War
It is tempting to imagine sapiens proudly unveiling their latest technological achievement.
“Behold—a finely shaped stone blade!”
The Neanderthal examined it.
“You spent all afternoon making that?”
“It has a symmetrical edge.”
“It’s a rock, Kevin.”
Both Neanderthals and modern humans made sophisticated tools. Neanderthals also used pigments, collected unusual objects and may have created symbolic markings or art of their own. The old caricature of the stupid, shambling caveman has not aged well.
Still, cultural rivalry must have been irresistible.
When sapiens began decorating caves with horses, bison and handprints, conservative Neanderthal critics may have complained that the work lacked realism.
“The bison’s legs are completely out of proportion.”
“It’s symbolic.”
“It’s rubbish. And now there’s ochre all over the wall.”
A Neanderthal father might have watched his daughter admire a sapiens artist and muttered, “He calls himself a painter, but he still lives in a cave.”
Love Across the Species Line
We do not know whether prehistoric groups disapproved of interbreeding. No Pleistocene advice columns survive, and archaeology has yet to uncover a stone tablet reading, “Over my dead body will you marry a Denisovan.”
Indeed, the word species becomes rather slippery here. Neanderthals, Denisovans and modern humans were distinct populations, but they could have fertile offspring. Whether we call them separate species, subspecies or humanity’s extremely distant regional cousins depends partly on how we define species.
Whatever terminology we choose, their DNA tells us the relationships occurred repeatedly.
The courtship process remains unknown.
Perhaps sapiens males impressed Neanderthal women with beads, music and long-distance trade networks. Perhaps Neanderthal males impressed sapiens women by lifting an entire elk without bending their knees.
One imagines the parental objections:
“He’s not our kind.”
“He’s thoughtful.”
“He has no chin.”
“That’s superficial.”
“He lives beyond the third glacier!”
“I love him.”
“Oh, wonderful. Mixed children. What will the mammoths say?”
Yet the children survived. Some had children of their own. Their genetic traces traveled through thousands of generations and now influence aspects of immunity, skin biology, metabolism and disease susceptibility.
Every family has secrets. Ours are encoded in base pairs.
The Denisovan Scandal
Denisovans are even more mysterious. We know them largely from DNA extracted from a few bones and teeth found in Asia. They left far fewer recognizable fossils than Neanderthals, yet their genetic legacy survives strongly in some populations, particularly in Papua New Guinea and other parts of Oceania.
Denisovan ancestry also contributed useful adaptations. One famous example is a version of the EPAS1 gene that helps Tibetans function at high altitude.
Thus, an ancient liaison with Denisovans eventually enabled people to live comfortably where sea-level visitors become breathless carrying a sandwich upstairs.
Evolution is nothing if not practical.
The Denisovans themselves had apparently interbred with even older, unidentified archaic humans—a “ghost population” that may have been related to Homo erectus or another ancient lineage.
This means Denisovans were criticizing their children’s romantic choices while concealing archaic entanglements of their own.
“You cannot marry a sapiens!”
“But Grandmother says your side of the family has erectus DNA.”
“Grandmother talks too much.”
We have no direct genetic proof that Homo erectus interbred with modern humans. But genetic studies suggest that some Denisovan ancestors mated with a very ancient human lineage. If that lineage was Homo erectus or something similar, then the prehistoric family tree was less a tree than a hedge after several glasses of wine.
Homo erectus: The Original Wanderer
Long before modern humans began congratulating themselves for leaving Africa, Homo erectus had already done it—perhaps nearly two million years ago.
They spread across enormous distances and survived far longer than our species has existed so far. If longevity were the measure of evolutionary success, Homo erectus could look at Homo sapiens and say, “Call me when you’ve made it past the million-year mark.”
Whether erectus engaged in genetic unfaithfulness with later human groups remains uncertain. But considering the record of every human population for which we possess DNA, the safest conclusion is that when two hominin groups met, they sometimes fought, sometimes exchanged technology and occasionally exchanged phone numbers—metaphorically speaking, since the Pleistocene cellular network was terrible.
The Shame of the Fusion Family
Did mixed families experience prejudice? Possibly, but we cannot know. Small hunter-gatherer groups depended heavily on cooperation, and outsiders could be regarded as competitors, allies, trading partners or potential spouses. A newcomer’s ancestry probably mattered less than whether he could find water, butcher an animal and refrain from attracting hyenas.
Still, one can imagine the social discomfort.
At the annual solstice gathering, a mixed sapiens-Neanderthal family arrives.
The children are robust, clever and slightly hairier than average.
Everyone tries not to stare.
A sapiens aunt whispers, “They have his brow ridge.”
The Neanderthal grandmother replies, “At least they have shoulders.”
Then the children demonstrate a better hunting technique, decorate the cave and survive the next epidemic. Within a few generations, everybody has forgotten why the match was controversial.
That is the great joke hidden in our DNA: the supposedly improper unions were not evolutionary mistakes. Some produced descendants who were better adapted to local diseases, climates and environments.
The scandal became an advantage.
We Are All the Result of Questionable Decisions
Eventually, Neanderthals disappeared as a distinct population. Denisovans vanished from the visible archaeological record. Other human lineages faded as well. Some were displaced, some absorbed and some probably succumbed to combinations of climate change, small population size, competition and plain bad luck.
But they did not disappear completely.
They survive inside us—in immune responses, skin traits, metabolic quirks and tiny stretches of chromosomes passed down from lovers who met tens of thousands of years ago.
We like to tell human evolution as a heroic story in which Homo sapiens marched out of Africa, defeated inferior rivals and inherited the Earth through superior intelligence.
The DNA tells a messier and much more entertaining story.
We wandered out of Africa in multiple waves. We met people who looked different from us. We competed with them, borrowed from them and had children with them. Some families were undoubtedly appalled. Some neighbors probably gossiped. A few fathers sat silently in the corner sharpening hand axes.
And yet the children endured.
So the next time someone complains about intermarriage, cultural mixing or newcomers entering the neighborhood, remind them that human beings have been upsetting the in-laws since the Ice Age.
Then show them their genome.
There is probably a Neanderthal in it, quietly raising one enormous eyebrow.