At a laboratory in Germany, a researcher sits stunned. Surely it can’t be true? It looks like something scientists thought they would never find. But there’s no mistake here: this girl was born from two entirely different species. Once the startled experts uncovered the true significance of the unsettling discovery, they realized there was no going back. The mysterious girl, and our own forgotten past, would never be the same.
Scientists have long suspected that there was interbreeding between ancient humans. The chances of uncovering proof of this? Slim at best, or so the experts thought. Then researchers in a cave in Siberia stumbled upon a tiny fragment of bone. Initially, the team didn’t even realize that this came from a hominin – a term that just means “all the species regarded as human.” But soon an incredible story began to unfold.
Although the bone languished in obscurity for years, one intrepid researcher eventually found it and began to inspect it. The Max Planck Institute for Evolutionary Anthropology’s Viviane Slon also decided to try to extract DNA from the artifact. And what she found has turned decades of research on its head. Now, we have some exciting new truths about how ancient humans made their way in the world.
First Confirmed Hybrid
Why was the bone so important? Well, we know that a number of different species walked the Earth before and even alongside modern humans. This particular discovery marked the first time that a direct hybrid had been unearthed, however. It was history in the making, and so it’s no wonder that researchers reacted to the news with delight.
And there’s an incredible tale behind this fragment of bone – a story all about the human race. Today, all people belong to the same species, Homo sapiens, which first emerged approximately 200,000 to 300,000 years ago. But as prehistory buffs know, that hasn’t always been the case.
The earliest known human ancestors were actually the Australopithecines. These were a number of different species that were capable of both climbing and walking on two legs. According to research, these distant relatives of Homo sapiens first emerged in Africa more than four million years ago. And, of course, they would have looked very different from how we appear today.
The Homo Species Begins
Next, scientists believe, the various species of Homo began to emerge. At first, they evolved longer legs that were better suited to running and walking. Then their brains began to grow. And these adaptations may have signaled a change in behavior, as these early humans began to hunt and take on a more carnivorous diet.
A New Look
Then, about 700,000 years ago, the species known as Homo heidelbergensis emerged in Africa and Eurasia. And experts have suggested that these hominins were much more like modern people in their appearance, laying the groundwork for how their descendants would evolve. They acted pretty differently from their predecessors, too.
Apparently, Homo heidelbergensis was likely more intelligent than those who had come before. Members used advanced tools and honed their hunting techniques, for example. Some even believe that individuals may have teamed up to bring down larger animals, which indicates a degree of social cohesion. But despite Homo heidelbergensis’ many strengths, the species still died out.
Modern Humans Emerge
You should know, though, that Homo heidelbergensis didn’t disappear from the Earth without leaving a trace. Far from it, in fact. An estimated 390,000 years ago, in the Middle Pleistocene era, a number of different species began to split off from this common ancestor. And from these branches, modern humans would ultimately emerge.
Of course, the story of mankind is one of rich and varied evolutionary history, with many species of hominin thought to have coexisted alongside one another over the millennia. And they didn’t just tolerate one another’s presence. For years, researchers have known that a degree of interbreeding took place between these different groups of early humans. Until the German discovery, however, no one could prove this for sure.
And, yes, although it’s often said that members of different species cannot successfully interbreed, this is far from an established fact. As Forbes’ Michael Marshall pointed out in a 2018 article, while a mule born from a donkey and a horse is always infertile, the outcome of other inter-species pairings could vary from animal to animal.
Odd Chromosome Out
Apparently, it’s all to do with DNA. You see, a mule is the product of a horse, which has 64 chromosomes, mating with a donkey, which has 62. So, the offspring of the two animals ends up with 63 chromosomes – an odd number. And, naturally, this has consequences. As the mechanics of sexual reproduction require an egg and a sperm to each contain 50 percent of an animal’s chromosomes, this non-even number means the creature has a “defective” genetic code – one that prevents it from reproducing further.
Two Evens Make It Possible
But some primate species, such as gorillas and orangutans, share identical numbers of chromosomes. Some researchers have theorized, then, that it could be easier for them to produce healthy offspring. There’s even evidence that bonobos and chimpanzees have interbred at various points throughout their history.
The Liger Example
Interestingly, this theory could also explain why some big cats are able to successfully interbreed. The much-touted liger never actually occurs in nature, as lions and tigers’ natural habitats are typically too far apart for mating to occur. But several zoos around the world now house examples of this large creature, which, as an adult, is usually bigger than either of its parents. Ligers can also go on to produce their own offspring.
And, crucially, early humans are also thought to have shared the same number of chromosomes. That meant the different species were able to interbreed. Experts believe that Homo sapiens began mating with Homo neanderthal not long after migrating from Africa and spreading out around the world.
That’s why most modern humans from Asia and Europe have about 2 percent Neanderthal DNA. Perhaps you noticed this in your own genetic test? But Homo sapiens wasn’t just coupling with Homo neanderthal. Apparently, members of the species also mated with those from another branch of the human family tree. They’re known as the Denisovans.
The Denisovans are a fairly recent discovery in the field of evolutionary studies. In fact, definite evidence of their existence has only come to light in the 21st century. In 2010 a team of scientists, also from the Max Planck Institute, announced the results of their latest research. After having analyzed a tooth and a finger bone found in the Altai Mountains in Siberia, they had found evidence of a new species of early human.
Pleased with their monumental breakthrough, the researchers dubbed the species Denisova in honor of the cave in which the specimens were found. But aside from what could be interpreted from DNA, little was known about this human ancestor. Then, in 2012, research at the same site in Siberia turned up another small fragment of bone.
At first, researchers lumped the unremarkable fragment in with the countless animal fossils that were also retrieved from the cave. And it wasn’t until years later, when the University of Oxford’s Samantha Brown took a closer look, that its true nature was revealed. Tasked with cataloging the artifacts, Brown analyzed the proteins inside this bone – and realized that it had come from an ancient human.
After that, the bone passed to Slon, a paleogeneticist. And in order to learn more about this mystery hominin, she, too, began to investigate the DNA contained within the fragment. But in the end, she found more than anyone was expecting.
A 13-Year-Old Girl
At first, it seems, the bone did not appear to be anything particularly remarkable. Just one inch in length, it is believed to have come from a teenage girl who was probably around 13 years of age. It’s thought that she died approximately 90,000 years ago, when the Denisovans populated this small corner of the Altai Mountains.
But when Slon analyzed the DNA present in the bone’s mitochondria, she was in for a big surprise. As anyone with a keen interest in genetics knows, this type of cellular structure contains material that a child inherits only from their mother. And in this case, it indicated that the teenager was descended from a female Neanderthal.
“This was already very exciting,” Slon told National Geographic in 2018. “It only got more exciting when we started looking at the nuclear DNA.” Our knowledge of genetics tells us that this material is passed down through both the male and female lines, and it allowed scientists to learn more about the father of this ancient teenager.
“That’s when we realized there was something a bit funky about this bone,” Slon continued. In fact, the results were so shocking that she was initially convinced she had made a mistake. Had she somehow skewed the data without realizing it? Or had the sample perhaps been corrupted in the laboratory?
Eventually, though, Slon realized that there was no mistake. Although the teenager’s mother had Neanderthal DNA, her father, according to the analysis, had been a Denisovan. And that wasn’t all. While analyzing the bone fragment, the paleogeneticist also discovered that the girl’s genetic makeup was remarkably varied as a whole.
But what does that mean in layman’s terms? Well, it’s all to do with a concept known as heterozygosity. Essentially, if your parents were closely related – let’s say, second cousins, for example – the amount of heterozygosity present in your genes would be relatively meager. If you were the result of inter-species breeding, on the other hand, those levels would be sky-high. Make sense?
And with the bone found in Denisova Cave, it was definitely a case of the latter. Speaking to National Geographic, computational biologist Richard E. Green explained of the ancient DNA, “It’s heterozygous out the wazoo. That’s really what nails it.” Amazingly, Slon had discovered one of the holy grails of human evolution: a first-generation child born of interbreeding between species.
“We knew from previous studies that Neanderthals and Denisovans must have occasionally had children together,” Slon told London newspaper the Evening Standard in 2018. “But I never thought we would be so lucky as to find an actual offspring of the two groups.” And at Harvard University, geneticist David Reich agreed.
“It’s amazing to be able to find something like this,” Reich said to National Geographic. “It seemed unlikely that we would be able to catch it happening in the act – an individual that’s really the product of a first-generation hybrid.” The discovery was so fortuitous, in fact, that it has raised questions about how common such interbreeding really was.
“It is striking that we find this Denisovan/Neanderthal child among the handful of ancient individuals whose genomes have been sequenced,” the Max Planck Institute’s Svante Pääbo told the Evening Standard. “Neanderthals and Denisovans may not have had many opportunities to meet. But when they did, they must have mated frequently – much more so than we previously thought.”
Adjusting Our Historical Views
Although it’s possible that the discovery was little more than a lucky break, researchers are considering other explanations. One of these is that the two species of hominin actually interacted – and interbred – with each other on a regular basis. And if this theory is true, it would turn our previous understanding of the ancient world on its head.
But the bone of the teenager – who has been dubbed Denny – isn’t the only evidence that lends support to this hypothesis. Up to 2018, scientists had only conducted genetic research on a relatively small number of ancient humans – 23, to be precise. Still, even within this tiny sample, there were at least two specimens that showed evidence of interbreeding between species.
Take the individual known as Oase 1, for instance. Identified by their lower jaw, this member of Homo sapiens is believed to have walked the planet about 37,000 years ago. But despite their relatively recent place on the human family tree, they were found to be carrying Neanderthal DNA.
Few And Far Between
And we’re not talking about the very distant past, either. According to a report published in the journal Nature in 2015, Oase 1’s Neanderthal forebears may have been alive only four to six generations previously. If interbreeding between species had only occurred sporadically, Pääbo reasoned, discoveries such as this should be few and far between.
On top of that, the study from the Max Planck Institute noticed something else about Denny. Apparently, the teenager’s father also had Neanderthal DNA combined with his Denisovan genes. And that’s incredibly revealing. According to Pääbo, “It suggests that these groups, when they met, mixed quite freely with each other.”
Previously, most researchers assumed that interactions between these different groups had happened only infrequently. So, how do these latest developments alter our view of ancient humans and their evolving society? Speaking to National Geographic, Reich explained, “[It]... qualitatively transforms and changes our understanding of the world. And that’s really exciting.”
Of course, there could be other explanations as to why a first-generation hybrid has already appeared in such a limited sample size. In Green’s opinion, caves such as the one in the Altai Mountains could simply have been popular meeting points for ancient humans, bringing sampling bias into the equation. Or, as the specialist neatly put it in an interview with National Geographic, “They’re the singles bars of the Pleistocene Eurasia.”
Key To Survival
But was it simply proximity that inspired the Denisovans and the Neanderthals to interbreed? Or was something else at play? Well, according to the University of Tübingen’s Katerina Harvati-Papatheodorou, such cross-species interactions could have formed a vital part of survival. Speaking to New Scientist, the German academic explained, “Human groups were very small and vulnerable to drastic mortality.”
Died Out Or Assimilated
And as more information emerges, scientists hope to solve some of the mysteries that have long puzzled those who study human evolution. Did the Denisovans and Neanderthals quickly die out as Homo sapiens began to thrive? Or were they simply assimilated into the dominant species? In an interview with New Scientist, Princeton University’s Joshua Akey admitted that Denny’s DNA points to the second of those assumptions, although we are a long way from a definitive answer.