Awesome - Geneticists Studying Ancient DNA Discovered A Girl Whose Parents Were Two Different Species
At a laboratory in Leipzig, Germany, a postdoctoral researcher is puzzling over a piece of ancient bone. According to DNA tests, this relic represents something that paleontologists specializing in genetics thought they would never find: a hybrid between two of humanity’s early relatives. At first, she suspects that an error has been made, but soon the truth becomes apparent. These remains come from a girl whose parents were from two entirely different species.
Although scientists have long suspected that interbreeding between ancient humans occurred, the chances of finding a first-generation offspring were considered slim. But then, researchers in a cave in Siberia stumbled upon a tiny fragment of bone. Initially, they didn’t even realize that it came from a hominin – a catch-all term which just means “all the species regarded as human” – but then an incredible story began to unfold.
After its 2012 discovery, for years the bone had languished in obscurity, with nobody realizing its significance. But then, Viviane Slon at the Max Planck Institute began extracting its DNA. And what she found has turned decades of research on its head, revealing exciting new truths about how ancient humans made their way in the world.
Ever since Neanderthal remains were first identified in Germany in the 1850s, scientists have been battling to understand more about our ancient ancestors. And now, we know that a number of different species walked the Earth both before and indeed alongside modern humans. But this discovery, whose findings were first reported in August 2018, marked the first time that a direct hybrid had been discovered, and researchers reacted with delight.
So what is the story behind this fragment of bone – and the girl to whom it once belonged? As it turns out, her tale is intertwined with that of the human race. Today, all people belong to the same species, Homo sapiens, which first appeared some 200,000 to 300,000 years ago. But that hasn’t always been the case.
In fact, the earliest-known human ancestors were the Australopithecines, a number of different species which were capable of climbing as well as walking on two legs. According to research, these first emerged in Africa more than 4 million years ago. But of course, they would have looked very different to the way we look today.
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. And then, their brains began to grow. According to some, these adaptations signaled a change in behavior, as these early humans began to hunt and consume a heavily carnivorous diet.
Then, about 700,000 years ago, the species known as Homo heidelbergensis emerged in Africa and Eurasia. According to researchers, these hominins were much more like you or I in their appearance, laying the groundwork for how their descendants would evolve. And they didn’t just look different: they acted differently as well.
Apparently, Homo heidelbergensis were likely more intelligent than their predecessors, using advanced tools and honing their hunting techniques. In fact, some believe that these early humans may have teamed up with one another to bring down larger animals, indicating a degree of social cohesion. Despite its advantages, though, this species eventually died out.
But 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 features a rich and varied evolutionary history, and many species of hominin are believed 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.
Today, there is an oft-repeated claim that members of different species cannot successfully interbreed. But this is far from an established fact. And, as Forbes magazine points out, although a mule born from a donkey and a horse is always infertile, the outcome could vary from animal to animal.
Apparently, it’s all to do with DNA. In the case of the mule, it is the product of a horse, which has 64 chromosomes, mating with a donkey, which has 62. And as a result, the offspring of the two animals is lumped with 63 – an odd number. The mechanics of sexual reproduction require eggs and sperm to carry precisely half of an animal’s chromosomes, so this non-even number means the creature has a “defective” genetic code which prevents it from reproducing further.
But many species, such as gorillas and orangutans, share the same number of chromosomes. And as such, some researchers have theorized, it could be easier for them to produce healthy offspring. For example, sticking with primates, there is evidence that bonobos and chimpanzees have interbred at various points throughout their history.
Interestingly, this theory could also explain why some big cats, such as lions and tigers, are able to successfully interbreed. Known as a liger, this hybrid never occurs in nature, as the animals’ habitats are too far apart for mating to occur. But several zoos around the world now house examples of this large creature, which grows bigger than either of its parents and can go on to produce its own offspring.
Like the big cats and the great apes, early humans are also thought to have shared the same number of chromosomes. And throughout history, the different species were known to interbreed. In fact, experts believe that Home sapiens began mating with Homo neanderthal not long after leaving Africa and spreading out around the world.
As a result of this interbreeding, it’s believed that most modern humans from Asia and Europe have about 2 percent Neanderthal DNA. And that wasn’t the only inter-species action that was going on. According to researchers, Homo sapiens also mated with another of our early ancestors, known as the Denisovans.
Compared to the Neanderthals, the Denisovans are a fairly recent discovery in the field of evolutionary studies. In 2010 a team of scientists, also from the Max Planck Institute, announced the results of their latest research. After analyzing 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 discovery, the researchers dubbed the species Denisovan in honor of the cave in which the specimens were found. But, aside from its DNA, little was known about this early human. 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 cataloguing the artifacts, she 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 at the Institute’s Evolutionary Anthropology department. And in order to learn more about this mystery hominin, she began to study the DNA contained within the fragment. But in the end, she found more than anyone was expecting.
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 the leg or arm of a teenage girl, probably roughly 13 years of age. And 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 magazine 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. In this case, 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? Or had the sample perhaps been contaminated 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 its genetic makeup was strikingly 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 low. But if you were the result of inter-species breeding, those levels would be sky-high.
And with the bone found in Denisovan cave, it was definitely a case of the latter. Speaking to National Geographic, computational biologist Richard E. Green explained, “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 offspring 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.” Meanwhile, at Harvard University, geneticist David Reich agreed.
“It’s amazing to be able to find something like this,” Reich told 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.” In fact, the discovery was so fortuitous that it has raised some questions about just 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.”
Although it’s possible, of course, that the discovery was little more than a lucky break, researchers are considering other explanations. And one of these is that the two species of early humans actually interacted – and interbred – with each other on a regular basis. If true, this would turn our previous understanding of the ancient world on its head.
But it’s not just Slon’s discovery, dubbed Denny, that has been lending support to this theory. Up to 2018 scientists had only conducted genetic research on a relatively small number of early humans: 23, to be precise. Yet within this tiny sample, there were at least two specimens which showed evidence of interbreeding between species.
As well as Denny, it seems, there is the individual known as Oase 1. Identified by their lower jaw, this Homo sapiens is believed to have lived about 37,000 years ago. But despite their relatively recent place on the human family tree, they were found to be carrying Neanderthal DNA.
And we’re not talking far back, either, like the DNA traces found in modern humans today. According to a report published in the journal Nature in 2015, Oase 1’s Neanderthal ancestors were as recent as some four to six generations back. 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 also revealed something else about Denny. Apparently, the teenager’s father also had Neanderthal DNA mixed in with his Denisovan genes. 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. According to Green, caves such as the one at Denisovan could simply have been popular meeting points for ancient humans, bringing sampling bias into the equation. Or, as he put it in an interview with National Geographic, “They’re the singles bars of the Pleistocene Eurasia.”
But was it simply proximity that inspired the Denisovans and the Neanderthals to interbreed? Or was something else at play? 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.”
As more information about ancient humans emerges, scientists hope to solve some of the mysteries that have long puzzled those who study human evolution. Did the Denisovans and Neanderthals suffer a violent extinction 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 Ackey admitted that Denny’s DNA points to the latter, although we are a long way from a definitive answer.
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