Modern humans evolved from Homo sapiens, and a small percentage of us have Neanderthal DNA. But did you know that there were more species of ancient humans? A newly-discovered girl had parents from two different species-- one Neanderthal, and another one that was recently discovered. This discovery will change our perception of human evolution.
Not Just Homo Sapiens And Neanderthals
Thanks to modern DNA tests, people can see whether they are part Neanderthal. Neanderthals are an extinct subspecies of humans that went extinct 40,000 years ago. Today, only 2% of Eurasians have Neanderthal DNA.
But there are more species of humans that existed thousands of years ago. A few recent discoveries have found a third species of ancient human that might have contributed to our ancestry. But how is that possible, and what happened to them?
The Third Species Of Human, Denisovans
In southern Siberia, Russia, there is a cave near the Altai Mountains called the Denisova Cave. It was named after an 18th-century hermit who lived there. In 2008, archaeologists from the Russian Academy of Sciences unearthed several bone fragments there.
Scientists dated the oldest bones to at least 51,000 years ago. But it was not a Neanderthal or Homo sapien; researchers from the Max Planck Institute announced that it was a new species of human, the Denisovan.
Who Were The Denisovans?
Beyond their DNA sequencing, little is known about the Denisovans. We know that they lived as far back as 217,000 years ago, but few Denisovan bone fragments have been found. Only five viable specimens have been analyzed to date.
But scientists had no idea that a new discovery would come out only a few years later. This ancient human was not just Denisovan; she was an interbred between two different human species, which challenges what we previously knew about human evolution.
Can Human Species Interbreed?
Interbreeding can be confusing. For instance, why do Neanderthals and Homo sapiens successfully interbreed, but mules (a cross between a horse and a donkey) become infertile? The answer lies in DNA.
A horse has 64 chromosomes, and a donkey has 62. When a mule is born, it gets 63 chromosomes-- an odd number, which is a "defective" genetic code. DNA needs to latch onto an even amount of chromosomes, 50% of the father's and 50% of the mother's.
The Key To Successful Interbreeding: Fertility
Although mules are infertile, plenty of other species can create fertile offspring. For instance, a liger (mix between a tiger and a lion) is fertile. These species are genetically compatible, just as many primates are also compatible.
Knowing this, it makes sense why Neanderthals and Homo sapiens could successfully interbreed. Researchers predicted that Denisovans also interbred with other human species, whether their offspring became fertile or not. However, they never found proof of this hypothesis until 2018.
In 2012, Archaeologists Had No Idea What They Would Find
In 2012, Russian archaeologists were once again examining the Denisova Cave. They uncovered multiple bone fragments, but they could not identify most of them. The archaeologists gathered a collection of 2,000 bone fragments and sent them to a lab.
These bone fragments sat untouched and unresearched for several years. Then, in 2016, a student at the University of Oxford discovered that one of these bone fragments was not like anything scientists had ever seen before.
Not Your Average Two-Centimeter Bone
Samantha Brown, an MSc student at the University of Oxford, was analyzing the 2,000 bone fragments in 2016. She was testing the DNA to determine what type of animal each bone belonged to. But to her surprise, at least one bone, only two centimeters long, ended up being human.
Knowing that Denisova Cave has a history of housing human remains, Brown looked closer. What she found was so shocking that she initially believed she had made a mistake.
What Did This College Student Discover?
Brown discovered that this human fragment was a unique result of interbreeding. The person was part Neanderthal and part Denisovan. If her results were accurate, then this would be the first evidence of first-generation breeding between two human species.
Brown contacted the head of the department, and then the bone fragments were sent to the the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. This Institute had the technology to closely examine the bone fragment and confirm Brown's results.
Even Researchers Couldn't Believe It
Brown was not the only person who was surprised. A team of paleogeneticists, Viviane Slon and Svante Pääbo, conducted a genome analysis of the bones. They isolated each DNA sequence to identify which parent was connected to each species.
To their shock, both researchers confirmed that Brown was right. This bone was indeed the first-ever Denisovan-Neanderthal hybrid discovered. "My first reaction was disbelief," Slon later admitted. The chances of finding a first-generation interbred human are especially low.
Nobody Expected This Very Rare Find
Scientists predicted that Denisovans mated with other ancient humans, but they never expected to find proof this quickly. "We knew from previous studies that Neanderthals and Denisovans must have occasionally had children together," Slon explained. "But I never thought we would be so lucky as to find an actual offspring of the two groups."
However, this DNA test was only the tip of the iceberg. Who was this ancient human? And how did these two species end up mating in the first place?
Who Was This Ancient Human?
When Slon and Pääbo further examined the bone fragment, they learned more about this ancient human. They knew that this person was a girl, and based on the width of her bones, they estimated that she was around 13 years old.
According to radiocarbon dating, this girl likely died 90,000 years ago. The initial bone that Brown examined was the tip of her finger. The researchers named her Denny, after the cave that she was found in.
Who Were Her Mother And Father?
To uncover Denny's parentage, the researchers were able to separate DNA sequences. They determined that the mitochondrial DNA came from a Neanderthal. Mitochondrial DNA comes from the mother, meaning that her mother was a Neanderthal and her father was a Denisovan.
"An interesting aspect of this genome is that it allows us to learn things about two populations-- the Neanderthals from the mother's side, and the Denisovans from the father's side," explained geneticist Fabrizio Mafessoni. The next question was how they came together.
However, Scientists Still Couldn't Believe Their Findings
Despite all of these findings, the research team still needed several people to confirm. "I thought they must have screwed up something," Pääbo recalled. For a third analysis, they sent the bones to the the Francis Crick Institute in London.
Population geneticist Pontus Skoglund concluded that these findings were correct. "They nail it," he said. “There seems to be no uncertainty at all.” But why were the scientists so uncertain about these findings? Was it really that shocking?
First-Generation Human Interbreeding Is Quite Rare
This ancient human was a result of first-generation interbreeding, meaning that their parents were fully Neanderthal and fully Denisovan. This is an incredibly rare find. Even a first-generation Neanderthal-Homo Sapien bone fragment has yet to be discovered.
Skoglund later explained, "To find a first-generation person of mixed ancestry from these groups is absolutely extraordinary. It's really great science coupled with a little bit of luck... I think it's going to go into the textbooks right away."
Denny's Life Is Still An Enigma
Although Denny's parentage is clear-cut, the rest of her life is an enigma. "Since her known remains consist of an unidentifiable bone fragment, it is very difficult to say anything about her daily life, activities, health, or subsistence," says Katerina Harvati-Papatheodorou at the University of Tübingen, Germany.
On top of this, Denny's paternal family history is largely unknown. Because Denisovans were a recent discovery, scientists have much to learn about this species' history and how they connect to modern humans.
How Did Denny's Parents Meet?
Although interbreeding is possible, it will not happen if the species live far apart from each other. For example, ligers don't naturally occur in the wild because lions and tigers do not live close to one another. Denny poses the same problem.
Harris believes that these groups traveled far before mating. "[Neanderthals] didn't just stay in one place for thousands of years," he explained. If this is true, then Denny's genome is truly unique compared to others during that time.
Perhaps Hybrids Lived In A Colony Together
Another possibility is that Denny was one of many who lived in a Neanderthal-Denisovan colony. That would explain why archaeologists have found so few Denisovan bones. The number of pure Denisovan bones that have been found I can count on one hand," Harris says.
If this is true, then there were few pure-blood Denisovans around. Their genes might have mixed into the Neanderthal bloodline, and it might have eventually disappeared. But then again, this is just a theory.
But Her DNA Suggests That Her Parents Didn't Live Together
Denny is not a pure hybrid. Although her DNA is mostly Denisovan and Neanderthal, she has some other hominem genes, too. According to the tests, Denny has high heterozygosity.
Heterozygosity is the amount of genetic variation in a person. For instance, if two cousins had a kid, that kid would have low heterozygosity. But if two people from different countries and ethnicities had a kid, they would have high heterozygosity. So why was Denny the latter?
Denny's Parents Lived Far, Far Apart From Each Other
During an interview with National Geographic, computational biologist Richard E. Green claimed that Denny is "heterozygous out the wazoo." In other words, Denny's parents were people from two very different areas who rarely ever mated.
According to her DNA sequence, Denny is more closely related to a 55,000-year-old Neanderthal in Croatia than she is to the 33,000-year-old Neanderthals near the Denosiva Cave. So how did her parents meet?
Perhaps They Met In The Cave
One possibility is that Denny's parents met in the cave. Most Neanderthals lived in Eurasia, while Denisovans were around Siberia. The Altai Mountains, where the cave discoveries were found, lies in between these two territories.
"I think any Neanderthal that lived west of the Urals would never ever meet a Denisovan in their life," Pääbo theorizes, since the two species lived close to each other. If this is true, is it possible that modern humans have Denisovan blood?
Denny Challenges Our Perception Of Human Evolution
The discovery of Denny suggests that interbreeding might have been more common than researchers previously thought. Until 2018, scientists had only studied 23 ancient human specimens. Of those, only two were the results of interbreeding, which lead experts to believe that it was rare.
But perhaps they were wrong. "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," Pääbo told the Evening Standard.
And She Was Not The First Evidence Of This
Denny inspired researchers to re-examine other human specimens. One example is Oase 1, an ancient human's lower jaw that dates back 42,000 years. Although Oase seemed to be a full-blood Homo sapien, closer analysis showed that Oase had a small amount of Neanderthal DNA.
How is this possible? Oase's Neanderthal ancestor likely dated back four to six generations. If interbreeding were truly rare, Pääbo said, then these discoveries would be few and far in between. But both Oase and Denny are evidence to the contrary.
Hybrid Breeding Had Biological Advantages
According to palaeoanthropologist Chris Stringer, Neanderthal-Denisovan pairings had evolutionary advantages. Humans who had genetic diversity tended to survive better. Plus, both humans and animals thrive in numbers.
"Human groups were very small and vulnerable to drastic mortality," explains Harvati-Papatheodorou. As long as mates stuck together, their children would have a higher chance of survival. Because ancient human groups migrated often, they inevitably ended up around other species. Perhaps this is how Denny came to be.
One Theory For Why The Denisovans Went Extinct
Interbreeding provides an interesting theory for how the Denisovans went extinct. "Neanderthals and Denisovans may not have become violently extinct," Pääbo suggests, "but may have become absorbed into modern human populations."
Every time a Denisovan mated with another species, their genetic percentage decreased. Over time, this species might have dwindled due to interbreeding. If this is true, then modern humans developed in part to the Denisovans. As with many other ideas, this has not been proven yet.
Did The Denisovans Go Extinct From Infertility?
If the Denisovans and Neanderthals mated frequently, why did they remain apart for hundreds of thousands of years? Harris has one explanation. She suggested that Denisovan-Neanderthal offspring might have been infertile.
This would explain how the Denisovans went extinct and why Neandeerthal-Homo sapien offspring survived. Unfortunately, there is no way to know whether Denny was infertile. And if she was fertile, how did Denisovan DNA die out? Denny presents more questions than answers, but we are getting closer to the truth about human evolution.
Or, They Went Extinct From Competition
Although it is possible that the Denisovans assimilated into other human species, Joshua Akey of Princeton University proposed a different idea. "[Denny's discovery] does not rule out a more complicated mixture of factors, including competition," he said.
In 2020, European scientists confirmed that competition--whether it be over food or mates--can make a species go extinct. If that species loses the competition, then they will struggle to survive. Did the Homo sapiens or Neanderthals beat the Denisovans in competition?
What This Says About Modern Humans
So what does this mean about modern humans? Initially, scientists believed that Homo sapiens came from a mysterious hominem species and mixed with Neanderthals. But Denny suggests that the creation of modern humans was much more complicated.
According to the research report in Evolutionary Biology, "recent genomic research has shown that hybridization between substantially diverged lineages is the rule, not the exception, in human evolution." In other words, modern humans are the results of several different species interbreeding.
In 2019, Another Major Discovery Was Made
Reports about Denny's DNA came out in 2018. Then, in January 2019, another discovery was made. Researchers reported that several species of humans inhabited the Denisova Cave in Siberia over thousands of years.
It is unknown when different species were there at the same time. Researchers also cannot tell if Neanderthals or Denisovans lived there or simply visited. However, further excavation might reveal more ancient humans that will unlock the mystery of our ancestors. Only time will tell.
A Fourth Species Of Human Might Be In Our DNA
In February 2019, another discovery challenged our notions of human evolution. Scientists from the Center for Genomic Regulation applied artificial intelligence to the human genome, which identified a fourth human ancestor. The problem? We don't know who that is.
This unidentified human ancestor might have contributed to our modern DNA, along with the Neanderthals and Denisovans. But even though Denny has given us many discoveries, we still have a lot of unanswered questions about how human beings came to be.
We Still Don't Know Whether Modern Humans Are Denisovan
Denny's discovery eventually leads to the question: are modern humans descendants of the Denisovans? Scientists still don't know. A Homo sapien-Denisovan hybrid has yet to be found, and even if it were, this still would not confirm whether or not these hybrids were fertile.
It is possible that future studies will find bits of Denisovan DNA in modern humans. There could be many people living today with Denisovan blood in their veins, but we may never know for sure.