A researcher in a lab somewhere in Germany has just discovered something shocking. She checks her notes and then checks them again, to make sure she hasn’t made a mistake — but her findings seem to be entirely accurate. What she has found is the remains of a human teenager that seems to have been born from two different species — a discovery that will forever change human knowledge of science and evolution.
The story starts many years back in Siberia, with a team of researchers finding a small fragment of a bone. At the time, they didn’t realize the bone was of human origin, so they didn’t pay very much attention to it. Much later, a paleontologist named Viviane Slon decided to try and extract DNA from the bone, as she was interested in its origins. She discovered that the bone had belonged to a hybrid human — something that had never before been discovered, making history.
For context, we must go back to prehistoric times, to learn about two species very similar to humans. The first one, the Australopithecines, had first appeared about 4 million years ago in Africa. While they had the ability to walk and climb, in most respects they were very different from what we now refer to as Homo sapiens. Gradually, changes arose — things such as bigger brains, the ability to hunt and eat meat, and longer legs to walk and run faster.
The second — an offshoot of the Homo species that slowly grew to resemble the modern human more and more — was known as Homo Heidelbergensis. This variant first appeared 700,000 years ago in Africa and Eurasia, and was even more different from the ones that came before.
One of the biggest differences was that Homo Heidelbergensis was smarter than previous iterations. They were much better at hunting as well, with the ability to create tools and work together to catch larger prey. Eventually, though, even Homo Heidelbergensis ended up dying out. However, by that point, even more species had grown and evolved from them — and one of these branches is where modern humans will finally come from.
Despite the knowledge that multiple different species of hominids existed at the same time, and the possibility that some from different species would have mated with each other at some point, there was no concrete proof of this assumption. However, all of that changed upon Viviane Slon’s amazing discovery.
Many people like to say that different species cannot mate with each other — but that’s actually not the case at all. One example of this happening is mules, which are a hybrid of a horse and a donkey. There is one small problem with this, though — in the case of mules, they are often sterile due to having an odd number of chromosomes. This is because reproduction takes 50% of each chromosome from the sperm and the egg. A donkey has 62 chromosomes while a horse has 64, leaving the resulting mule with 63 and therefore infertile.
That said, it’s not entirely impossible for the offspring of interspecies mating to be fertile. One example of this is the liger, which is the offspring of a lion and a tiger. Since lions and tigers, as well as many other large cats, have the same number of chromosomes, ligers are capable of reproducing as well. Many species of primates also have the same number of chromosomes, so any offspring resulting from interspecies mating in these cases would also be able to reproduce.
The assumption in this situation, of course, is that the different species of early humans had the same number of chromosomes. This would allow their offspring to reproduce as well, and carry on their lineage into the future. In fact, though there was no certain proof at the time, some scientists already believed that early Homo sapiens had mated with Neanderthals and produced fertile offspring. This would explain why so many Asians and Europeans have a small amount of Neanderthal DNA.
As it turns out, Homo sapiens did not just mate with Neanderthals. In fact, there was another species of hominid that was only discovered recently in 2010: the Denisovans. They were discovered when researchers in Germany studied bones found in a Siberian cave — Denisova’s cave, which the new species was named after — and found proof of their existence.
Of course, this discovery was a great leap forward for science, but there wasn’t a lot that could be found about the Denisovans just yet, besides what could be gleaned from analyzing the DNA of the bones that had already been found. To try to learn more about this new species, another trip occurred in 2012 into the cave where the first bone was found.
Another bone was in fact found during this trip — though it wasn’t immediately recognized as human, since it was discovered alongside hundreds of animal fossils as well. It wasn’t until a few years later that Samantha Brown found out the truth as she was cataloging the artifacts that had been found in the cave.
When she realized the true origins of the bone, she sent it to Viviane Slon. Slon then analyzed the DNA in the bone mitochondria — which carries DNA passed down from the mother — and made her shocking discovery. The evidence she found in her research suggested that the mother of the teenager the bone belonged to was a Neanderthal.
This was a huge discovery — it could provide solid proof of past interspecies mating in ancient humans. However, Slon would need to find evidence of the father’s species as well, so she searched through the nuclear DNA, which was passed down from the father as well as the mother. In doing this, Slon found proof that the teenager’s father was in fact a Denisovan. Even after finding this evidence, she kept on looking for more. She had to be certain.
Heterozygosity is known as the inheritance in a child of differing forms of specific genes from each parent. When interspecies mating occurs — in any species, not just humans — the resulting child will have much higher than usual levels of heterozygosity. In the bone fragment, Slon found just that, confirming for certain that the teenager — who scientists named ‘Denny’ — was in fact a hybrid human.
Many scientists specializing in evolution were amazed by this new information, as it confirmed what had until now only been just a hypothesis. They wanted to look into it further, since if this interspecies mating was a repeated occurrence, it could completely change how we view evolution as a whole.
As a result of this discovery, many scientists began researching and analyzing a small number of ancient human remains. They needed to know exactly how this happened, and if it was a frequent occurrence. What they discovered was that out of 23 remains of humans they studied, at least two had resulted from interspecies mating. And that was just a small sample. If they studied a larger sample, the results might be much greater.
One of the cases that scientists studied was that of a human named Oase 1. When scientists analyzed Oase 1’s DNA, they discovered Neanderthal genes. The implication of this, that interspecies mating was a regular occurrence, led them to do more research on Denny as well. There, they found that Denny’s father had Neanderthal DNA as well — which meant that he was also a hybrid. This provided even more evidence for interspecies mating being relatively frequent, contrary to what scientists had initially believed.
To provide more context for all this, it is time to go back to Denisova’s cave, where Denny was originally discovered. Scientists have suggested that such a cave would be quite useful for shelter. Therefore, it made sense that two groups of different species that lived nearby would eventually meet. That explains the ‘how’ — but what about the ‘why’?
Due to the high death rate and small group size of ancient humans, it’s entirely possible that this interspecies mating was a way to help humanity survive in the long run. If these ancient humans were aware of their low chance of survival, it would make sense that they would try to increase their numbers in any way possible — including mating with another, similar species.
Of course, there are many questions that are still unanswered. What happened to the hybrid species in the end? Did they die out like so many others, or are they in fact our ancestors? We don’t yet know the answers to these questions. However, scientists are doing their best to find the truth, in hopes that these mysteries and more will one day be solved.