Varying Outcomes

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.

Even Species

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.

Denisovans

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.

New Evidence

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.

Another Fragment

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.

Human 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.