How could humans have interbred with Neanderthals if we're a different species?

How could humans have interbred with Neanderthals if we're a different species?

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To be clear, I'm not doubting that homo sapiens and homo neanderthalensis did interbreed: of that much I'm convinced.

Within the past few years I've seen an upcropping of pop-sci articles discussing the interbreeding between pre-historic species of humans. In everything that I see in these articles, as well as in scientific literature (my college Bio textbook, among others), I see these different humans being referred to as separate species.

This conflicts with my understanding of a species. Given the following definition, wouldn't homo sapiens and homo neanderthalensis be the same species?

A species is often defined as the largest group of organisms where two hybrids are capable of reproducing fertile offspring, typically using sexual reproduction. ~Wikipedia

  • Is this definition incorrect?
  • Are the publications using "species" colloquially, as opposed to scientifically?
  • Is "species" still a poorly defined concept? (see Ring Species)


Short answer

The concept of species is poorly defined and is often misleading. The concepts of lineage and clade / monophyletic group are much more helpful. IMO, the only usefulness of this poorly defined concept that is the "species" is to have a common vocabulary for naming lineages.

Note that Homo neanderthalis is sometimes (although it is rare) called H. sapiens neanderthalis though highlighting that some would consider neanderthals and modern humans as being part of the same species.

Long answer

Are neanderthals and modern humans really considered different species?

Often, yes they are considered as different species, neanderthals being called Homo neanderthalis and modern humans are being called Homo sapiens. However, some authors prefer to call neanderthals Homo sapiens neanderthalis and modern humans Homo sapiens sapiens, putting both lineages in the same species (but different subspecies).

How common were interbreeding between H. sapiens and H. neanderthalis

Please, have a look at @iayork's answer.

The rest of the post is here to highlight that whether you consider H. sapiens and H. neanderthalis to be the same species or not is mainly a matter of personal preference given that the concept of species is mainly arbitrary.

Short history of the concept of species

To my knowledge, the concept of species has first been used in the antiquity. At this time, most people viewed species as fixed entities, unable to change through time and without within-population variance (see Aristotle and Plato's thoughts). For some reason, we stuck to this concept even though it sometimes appears to not be very useful.

Charles Darwin already understood that as he says in On the Origin of Species (see here)

Certainly no clear line of demarcation has as yet been drawn between species and sub-species- that is, the forms which in the opinion of some naturalists come very near to, but do not quite arrive at the rank of species; or, again, between sub-species and well-marked varieties, or between lesser varieties and individual differences. These differences blend into each other in an insensible series; and a series impresses the mind with the idea of an actual passage.

You might also want to have a look at the post Why are there species instead of a continuum of various animals?

Several definitions of species

There are several definitions of species that yield me once again to argue that we should rather forget about this concept and just use the term lineage and use an accurate description of the reproductive barriers or genetic/functional divergence between lineage rather than using this made-up word that is "species".

I will below discuss the most commonly used definition (the one you cite) that is called the Biological species concept.

Problems with the definition you cite

A species is often defined as the largest group of organisms where two hybrids are capable of reproducing fertile offspring, typically using sexual reproduction.

Only applies to species that reproduce sexually

Of course, this definition only applies to lineages that use sexual reproduction. If we were to use this definition for asexual lineages, then every single individual would be its own species.

In practice

In general, everybody refers to this definition when talking about sexual lineages but IMO few people are correctly applying for practical reasons of communicating effectively.

How low the fitness of the hybrids need to be?

One has to arbitrarily define a limit of the minimal fitness (or maximal outbreeding depression) to get an accurate definition. Such boundary can be defined in absolute terms or in relative terms (relative to the fitness of the "parent lineages"). If, the hybrid has a fitness that is 100 times lower than any of the two parent lineages, then would you consider the two parent lineages to belong to the same species?

Type of reproductive isolation

We generally categorize the types of reproductive isolation into post-zygotic and pre-zygotic reproductive isolation (see wiki). There is a lot to say on this subject but let's just focus on two interesting hypothetical cases:

  • Let's consider two lineages of birds. One lineage has blue feathers while the other has red feathers. They absolutely never interbreed because the blue birds don't like the red and the red birds don't like the blue. But if you artificially fuse their gametes, then you get a viable and fertile offspring. Are they of the same species?

  • Let's imagine we have two lineages of mosquitoes living in the same geographic region. One flying between 6 pm and 8 pm while the other is flying between 1 am and 3 am. They never see each other. But if they were to meet while flying they would mate together and have viable and fertile offsprings. Are they of the same species?

Under what condition is the hybrids survival and fertility measured

Modern biology can do great stuff! Does it count if the hybrid can't develop in the mother's uterus (let's assume we are talking about mammals) but can develop in some other environment and then become a healthy adult?

Ring species in space

As you said in your question, ring species is another good example as to why the concept of species is not very helpful (see the post Transitivity of Species Definitions). Ensatina eschscholtzii (a salamander; see DeVitt et al. 2011 and other articles from the same group) is a classic example of ring species.

Species transition through time

Many modern lineages cannot interbreed with their ancestors. So, then people might be asking, when exactly did the species change occurred? What generation of parent where part of species A and offspring where part of species B. Of course, there is no such clearly defined time in which transition occurred. It is more a smooth transition from being clearly reproductively isolated (if they were placed to each other) from being clearly the same species.

Practical issue - Renaming lineages

How boring it would be if every time we discover the two species can in some circumstances interbreed, we had to rename them! That would be a mess.


Of course, when we talk about a species we refer to a group of individuals at a given time. However, we don't want to rename the group of individuals of interest every time a single individual die and get born. This notion yield to the question of how long in time can a single species exist. Consider a lineage that has not split for 60,000 years. Was the population 60,000 years ago the same species as the one today? The two groups may differ a lot phenotypically and may actually be reproductively isolated if they were to exist at the same time.

Special cases

When considering a few special cases, the concept of species become even harder to apply.

The Amazon molly (a fish) is a "species" that have "sexual intercourse" without having "sexual reproduction" and there are no males in the species! How is it possible? The females have to seek for sperm in a sister species in order to activate the development of the eggs but the genes of the father from the sister species are not used (Kokko et al. (2008)).

In an ant "species", males and females can both reproduce by parthenogenesis (some kind of cloning but with meiosis and cross-over) and don't need each other to reproduce. In this respect, males could actually be called females. But they still meet to reproduce together. The offsprings of a male and a female (via sexual reproduction) are sterile workers. So males and females are just like two sister species that reproduce sexually to create a sterile army to protect and feed them (Fournier et al. (2005)).


It often brings fame to discover a large new species. In consequence, scientists might tend to apply a definition of species that allow them to tell that their species is a new one. A typical example of such eventual bias concern dinosaurs where many new fossils are abusively called a new species while they sometimes are just the same species but at a different stage of development (according to this TED).

So why do we still use the concept of species?


IMO, its only usefulness is that it allows us to name lineages. And it is very important that we have the appropriate vocabulary to name different lineages even if this brings us to make a few mistakes and use some bad definitions.

The alternative use of the concept of lineage

It is important though that we are aware that the concept of species is poorly defined and that if we need to be accurate that we can talk in terms of lineages. The main issue with the term lineage is not semantic and comes about the fact that gene lineages may well differ considerably from what one would consider being the "species lineage" as defined by the "lineages of most sequences"… but this is a story for another time.

In consequence

In consequence to the above issues, we often call two lineages that can interbreed to some extent by different species names. On the other hand, two lineages that can hardly interbreed are sometimes called by the same species name but I would expect this case to be rarer (as discussed by @DarrelHoffman and @AMR in the comments).

Homo lineages

I hope it makes sense from the above that the question is really not related to the special case of the interbreeding between the Homo sapiens and the Homo neanderthalis lineages. The issue is a matter of the definition of species.

Video and podcast

SciShow made a video on the subject: What Makes a Species a Species?

For the French speakers, you will find an interesting (one hour long) podcast on the consequence of the false belief that the concept of species is an objective concept on conservation science at > La biodiversité - plus qu'une simple question de conservation > Pierre-Henry Gouyon

Here is a related answer

The definition of species is open for debate, and this is especially the case when you try to define it from a paleontology perspective.

Homo neanderthalensis was first discovered and defined in the 1860's, long before we were able to sequence their genome, which was published in 2010. There genome was different enough that most scientists would still say that they are distinct from modern humans, but that doesn't necessarily mean that those distinctions were enough that it affected the ability to produce fertile offspring between sapiens/neanderthalensis matings.

In fact on the wikipedia article I linked to the alternative Homo sapiens neanderthalensis is suggested as a synonym making us Homo sapiens sapiens for anatomically modern humans. This is similar to distinctions made in the wolf lineages where you have Canis lupus for grey wolves, Canis lupus familiaris for Domestic dogs, Canis lupus dingo for the wild dogs of Australia, and on and on for many other sub species of Canis lupus. Wolf/Dog hybrids are known to be fertile.

Female Horses and Male Donkeys can breed to create sterile offspring, Mules. The sterility is likely due to the fact that there is a greater phylogenetic distance between Horses and Donkey than there is for sapiens/neanderthalensis and also due to the fact that Horses have a different number of chromosomes to Donkey's , where as sapiens/neanderthalensis have the same number.

Another thing you have to remember is that the discovery of Homo neanderthalensis occurred about 5 years after Darwin created an uproar about Humans being evolved primates. Now you were faced with a skeleton that clearly was different from modern human but was clearly hominid. The closely held view that humans were special and therefore held dominion over all other creatures made it difficult to accept the fact that we may actually have evolved.

In addition to @Remi.b's answer on the species concept, and the perils of using human definitions to try to encompass biological reality, you need to understand what "interbreeding" meant to humans and neanderthals. Fertile crosses between sapiens and neandertalis were very rare, probably less than one successful cross per generation, and there's some evidence that male hybrids were almost all sterile (or that this was a fatal condition). This is probably less successful interbreeding than horse-donkey crosses, where the two partners are unambiguously different species by just about any definition (they have different chromosome numbers!). So it's not like sapiens and neandertalis could effortlessly hybridize; it was very rare and generally unsuccessful.


We find that observed low levels of Neanderthal ancestry in Eurasians are compatible with a very low rate of interbreeding (<2%), potentially attributable to a very strong avoidance of interspecific matings, a low fitness of hybrids, or both. These results suggesting the presence of very effective barriers to gene flow between the two species are robust to uncertainties about the exact demography of the Paleolithic populations, and they are also found to be compatible with the observed lack of mtDNA introgression. Our model additionally suggests that similarly low levels of introgression in Europe and Asia may result from distinct admixture events having occurred beyond the Middle East, after the split of Europeans and Asians.

--Strong reproductive isolation between humans and Neanderthals inferred from observed patterns of introgression. Currat M, Excoffier L. Proc Natl Acad Sci U S A. 2011 Sep 13;108(37):15129-34

Our results indicate that the amount of Neanderthal DNA in living non-Africans can be explained with maximum probability by the exchange of a single pair of individuals between the subpopulations at each 77 generations, but larger exchange frequencies are also allowed with sizeable probability.

--Extremely Rare Interbreeding Events Can Explain Neanderthal DNA in Living Humans. Neves AGM, Serva M PLoS ONE 2012 7(10): e47076

These results suggest that part of the explanation for genomic regions of reduced Neanderthal ancestry is Neanderthal alleles that caused decreased fertility in males when moved to a modern human genetic background.

--The genomic landscape of Neanderthal ancestry in present-day humans Sankararaman et al Nature 2014 507:354-357

The same way lions and tiger are different species but can interbreed. They do so poorly, not very often, and the male hybrids are infertile, although the female hybrids retain some fertility. Gene flow between the two species is possible but very limited. We call lions and tigers two species because gene flow between the two is limited. Free flow of genes between the two species is not possible.

If we look at the papers coming out it is obvious that the flow of genes between Homo Sapiens and Neanderthals was limited. It was so limited that we can say that two species were on the verge of cutting all ties between each other.

Firstly, there are no Neanderthal mitochrondria, meaning successful mating involving Neanderthals was mainly between human females and male Neanderthals. Next there are no Neanderthal Y chromosomes, meaning male hybrids were sterile.

Although there are Neanderthal gene in autosomes, there are no Neanderthal genes on X chromosomes, which house many of the genes regulating intelligence and fertility. So natural selection has selectively weeded out Neanderthal genes in those two areas, probably because they don't play well with their human counterparts (i.e. they cause a fitness disadvantage).

Also, the Neanderthal gene segments, although found in all non-Africans, are in small pieces. Individual have less than 2% (although as much as 35-60% of the Neanderthal genome remains in the non-African human population). So it means that the hybridization event occurred rarely and far in the past - probably when the first Homo sapiens left Africa.

The concept of species is imperfect in any definition. But the oldest (and probably most function related definition) is that ability to cross breed to have viable offspring. So under this definition, if the two populations can cross breed than they are not separate species.

Note also, that a few decades ago Neanderthal and "Cro Magnon" were considered to be part of the same species, homo sapiens. Cro Magnon would be Homo sapiens sapiens (subspecies) and Neanderthal would be Homo sapiens neanderthalis. So under the traditional nomenclature, it would not even be so shocking to have seen some cross breeding!

Lately the fashion for species naming has departed some from the functional definition to other definitions. In general ones that make species out of former subspecies. Sometimes this is for protecting certain creatures (legal issues of protection). Often it is because academic biologists gain prestige from "discovering" species. (Very few of which lately are true in the field find a new animal discoveries, much more commonly being just nomenclature debates.)

[And don't get me started on mitochondrial DNA ("genetic proof") which many everyday people, like on Wikipedia, don't realize is not "the DNA" of chromosomes and of phenotypic recognition. And for species with wandering males, the actual "genes" can be well mixed, but if the females don't travel much, you will see different mito DNA populations.]

*For example, look up issue of "ring species".

Neanderthals, Humans Interbred—First Solid DNA Evidence

The next time you're tempted to call some oaf a Neanderthal, you might want to take a look in the mirror.

According to a new DNA study, most humans have a little Neanderthal in them—at least 1 to 4 percent of a person's genetic makeup.

The study uncovered the first solid genetic evidence that "modern" humans—or Homo sapiens—interbred with their Neanderthal neighbors, who mysteriously died out about 30,000 years ago.

What's more, the Neanderthal-modern human mating apparently took place in the Middle East, shortly after modern humans had left Africa, not in Europe—as has long been suspected.

"We can now say that, in all probability, there was gene flow from Neanderthals to modern humans," lead study author Ed Green of the University of California, Santa Cruz, said in a prepared statement.

That's no surprise to anthropologist Erik Trinkhaus, whose skeleton-based claims of Neanderthal-modern human interbreeding—previously contradicted with DNA evidence—appear to have been vindicated by the new gene study, to be published tomorrow in the journal Science.

"They've finally seen the light . because it's been obvious to many us that this happened," said Trinkaus, of Washington University in St. Louis, Missouri, who wasn't part of the new study.

Trinkhaus adds that most living humans probably have much more Neanderthal DNA than the new study suggests.

"One to 4 percent is truly a minimum," Trinkaus added. "But is it 10 percent? Twenty percent? I have no idea."

Surprising Spot for Neanderthal-Human Mating

The genetic study team reached their conclusion after comparing the genomes of five living humans—from China, France, Papua New Guinea, southern Africa, and western Africa—against the available "rough draft" of the Neanderthal genome. (Get the basics on genetics.)

The results showed that Neanderthal DNA is 99.7 percent identical to modern human DNA, versus, for example, 98.8 percent for modern humans and chimps, according to the study. (Related: "Neanderthals Had Same 'Language Gene' as Modern Humans.")

In addition, all modern ethnic groups, other than Africans, carry traces of Neanderthal DNA in their genomes, the study says—which at first puzzled the scientists. Though no fossil evidence has been found for Neanderthals and modern humans coexisting in Africa, Neanderthals, like modern humans, are thought to have arisen on the continent.

"If you told an archaeologist that you'd found evidence of gene exchange between Neanderthals and modern humans and asked them to guess which [living] population it was found in, most would say Europeans, because there's well documented archaeological evidence that they lived side by side for several thousand years," said study team member David Reich.

For another thing, Neanderthals never lived in China or Papua New Guinea, in the Pacific region of Melanesia, according to the archaeological record. (See "Neanderthals Ranged Much Farther East Than Thought.")

"But the fact is that Chinese and Melanesians are as closely related to Neanderthals" as Europeans, said Reich, a population geneticist at the Broad Institute of MIT and Harvard University.

Neanderthal-Human One-Night Stand?

So how did modern humans with Neanderthal DNA end up in Asia and Melanesia?

Neanderthals, the study team says, probably mixed with early Homo sapiens just after they'd left Africa but before Homo sapiens split into different ethnic groups and scattered around the globe.

The first opportunity for interbreeding probably occurred about 60,000 years ago in Middle Eastern regions adjacent to Africa, where archaeological evidence shows the two species overlapped for a time, the team says.

And it wouldn't have taken much mating to make an impact, according to study co-author Reich. The results could stem from a Neanderthal-modern human one-night stand or from thousands of interspecies assignations, he said.

More DNA Evidence for Neanderthal-Human Mating

The new study isn't alone in finding genetic hints of Homo sapiens-Homo neanderthalensis interbreeding.

Genetic anthropologist Jeffrey Long, who calls the Science study "very exciting," co-authored a new, not yet published study that found DNA evidence of interbreeding between early modern humans and an "archaic human" species, though it's not clear which. He presented his team's findings at a meeting of the American Association of Physical Anthropologists in Albuquerque, New Mexico, last month.

Long's team reached its conclusions after searching the genomes of hundreds of modern humans for "signatures of different evolutionary processes in DNA variation."

Like the new Science paper, Long's study speculates that interbreeding occurred just after our species had left Africa, but Long's study didn't include analysis of the Neanderthal genome.

"At the time we started the project, I never imagined I'd ever see an empirical confirmation of it," said Long, referring to the Science team's Neanderthal-DNA evidence, "so I'm pretty happy to see it."

  • Genetic similarity of humans and Neanderthals was studied by Oxford scientists
  • Two species were closer than wolves and coyotes, and brown and polar bears
  • This genetic similarity allowed them to have fertile and healthy offspring

Published: 10:24 BST, 4 June 2020 | Updated: 16:53 BST, 4 June 2020

Neanderthals, Denisovans and Homo sapiens were so closely related that they were able to interbreed easily and have offspring that were fertile and healthy.

Analysis from the University of Oxford reveals the species were more genetically similar than brown bears and polar bears are today.

This significant overlap provided genetic compatibility and allowed the branches of humanity to have fertile and healthy chimeric children.

In the biological world, lots of animals are able to breach the species barrier and interbreed and produce healthy offspring, but it is rare for the offspring to be fertile.

Normally, hybrids such as the liger and the mule struggle to reproduce themselves, due to incompatible genetics often caused by differing amounts of chromosomes.

Neanderthals, Denisovans and Homo sapiens were so closely related that they were able to interbreed and have offspring that were fertile and healthy, a study suggests. The species were more genetically similar than brown and polar bears are today (file photo)

Humans, Neanderthals and Denisovans interbred in prehistory, and the DNA from the now-extinct Neanderthals and Denisovans can be found in human genomes to this day. Humans out-competed Neanderthals in Eurasia, leading to their extinction around 40,000 years ago

Producing viable offspring is a defining aspect of a species

A species is the most specific classification of a group of animals.

Within this group there is still variation, but they are all considered similar enough to be the same species.

For example, dogs have a variety of breeds, coming in different sizes, colours and temperaments. But they are all one species, known as Canis lupus familiaris.

One defining characteristic of what makes a species, is everything that exists inside the classification must be able to produce 'viable offspring'.

That is to say, if two individuals mate, their offspring should be fertile and also able to reproduce.

There are some exceptions, as with all aspects of biology, but the rule is largely true.

For example, a great Dane can produce viable offspring with a Labrador, despite looking very different. This makes them part of the same species.

However, a rhino cannot have viable offspring with a hippopotamus, making them different species.

However, a study has now found prehistoric humans and their cousins did not have this issue.

They analysed the genomes of closely-related species and used computer modelling to determine how similar or distant they were from one another.

This then provided each pair with a value for their 'genetic distance' which could be used to compare various species on a like-for-like basis.

Extant species known to be closely related and able to produce viable offspring, such as coyotes and wolves, and brown bears and polar bears, were included in the analysis.

Genetic distance values were expressed as a percentage difference and the researchers found Neanderthals and modern humans had a 1.6 per cent difference.

In contrast, polar and brown bears were 2.4 per cent different.

Denisovans and modern humans are also 2.4 per cent different, while Neanderthal and Denisovans were slightly more distant with 2.6 per cent genetic difference.

However, all three species were able to have viable offspring, it is believed.

Preserved ancient DNA was taken from the remains of ancient humans, Neanderthals and Denisovans to make the study as accurate as possible.

The three species interbred in prehistory, and the DNA from the now-extinct Neanderthals and Denisovans can be found in human genomes to this day.

Genetic distance correlated to fertility of the hybrid offspring, it was found.

It confirmed that the greater the evolutionary distance between any two species, the less likely it is that the offspring between them would be fertile.

Professor Greger Larson of Oxford who is a senior author of the research, said: 'Our desire to categorise the world into discrete boxes has led us to think of species as completely separate units.

'Biology does not care about these rigid definitions, and lots of species, even those that are far apart evolutionarily, swap genes all the time.

'Our predictive metric allows for a quick and easy determination of how likely it is for any two species to produce fertile hybrid offspring.


We are the result of genetical engineering processes led by our Creators

FINALLY SOLVED: BECOMING HUMAN / INTELLIGENCE / AI. NEW COMPREHENSIVE THEORY STARTS FROM THE END by establishing the working theory of functioning of the human brain-IQ. That is the only way to solve this puzzle and here is the only picture/story that makes sense. The human evolution ( 7 million years, 30 hominins ) must perform / accomplish the evolution of intelligence (to the achievement C+IQ / collective intelligence with ability the speak ), but I have found only "the evolution of emotions". These three processes intersect at one point - baby / human infant that is incapable for independent survival for many years. That is not an evolutionary mistake, on the contrary, that is the key element of my research. By observing it’s mother’s behavior, a process called MSP /multi self-projection passively occurs in baby’s brain when child perceives guardians body as his own. MSP may be the most easily understood as a feeling similar to that of the apparent movement which we have when we are in a train that stands while we are looking through the window at another train that is moving. That way infant’s CNS immediately learns the shortest way to get something done, which enables the creation of many more similar thinking processes till the moment when a minimal number of thinking processes (Adam’s number) are required in order to effect of self-consciousness arise. To connect all that I have mentioned with a huge number of scientific data (Denisovans, Homo naledi, Scientific Adam, Mitochondrial Eve, autism, speech, pleasure in the presence of fire, dreams. ) required membership in the Mensa organization. The biggest picture (the framework) for all scientific data (even A.I. because start, origin of original, in making SAI/AGI/HAL 9000 is crucial / what has been missing) is FEST theory.

FINALLY SOLVED: BECOMING HUMAN / INTELLIGENCE / AI. NEW COMPREHENSIVE THEORY STARTS FROM THE END by establishing the working theory of functioning of the human brain-IQ. That is the only way to solve this puzzle and here is the only picture/story that makes sense. The human evolution ( 7 million years, 30 hominins ) must perform / accomplish the evolution of intelligence (to the achievement C+IQ / collective intelligence with ability the speak ), but I have found only "the evolution of emotions". These three processes intersect at one point - baby / human infant that is incapable for independent survival for many years. That is not an evolutionary mistake, on the contrary, that is the key element of my research. By observing it’s mother’s behavior, a process called MSP /multi self-projection passively occurs in baby’s brain when child perceives guardians body as his own. MSP may be the most easily understood as a feeling similar to that of the apparent movement which we have when we are in a train that stands while we are looking through the window at another train that is moving. That way infant’s CNS immediately learns the shortest way to get something done, which enables the creation of many more similar thinking processes till the moment when a minimal number of thinking processes (Adam’s number) are required in order to effect of self-consciousness arise. To connect all that I have mentioned with a huge number of scientific data (Denisovans, Homo naledi, Scientific Adam, Mitochondrial Eve, autism, speech, pleasure in the presence of fire, dreams. ) required membership in the Mensa organization. The biggest picture (the framework) for all scientific data (even A.I. because start, origin of original, in making SAI/AGI/HAL 9000 is crucial / what has been missing) is FEST theory.

This isn’t super surprising to me….I think modern humans were most definitely out of Africa 100,000 years ago. There was recently a tooth from a Homo sapien discovered at a cave in China that dated over 100,000 years. I tend to think that would also make more sense at the amount of time given for the first people to arrive in Australia. Very very cool stuff!

Dawn of Humanity

Decoding Neanderthals

Becoming Human Part 1

In August 1856, in the German valley of Neander—Neanderthal in German—men cutting limestone for the Prussian construction industry stumbled upon some bones in a cave. Looking vaguely human, the bones—a piece of a skull, portions of limbs, and fragments of shoulder blades and ribs—eventually made their way to an anatomist in Bonn named Hermann Schaafhausen.

Schaafhausen pored over the fossils, observing their crests and knobs. He noticed that the bones had the overall shape youɽ expect from a human skeleton. But some bones had strange features, too. The skullcap, for example, sported a heavy brow ridge, hanging over the eyes like a boney pair of goggles. It was, at once, human and not.

The Neanderthal Man challenged Schaafhausen with a simple yet profound question: Was it a human, or did it belong to another species?

It's been over 150 years since the bones first emerged from the Neander Valley—a time during which we've learned a vast amount about human evolution. Today, scientists can even scan the genomes of Neanderthals who died 50,000 years ago. And yet the debate still rages. It's a debate that extends beyond Neanderthals, forcing us to ask what it means to be a species at all.

Variations on a theme

The Neander Valley bones were a sensation as soon as Schaafhausen published his report on them in 1857, because nothing like them had been seen before. Earlier in the 1800s, cave explorers had found ancient human bones, sometimes lying next to fossils of cave bears and other extinct animals. Naturalists had a hazy sense from such bones that humanity had been around for quite a long time. But the idea that humans—or any other species—had evolved was scandalous. Darwin would not publish The Origin of Species for another two years. Instead, naturalists saw humans as a species distinct from chimpanzees, gorillas, and all other primate species. We were distinct today, and we had been distinct since creation.

The youngest Neanderthal fossils date to 28,000 years ago.

Within the human species, European anatomists divided people into races. They often ranked Europeans as the noblest race, considering the others barely better than apes. To justify this racist view of humanity, anatomists searched for clear-cut differences between the skeletons of different races—the size of skulls, the slopes of brows, the width of noses. Yet their attempts to neatly sort people into groups were bedeviled by the blurry variations in our species. Within a single so-called race, people varied in color, height, and facial features. Schaafhausen knew, for example, about a skull dug up from an ancient grave in Germany that "resembled that of a Negro," as he wrote.

A barbarian (with sword) attacking a Roman legionary in a second-century relief. Neanderthals wouldn't have been out of place amongst such savage sorts, Schaafhausen believed.

European savages

On this confusing landscape Schaafhausen tried find a place for the Neanderthal Man. He decided that its heavy brow didn't disqualify it as a human. To back up this diagnosis, he relied on stories of ancient European savagery. "Even of the Germans," Schaafhausen wrote in his 1857 report on the Neander Valley bones, "Caesar remarks that the Roman soldiers were unable to withstand their aspect and the flashing of their eyes, and that a sudden panic seized his army."

Schaafhausen searched historical records for other clues of Europe's monstrous past. "The Irish were voracious cannibals, and considered it praiseworthy to eat the bodies of their parents," he wrote. In the 1200s, ancient tribes in Scandinavia still lived in the mountains and forests, wearing animal skins, "uttering sounds more like the cries of wild beasts than human speech."

Surely, in such a savage place, this heavy-browed Neanderthal would have fit right in.

A distinctive creature

When Schaafhausen published his report, many other naturalists tried to make sense of the bones for themselves. After Darwin published his theory of evolution in 1859, new possibilities arose: Perhaps humans evolved from Neanderthals, or perhaps they were both descended from a common ancestor.

Thomas Huxley, Darwin's great champion in England, argued that Neanderthals were human, pointing to the thick foreheads of living Australian Aborigines. William King, an Irish geologist, disagreed. In an 1864 paper, "The Reputed Fossil Man of the Neanderthal," he pointed to a long list of traits that separated it from living humans—from its tightly curved ribs to the massive sinuses in its skull. Its braincase was so ape-like that it could not house a human-like brain.

Australian Aborigines have a prominent brow ridge, a fact that helped lead Thomas Huxley to argue that Neanderthals were indeed human.

"I feel myself constrained to believe that the thoughts and desires which once dwelt within it never soared beyond those of a brute," King wrote.

From all this evidence, King concluded that the Neanderthal Man was not simply an ancient European, as Schaafhausen had thought. It was a separate species. He even gave that species a name: Homo neanderthalensis.

Mounting evidence

King was certainly right that Neanderthals were distinct from living humans. Subsequent generations of fossil-hunters have found remains of Neanderthals from Spain to Israel to Russia. The youngest Neanderthal fossils date to 28,000 years ago. The oldest ones date back over 200,000 years. Like the original Neanderthal Man, they were stocky, with a heavy brow ridge and other singular traits. We can't know exactly what thoughts and desires soared in their heads, but they certainly left behind some telling clues—carefully engineered spear blades and stone knives painted shells that might have been used as jewelry. Neanderthals endured the comings and goings of ice ages in Europe and Asia, hunting for reindeer, rhinoceroses, and other big game.

As the fossils have emerged, paleoanthropologists have revisited the question of whether Neanderthals are part of our own species—call them Homo sapiens neanderthalensis—or a separate Homo neanderthalensis. Some researchers argued that Neanderthals belonged to a single species of humans stretching across the Old World, one that evolved over the past million years from small-brained hominids into our big-brained form.

Europeans and Asians carry a small portion of DNA inherited from Neanderthals.

But some researchers challenged this view. They pointed out that for thousands of years, Europe was home to the burly Neanderthals as well as slender humans. Neanderthals didn't give rise to living Europeans, these scientists argued they were replaced by immigrants expanding out of Africa—perhaps even outcompeted into extinction.

Over the past 15 years, Svante Pääbo, a geneticist at the Max Planck Institute of Evolutionary Anthropology, and his colleagues have uncovered an entirely new source of evidence about the nature of Neanderthals: their DNA. Starting with those fossils from the Neander Valley, they extracted bits of genetic material that had survived tens of thousands of years. Eventually, they were able to assemble the fragments into the entire Neanderthal genome.

Populations of the same species that a river or other barrier divides can become unable to breed successfully with each other. Such an inability never occurred between Neanderthals and humans, who bred successfully at least once.

It's clearly different from the genome of any human alive today, sprinkled with many distinctive mutations. These mutations accumulated in a clock-like way, and by tallying them up, Pääbo and his colleagues estimate that Neanderthals and humans share a common ancestor that lived 800,000 years ago. It's possible that the ancestors of Neanderthals expanded out of Africa then, while our own ancestors stayed behind.

A question of breeding

That's a long time—long enough to reasonably ask if humans and Neanderthals are indeed two separate species. Old species split into new ones when some of their members get isolated from the rest. If a river cuts the range of a species of frog in two, for example, the frogs on one side of the river may only be able to mate with one another. Each population will evolve along its own path. If they are isolated long enough, they will have trouble interbreeding. They may even be unable to interbreed at all.

From these facts of evolution, the biologist Ernst Mayr developed what came to be known as the Biological Species Concept in the 1940s—namely, a species is made up of members of populations that actually or potentially interbreed in nature. Experiments on living animals have shown that barriers to this interbreeding can arise in tens of thousands, or even just thousands, of years.

Once the Neanderthal lineage left Africa 800,000 years ago, did humans and Neanderthals have enough time to become unable to interbreed? Pääbo's research provides an answer: no.

Does the late Ernst Mayr's notion of what constitutes a species, which held sway for many decades, need to be scrapped or substantially revised? Many biologists believe so.

Europeans and Asians carry with them a small portion of DNA inherited from Neanderthals—while Africans do not. The best explanation for our mixed genomes is that after humans expanded out of Africa, they encountered Neanderthals and interbred. Comparing the different Neanderthal-derived genes in different people, Pääbo and his colleagues estimate that this encounter occurred around 40,000 years ago. The tiny amount of Neanderthal DNA has been interpreted by some scientists as evidence that Neanderthals rarely mated with humans—perhaps just once, in fact. But as scientists sequence more genomes from more human populations, they're exploring the possibility that our ancestors mated with Neanderthals several different times.

A matter of survival

The presence of DNA from Neanderthals in human genomes is compelling evidence that humans and Neanderthals could mate and produce fertile offspring. If we stick to the Biological Species Concept, then we are a single species, as Schaafhausen originally thought. But some scientists reject this argument. They think that Mayr's Biological Species Concept has worn out its usefulness.

Homo neanderthalensis and Homo sapiens endured—at least until the Neanderthals became extinct.

With the advent of gene sequencing, scientists have found that many animal species regularly interbreed. It's easy for any safari tourist to tell the difference between olive baboons and yellow baboons that live in Kenya, for example. And yet the two species regularly produce hybrids in the places where their species overlap, and they've been doing it for a long time.

What will it take for experts to agree on whether Neanderthals (foreground) and modern humans are one and the same species?

So why haven't the two baboon species merged into a single hybrid olive-yellow species? The baboons produced by interbreeding may not survive as well as purebred ones. They produce fewer offspring of their own, and so the genes from one species don't spread easily in the other. Thus, despite interbreeding—breaking Ernst Mayr's rule, in other words—the olive and yellow baboons endure as separate species.

Perhaps humans and Neanderthals were the same: They only interbred rarely, and when they did, the hybrid children couldn't fuse the two kinds of humans together. That may be why human and Neanderthal fossils remained so different.

William King would probably have been horrified at the notion of human beings having sex with Neanderthal "brutes." But despite this intermingling, Homo neanderthalensis and Homo sapiens endured—at least until the Neanderthals became extinct, and we survived.

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If humans and neanderthals were two different species, how could we have interbred?

I understand that we could've had children with the neanderthals, but isn't part of the definition of two species being different that they cannot produce fertile offspring, and so therefore we could not be mixed with them today?

There are many many definitions of "species", with the biological species concept (the definition under which we say that two different populations are different species if their members cannot interbreed with one another) being only one. A full discussion of the difficulties with concepts of speciation and what we really mean when we talk about species would be way beyond the 10,000 character limit on reddit comments.

Instead, let me share a little vignette about fish, and then I'll comment on Neanderthals. Stickleback fish in a few different stream systems in British Columbia have split into two different kinds: a limnetic form, which is generally small and lives near the surface, and a benthic form which lives near the bottom of streams. These two different populations generally live very close to one another (i.e. in the same streams), but fill different ecological roles, and they don't interbreed. However, in recent years, it appears that the two species have begun interbreeding with one another, and have started to collapse into a single species. I'm not sure if the exact reason is known (I thought it had to do with agricultural runoff making it difficult for members of the two different species to distinguish one another visually, but I'm not finding any evidence for that in the few papers I've looked at and hunting through a whole bunch more to find out is a bit beyond my scope at the moment), but these fish were obviously not intrinsically, biologically isolated from one another, because they are able to produce healthy, fertile offspring. So what seemingly were "good", genetically isolated species ceased to be so once something in their environment changed. Presumably, if that environmental change hadn't happened, they would have gone on diverging from one another until eventually they would have been completely isolated, such that they wouldn't have been able to produce viable offspring at all, at which point they would have been permanently separated into different species. That's not what happened, but it could have. So these fish were part way down the path to being completely separate species, such that they had many of the characteristics weɽ normally associate with separate species, but hadn't completely finished "speciating".

Ok. So what about Neanderthals? Well, the original classifications of Neanderthals as separate species from us is essentially based on the "morphological species concept". This is most commonly applied to fossils, because it was until recently the only way we really have of classifying fossils into separate species (and that remains true for all but very recent fossils, like Neanderthals and Denisovans, where we can still get DNA from them). We can't test whether two fossil organisms could interbreed with one another, for obvious reasons. So, Neanderthal fossils are morphologically different from modern humans, and so some anthropologists saw this as enough to classify them as different species. Others didn't. There was a whole bunch of contentious debate about this which I'm not going to touch with a ten foot pole, other than to say that I don't think the debate was of much use.

With the advent of ancient DNA sequencing, we now know with relative certainty that Neanderthals and modern humans interbred, and that all non-Africans (and some Africans) carry DNA from those interbreeding events in their genome today. What we don't know are the circumstances of that interbreeding. It could be that modern humans and Neanderthals were actually quite reproductively isolated, to the extent that if they did try to mate, they usually (but not always) have offspring who were pretty badly screwed up, or were infertile, due to intrinsic genetic incompatibilities between the two groups, and so the Neanderthal DNA we find in human populations today is due to those few matings that actually worked. On the other hand, it could have been that the two groups were actually perfectly capable of producing perfectly fit hybrids, but maybe Neanderthals and modern humans didn't consider one another very attractive, and so didn't even try to mate with one another very often, and so only a small amount of interbreeding took place. A third possibilities is that they were both perfectly interfertile and willing to get it on 1 , but they actually didn't come into contact with one another all that often, and so didn't have that many opportunities.

Each of the above scenarios (this is a non-exhaustive list, by the way) corresponds to modern humans and Neanderthals being at some particular point down the path to becoming separate species, and yet each hypothesis is at least in principle consistent with the observation that modern humans and Neanderthals interbred, but only enough to leave a small trace of Neanderthal DNA (

I suspect an anthropologist could give you a more qualified opinion on which of those scenarios was more likely than I could but the truth is that we don't know yet. Hopefully as we get more genomes sequenced from more ancient individuals we might start to get a better idea.

Let me know if you have any follow up questions or if I did a poor job explaining anything.

How Common Was Consanguinity?

Even if the Altai individual descended from a long line of inbreds, that’s just one Siberian population, living over 4000 miles from what is considered to be the Neanderthal heartland in Europe. The fact that the Altai population mated with kin doesn’t mean that behavior was typical for the species. So what about European Neanderthals?

In 2017 researchers counted ROH in the genome of a roughly 50,000-year-old Neanderthal woman from Vindija, Croatia. This individual did not have abundant long tracts of matching DNA, indicating her parents were not close relatives. However, the quantity of short ROH (2.5 – 10 cM) was on par with the Altai Neanderthal and above present-day groups with a history of isolation and inbreeding.

Several lines of evidence suggest inbreeding among Neanderthals from El Sidrón, Spain. At this site, over 2,500 bone fragments have been recovered, constituting at least 13 individuals of both sexes and various ages. Archaeologists investigating El Sidrón believe the skeletons represent a close-knit Neanderthal group that died together around 50,000 years ago (these Neanderthals also show signs of cannibalism , but that’s besides the point).

Although full genomes aren’t available, portions of their DNA that have been sequenced are consistent with the group being close kin, who may have been too close. A 2011 paper , analyzing mitochondrial DNA (mtDNA) — a small loop of genetic code passed on only by mothers — revealed low diversity within the group. The adult males in particular seem to have been maternal kin. Another study fully sequenced chromosome 21 from one El Sidrón individual and found a similar pattern as in the Vindija Neanderthal: an abundance of short ROH .

And earlier this month , anthropologists reported hard — skeletal — evidence for consanguinity among the El Sidrón Neanderthals. The team identified 17 bones, belonging to at least 4 individuals, showing congenital abnormalities. These are conditions present at birth, as opposed to ones developed during life through injury, infection or nutritional deficiencies. In the El Sidrón remains, the congenital features included cleft or asymmetric vertebrae, a misshapen kneecap and a baby tooth retained into adulthood. The identified conditions are rare in living humans (between 3.8 to 0.00004 percent) and may be harmless, but they do occur more frequently in cases of inbreeding. In other words, these skeletal features suggest the parents were kin.

Humans mated with Neandertals much earlier and more frequently than thought

Members of our species had sex with Neandertals much earlier—and more often—than previously believed, according to a new study of ancient DNA. As some of the first bands of modern humans moved out of Africa, they met and mated with Neandertals about 100,000 years ago—perhaps in the fertile Nile Valley, along the coastal hills of the Middle East, or in the once-verdant Arabian Peninsula. This pushes back the earliest encounter between the two groups by tens of thousands of years and suggests that our ancestors were shaped in significant ways by swapping genes with other types of humans.

These early modern humans’ own lineages died out, and they are not among the ancestors of living people. But a small bit of their DNA survived in the toe bone of a Neandertal woman who lived more than 50,000 years ago in Denisova Cave in the Altai Mountains of Siberia, Russia. A new analysis of her ancient genome has found that this so-called “Altai” Neandertal inherited DNA from modern humans from Africa, including a gene that may have been involved in speech.

“This is the first genetic evidence that early modern humans met Neandertals and bred with them earlier than we thought,” says lead author Sergi Castellano, an evolutionary biologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

Ever since researchers sequenced the first full genome of Neandertals in 2010, they have known that the ancestors of European Neandertals interbred with modern humans. By comparing the Neandertal genome with that of modern humans, they found a curious pattern: Present-day Europeans and Asians have inherited about 1% to 3% of their DNA from Neandertals, but Africans have not. This suggested that encounters between modern humans and Neandertals were rare and happened in the Middle East or the Arabian Peninsula after modern humans swept out of Africa, but before they spread widely. When moderns did expand all over Eurasia, they carried that Neandertal DNA in their cells. Later studies of ancient DNA from a 45,000-year-old modern human in Romania helped pinpoint the timing of that encounter to between 50,000 and 65,000 years ago.

More ancient DNA, also from a bone from Siberia’s Denisova Cave, complicated the picture. By sequencing an ancient girl’s finger bone from the cave, researchers discovered a new type of human, the Denisovans, who are closely related to Neandertals but also mated with the ancestors of today’s Melanesians.

With every ancient genome, however, came new surprises. The Denisovan girl’s people had also mated with the ancestors of present-day Melanesians and some mainland Asians (who still carry small amounts of Denisovan DNA) sometime after modern humans encountered Neandertals 50,000 to 65,000 years ago.

So, modern humans had interbred at least twice with archaic humans—Neandertals and, later, Denisovans—after leaving Africa. What’s more, the Denisova girl seemed to also carry some ancient DNA from an even more archaic hominin, such as the direct human ancestor Homo erectus, which lived 1.8 million to roughly 200,000 years ago. Her ancestors had inherited this “super archaic” DNA within the past 400,000 years, but the Altai Neandertal did not have it.

This scenario shows how after human ancestors split with the ancestors of Neandertals, the two groups interbred at least twice—100,000 years ago, soon after modern humans emerged and first left Africa, and again between 47,000 and 65,000 years ago.

How to explain this pattern?

In the new study, published online today in Nature, Castellano and an international team of researchers first zeroed in on the chunks of modern DNA in the genomes of the Altai Neandertal and the Denisovan. By comparing it to key segments of the genomes of 504 Africans, they found that the Altai Neandertal had inherited DNA from modern humans who lived across Africa—and that this “African” DNA was inherited about 100,000 years ago. By contrast, the Denisovan girl and two other Neandertals from Europe (Croatia and Spain) had not inherited that ancient African DNA.

By using modeling to explain the patterns of DNA distribution, the researchers came up with the following scenario: After early modern humans emerged in Africa about 200,000 years ago, some eventually left the continent and mixed with Neandertals in the Middle East or the Arabian Peninsula, where fossils and stone tools of both groups date back to about 120,000 to 125,000 years. This group of modern humans went extinct, but their DNA persisted in the Neandertals that headed east to eventually settle in Siberia. Meanwhile, another group of modern humans left Africa much later and interbred 50,000 to 60,000 years ago with Neandertals that had headed south from Europe to the Middle East. In this later migration, Neandertals interbred with the ancestors of living Europeans and Asians, who then spread throughout Eurasia. Some of this group of modern humans also encountered Denisovans, picking up the DNA that persists today in Melanesians and some Asians.

All of this suggests that modern humans mixed with archaic humans at least three times after they migrated out of Africa. But that’s just a fraction of the intermingling that must have taken place. Neandertals also interbred with Denisovans. And the new study confirms that the Denisovans themselves did indeed interbred with a “superarchaic” hominin, possibly H. erectus, whom they encountered as early as 400,000 years ago. There are also hints that Denisovans interbred with modern humans in Asia more than once, based on different patterns in the distribution of Denisovan DNA in some Chinese and Melanesians. “One would think that mixing has occurred multiple times for a long time,” Castellano says.

The low levels of DNA exchanged by these encounters suggests that it came from only a few trysts—not whole-sale mate-swapping. But it was enough to pass on genes that may have spelled the difference between survival and extinction for modern humans, including Europeans who still have genes from Neandertals that are shaping their health today. The inbred Altai Neandertal also got modern human DNA that may have been involved in speech, the immune system, and the production of sperm, Castellano says. And that fits with the theory that interbreeding was an important and rapid source of genetic diversity that could have been crucial for adapting to new terrain as modern humans spread into foreign lands.

Now, it seems that modern humans picked up Neandertal genes—and passed some of their own DNA back to our close cousins—more than once, almost as soon as our species emerged from Africa, says computational geneticist Sriram Sankararaman of the University of California, Los Angeles, who was not a member of this study. “Admixture between modern humans and Neanderthals predates the out-of-Africa event to which present day non-Africans trace their ancestry.”

Homo sapiens Interbred with Various Neanderthal Populations, Study Says

Modern humans in Eurasia carry genetic material inherited from Altai Neanderthals, according to a study published in the journal Genetics. This is noteworthy because past research has shown that Neanderthals connected to a different location — the Vindija Cave in Croatia — have also contributed DNA to modern-day Eurasian populations.

Various Neanderthal populations contributed to extant human genetic variation in a population-specific manner.

“It’s not a single introgression of genetic material from Neanderthals,” said University at Buffalo’s Dr. Omer Gokcumen, senior author of the study.

“It’s just this spider web of interactions that happen over and over again, where different ancient hominins are interacting with each other, and our paper is adding to this picture.”

“This project will now add to an emerging chorus — we’ve been looking into this phenomenon for a couple of years, and there are a couple of papers that came out recently that deal with similar concepts.”

“The picture in my mind now is we have all these archaic hominin populations in Europe, in Asia, in Siberia, in Africa.”

“For one reason or another, the ancestors of modern humans in Africa start expanding in population, and as they expand their range, they meet with these other hominins and absorb their DNA, if you will.”

“We probably met different Neanderthal populations at different times in our expansion into other parts of the globe.”

Dr. Gokcumen and colleagues analyzed the DNA of hundreds of people of Eurasian ancestry.

The goal was to hunt for fragments of genetic material that may have been inherited from Neanderthals.

The researchers found that the Eurasian populations could trace some genetic material back to two different Neanderthal lineages: one represented by a Neanderthal whose remains were discovered in the Vindija cave in Croatia, and another represented by a Neanderthal whose remains were discovered in the Altai mountains.

They also discovered that the modern-day populations they studied also share genetic deletions — areas of DNA that are missing — with both the Vindija and Altai Neanderthal lineages.

The DNA of the Vindija and Altai Neanderthals, along with the modern human populations studied, were previously sequenced by different research teams.

“It seems like the story of human evolution is not so much like at tree with branches that just grow in different directions. It turns out that the branches have all these connections between them,” Dr. Gokcumen said.

“We are figuring out these connections, which is really exciting. The story is not as neat as it was before. Every single ancient genome that is sequenced seems to create a completely new perspective in our understanding of human evolution, and every new genome that’s sequenced in the future may completely change the story again.”

Ozgur Taskent et al. Analysis of Haplotypic Variation and Deletion Polymorphisms Point to Multiple Archaic Introgression Events, Including from Altai Neanderthal Lineage. Genetics, published online March 31, 2020 doi: 10.1534/genetics.120.303167

Unanswered Questions Remain

One of the questions rising from this new study, however, is how these small Neanderthal groups managed to spread all over Europe (from Spain to Siberia). Joshua Schreiber, a population geneticist at Temple University who was not involved in the research, is not sure how these genes could have spread across such a big territory back then. “It’s hard for genes to move when they don’t have cars and airplanes,” he tells The Verge , even though he agrees that the new theory makes a lot of sense and could reshape human history if further genetic analyses back it up.

Furthermore, Schreiber says that mitochondrial DNA is only a small part of the larger genetic puzzle and to confirm their analyses, the researchers will need nuclear DNA as well. What complicates things for the research team even more though, is that no nuclear DNA has been recovered from the ancient thigh bone, since it was chewed over by carnivores and contaminated with modern DNA. Nevertheless, Posth and his team hope that enough DNA samples may make it possible to retrace humans’ very early migration even without a fossil record, “We can track the human genes appearing among Neanderthals. It’s a nice parallelism with what happens later, with the Neanderthals inside of us,” he tells The Verge , implying that the research will continue for a long time to come.