Are Modern Humans and Neanderthals the Same Species?

 No, yes, maybe.

A species is typically defined as  a population, or group of populations, that can be recognized and distinguished from other such populations by a distinctive set of characteristics, and which is reproductively isolated from other such populations. 

Whether two populations of related organisms are classified as belonging to a single species or to two separate species is a decision made by taxonomists depending on two things: the degree to which the entities  can be distinguished from one another, and/or the degree to which they are reproductively isolated from one another. Fossils of modern humans, Neanderthals, and a third group known as Denisovians, are clearly recognized and distinguishable from one another and so are considered separate species by anthropologists.  This is consistent with how fossils in general are classified, as we typically have little or no information on reproductive isolation among them.

The growing evidence of genetic exchange between Neanderthals,  Denisovians, and modern humans some 40,000-60,000 years ago, however, suggests a possible alternate taxonomic decision - that they were all part of the same species. Though only modern humans remain today, the three groups overlapped  geographically for roughly 20,000 years after modern humans migrated into Eurasia. Genetic analysis confirms that all peoples of Eurasia possess between 1-4% Neanderthal DNA in their genetic makeup.

We know far less about Denisovians, who inhabited eastern Eurasia, but evidence is mounting that they too exchanged genes with modern humans, with their DNA now detected in indigenous human populations from central Asia to Papua New Guinea and Australia.  

The exchange and retention of Neanderthal DNA likely enhanced the ability of human immigrants from Africa to adapt to the colder climates of Eurasia. The February issue of National Geographic provides a fine review of our current knowledge of the human family and the interactions among the species. The article incudes a fascinating illustration, somewhat speculative of course, of a peaceful family scene of a Neanderthal man, his modern human wife, and their infant son huddled around a campfire. It hardly seems more remarkable than an interracial family today. 

Since they were not reproductively isolated from one another during that period, the three species of  humans might reasonably be combined into  a single species, and recognized as three subspecies. 

I boldfaced the word "decision" at the beginning of this post because classification is a human artifice, a matter of convenience for discussion and recording of information. Names, and taxonomic ranks are not a part of nature.  Taxonomists may not agree on how distinct a species must be from other species, and how complete the reproductive isolation must be. The category of species is of higher rank than subspecies, and therefore indicates a higher level of  character distinction and reproductive isolation. The history of plant taxonomy is full of conflicts between "lumpers" and "splitters," who disagreed on how broad species should be. Splitters recognized greater numbers of more narrowly-defined species, while lumpers were more tolerant of variation within species and recognized fewer species. 

Our expanding understanding of reproductive isolation has generally favored lumping, as we realized that apparently different species of plants or animals were mere variants of large, interbreeding populations. For example, the World Checklist of Vascular Plants currently recognizes 437,645 species, and over a million synonyms - formerly recognized species that have been lumped into the recognized species. As a more specific example, 46 species of the genus Rubus (blackberries and their relatives) are now recognized from South America, out of 110 that had been named by earlier taxonomists.

 Incidentally, I chose to use the human family as the focus of this discussion because it has an unusually extensive fossil record, as well solid evidence of genetic exchange among species, including some that are now extinct. Adding the time perspective illustrates the source of many taxonomic problems. We rarely have fossils of ancestral plants, and so plant taxonomy deals almost exclusively with present day plant populations. In theory, however, everything I've said and will say in this post applies to plants as well. 

Reproductive isolation prevents two species from sharing genes and blending back together, and so each species continues to develop its own characteristics. Reproductive isolation is a tricky factor, however. Closely related plant species can often hybridize. The list of garden plants that are of hybrid origin is extensive. In nature, however, closely related species don't normally interbreed. They are isolated geographically, by ecological preference, or in many plants (such as orchids) by pollinator specificity, all of which can be overcome by plant breeders. 

A liger at Jungle Island, Miami. Lions
and tigers naturally live in different
habitats and so rarely meet. If they do
happen to meet and interbreed, the
resulting offspring are unable to 
produce viable sperm or egg due
to chromosomal incompatibilities, and
so do not produce further generations.
The genetic integrity of the two species
thus remains intact. 
Photo by Maxitup16, CC by SA 3.0

 Closely related species may interbreed in nature where their geographical or ecological boundaries meet, but the resulting hybrid individuals are often less fit for survival outside of the narrow interface zone, and the alleles of hybrid origin usually do not spread back into the parent populations. So despite occasional hybridization, the parent species remain effectively isolated genetically. Animals like lions and tigers, or horses and donkeys, can mate upon rare natural encounters, or in the hands of zookeepers, but their offspring are sterile, revealing a more complete separation of species. 

So lions and tigers are unequivocally different species despite the occasional hybridization. Not so with the three human species that interbred in Asia. Hybrid individuals were able to reproduce and spread hybrid gene combinations throughout the parent populations. 

We can say that lions and tigers are further along in the biological process of speciation than the three species of humans were. Speciation is the real process of populations changing over time, in contrast with the arbitrary human process of classification. As species drift apart over time, chromosomal rearrangements arise that reinforce the tentative reproductive isolation caused by geographical separation, making further genetic exchange impossible. The ambiguities of this situation, and classification in general, are due in large part to the fact that groups of related species that we encounter in nature are at different stages of speciation. Some are recently separated and still capable of interbreeding, while others have become chromosomally incompatible and fully separated. 

 All species begin as a population that splits off of from a pre-existing species.  The Neanderthal/Denisovian lineage split off from an ancestral species in Africa that was also ancestral to modern Homo sapiens. So at the beginning, they were the same species. Members of one lineage later migrated to Eurasia, where they split into what we now recognize as H. neanderthalensis and H. densoviensis. As these Eurasian populations adapted to the very different environments they encountered they developed distinctive characteristics, but not solid reproductive isolation from their African cousins that would migrate later. If the three species had remained separate for another 100,000 year or more, they might have crossed the line into fuller genetic incompatibility, and then without doubt would be separate species.

Before that stage is reached, however, species may exchange genetic information temporarily or possibly merge back together, combining the best of each, and survive as an improved, hybrid population. When immigrants from Africa met their Eurasian cousins, something in-between happened. Genes acquired from Neanderthals for lighter skin, hair, and eye color, along with other things, helped those immigrants adapt to the harsher conditions they encountered. The outcome was apparently not so good for Neanderthals and Denisovians. For reasons still not fully understood, these groups became extinct soon after.

So the answer to our title question is still a matter of taxonomic opinion. The speciation process was not complete with respect to reproductive isolation, and so were the physical differences enough to warrant speciation? The outcome is of interest on many levels. If, hypothetically, a group of Neanderthal survivors were found today, would they be granted all the civil and religious rights we delegate to ourselves? For biology students, anyway, it is a vivid illustration of the speciation process, the dynamic nature of evolution, and the tenuous authority of formal taxonomy.

Incidentally, the very first member of the genus Homo, recognized by dint of some distinctly human characteristics, logically must have evolved from ancestors formally classified in a pre-existing genus. It is clear now that that genus was Australopithecus, which existed for several million years before Homo arose. That, by definition, makes Australopithecus a paraphyletic genus, something prohibited in clade-based phylogenetic taxonomy. The rule is that all formal taxa must be monophyletic - complete clades consisting of a common ancestor and all its descendants.  Anthropologists at first went through considerable taxonomic acrobatics to resolve this problem, splitting Australopithecus into ever finer monophyletic units. But in the end, there was no escaping the fact that, unless you believe in special creation, the first humans must have descended from something not quite human. 

[BTW - the answer to the old chicken or egg question is clear. The very first chicken hatched from an egg laid by an almost-chicken!]

Because of the need to provide formal names (binomials consisting of the genus name and the specific epithet) for the Australopithecines, paleontologists have come to relax the rule to greater or lesser extent. However, one must acknowledge that every genus ever identified by taxonomists logically began with a common ancestor that emerged from a pre-existing genus. The fact that we don't have fossils of that pre-existing genus, as we generally don't with plants, does not negate that inescapable conclusion. One proposed solution is to abandon formal taxonomic ranks like the genus, and simply name clades, but then it becomes very difficult to provide formal names for organisms. This is something I have been obsessed with throughout this blog series, and you can review my earlier attempts to clarify the situation:

The great botanical butter battle book (30 Aug 2012)
Making the ancestor problem go away (18 Oct 2012)
Minding your stems and crowns (3 Jun 2015)