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)

The Problem with Wookiees

Photo by Scott Ruether,
CC BY 2.0 

When making up alien creatures, imagination has no bounds. Alien characters in sci-fi stories are most commonly human-like, but with animal features. But be they scary, funny, or loyal comrades-in-arms, they exist purely for entertainment. We might laugh at the absurdity of a giant slug-like creature with a human face slithering around on a desert planet, but we don't care when engrossed in the story.

Nevertheless, the critique of aliens is a great exercise for biology students.  The fundamental rule is that evolution can only proceed through logical steps. Every feature of an alien needs to be explained in terms of adaptation to environmental conditions. In terms of this purely academic exercise, my favorite aliens, the Wookiees, raise a number of red flags.

I recently discovered a webpage purporting to describe the biology and evolution of Wookiees on their home world of Kashyyyk. Whether the facts stated therein were created by the writers of the Star Wars series or made up by imaginative fans, I don't know. But since they have been posted, they are fair game. As a botanist and biology teacher, I am obligated to respond! 

Wookiees qualify as human-like sentient beings. They are smart enough, and have the upright posture, flexible shoulder joints and grasping hands needed to manipulate weapons and fly spaceships. So their evolutionary history must account for both their body anatomy and the development of their large brains. We assume they acquired their human-like traits through an evolutionary history similar to ours. Indeed, they are described as tree-dwelling primates.

For our own species, however, the path to humanity required both an arboreal (tree-dwelling) phase and a hunting/gathering savanna phase. The arboreal phase evolved in tropical rainforests, which provided a rich diet of flowers, fruits, seeds, leafy shoots, and insects or other small animals. Accessing these food sources required efficient mobility to move around a tree canopy and from one tree to another. The interlaced system of slender branches and vines of the tropical rain forest fostered such mobility, resulting in grasping hands and flexible shoulder joints. 

The hands of our ancestors were uniquely claw-free. Claws as found in other animals, were replaced in our arboreal ancestors by flat fingernails and soft, sensitive undersides, allowed them to grasp relatively slender branches and vines firmly as they swung, ambled, and leapt about. They evolved uniquely twistable,  double-boned forearms that could turn the hands upward or downward, aiding in locomotion, as well as for reaching, picking, and manipulating food items.   

Shoulders could rotate, allowing us to reach in multiple directions for food items, or branches by which to "swing through the trees with the greatest of ease." Such anatomical features would later be essential for wielding sticks and stones, for playing baseball and ultimately assembling watches and cell phones. So among the denizens of the earth only primates have the dexterity to play baseball. Though it might be a nightmare to keep them focused on the game, a match between between the Chicago Chimps and the Green Bay Gorillas is at least technically possible, because primates have arms and hands capable of throwing things. I've seen annoyed gorillas in a zoo throw something much nastier than a baseball at heckling tourists. Non-arboreal animals are even more severely limited. Shoulder joints and paws (or hooves) are rigidly limited to their walking/running function. Though squirrels, cats, and raccoons can hold food items with their front paws, they cannot grasp, and cannot throw rocks. 

The second phase of human evolution came in the radically different, wide-open vegetation of the African savanna, where we had to make a leap in intelligence and brain size.  Water and food resources in the savanna were widely scattered and highly seasonal and we had to remember the places where they could be found. The fruits, vegetables, and occasional beetle grub so abundant in the rainforest were too scarce in the savanna to sustain us, and so we had turn to hunting small animals, who generally did their best to avoid becoming our dinner. We did not have the strength, speed, claws, or toothy jaws of our competitors, and so could only survive on our wits. We had to learn to use sticks and stones to catch and kill prey, as well as to defend ourselves against the bullies further up the food chain who considered us as slow-moving food items. 

Sticks and stones led to bows and arrows, houses, wheels, and ultimately space ships. Our brains grew to human proportions and capabilities as we adapted to the harsh realities of the savanna, but would not have been possible without the flexible anatomy we inherited from our arboreal ancestors.  

Koalas are arboreal animals with claws and some
grasping ability. As slow-moving leaf-eaters, 
there was less selective pressure for greater agility
and mobility, not to mention for intelligence. 
Photo from GreenLeft.org

Red flag #1: Though Wookiees appear to have grasping hands and rotatable arms, they are also said to have retractable claws for climbing up trees. Claws big enough to support the considerable weight of an average Wookiee adult could not be retracted enough for delicate work like pressing the triggers of weapons, operating the console of a space ship, or using tools to repair a space ship. In clawed animals, bone structure and musculature are focused on supporting the claws, which must bear the weight of the animal.  There are arboreal animals that climb with claws: squirrels, sloths, koalas, etc., but of these, only squirrels are really agile in trees. They, however, are small and light-weight, depending more leaping rather than grasping and swinging to move through a forest. The others are far more limited in mobility and depend on more limited diets. 

Red flag #2: The forests of Kashyyyk are said to be dominated by coniferous Wroshyr trees, sounding more like a boreal forest than a tropical rain forest, and would provide neither the variety of food sources nor the 3-dimensional jungle gym structure to move around in.  Also, Wookiee ancestors are said to have been carnivores. However, all known arboreal animals are vegetarian or omnivorous. It's too difficult to run down prey larger than insects, frogs or lizards in the forest canopy, unless you're a bird of prey, So it is unlikely that Wroshyr forests could have fostered the evolution of the flexible anatomy required for the later development of technology.  

Wroshyr trees are also said to be massive, supporting Wookiee communities within their trunks and large branches. Such trees are said to average 300-400 meters in height, with some varieties as tall as several kilometers. The tallest trees on Earth reach a little more than 120 meters, and are pushing the limit of the physical force of transpiration to lift water against the pull of gravity. If gravity were a little less on this planet, trees might be a bit taller, but if gravity were substantially weaker, the planet could no longer hold onto its liquid water or atmosphere. So  trees significantly larger than those on Earth are highly unlikely.

Red flag #3. It appears that Wookiees stayed in the forests. They did not face the challenges of the savanna or any other environment that could drive the evolution of upright posture and higher intelligence. Why is this important? If they indeed lived continuously in forests, they would have advanced little more than the great apes of Earth. The forests did not provide the necessary challenges.

Red flag #4: The dense hairy coat of the Wookiees, while fine for primates who never left the rain forest, would be a real liability in the savannas, in particular for developing greater intelligence, i.e. bigger brains. The brain is the most heat-generating organ of the body. As it increased in size, we required an improved cooling system. To provide cooling surface for all that brain heat, along with heat from the scorching savanna sun, we lost body fur and enriched our bare skin with fine sub-surface capillaries and  a high density of sweat glands. We retained hair on top of our heads to protect our brains from direct heating, but the rest of the body was freed up for cooling the blood. 

As a unique, omnivorous species in the savanna, we could not nap in the shade of trees after a big kill, because hunting and gathering had to continue all day long. So we became "naked apes." This is nicely explained in a 2010 article in Scientific American, by Nina G. Jablonski. Being naked therefore was also essential for breaking through to human-level intelligence. This also applies, incidentally, to our homegrown hairy aliens, Sasquatch and Yeti.

So we are left to ponder how the Wookiees became intelligent beings. If they did not follow the human game plan, what evolutionary history did they have? I keep coming back to my conviction that if we were to discover intelligent, technologically capable, alien life forms, they would have had to go through a similar evolutionary pathway as ours, and would look boringly like us (see my post on the inevitability of humans. Even with very human-like aliens like Vulcans, we have to explain pointy ears, green blood, and internal organs that are somehow different. If you are a teacher, try this with your students.  It's a topic that's guaranteed to wake up that guy in the third row who's been sleeping since the lecture on Cyanobacteria. Oh, and less you wonder if technological aliens could have evolved from sea creatures, have you ever tried to throw a baseball under water?