I came across the following post by Professor David Hillis of the University of Texas at Austin, an evolutionist I much admire for his perspicacity and malleability: his ability to contribute to several different fields, including herpetology. (He also breeds longhorn cattle on his Double Helix Ranch in the Hill Country.) Of course one reason I respect his work is because many of his conclusions agree with mine!
One of them is his adherence to the Biological Species Concept formulated by Ernst Mayr, which basically states that two populations belong to the same species if, when they contact each other in nature, they can produce fertile hybrids. If they can’t for any reason, then they are members of different species. Nearly all evolutionists adhere to some form of the BSC, but I won’t go into the concept in depth as it, its problems and problems with alternative species concepts are covered in detail in my book Speciation written with Allen Orr.
One long-standing question in evolutionary anthropology is whether Neanderthals were a different species from “modern” Homo sapiens. Some anthropologists say they were different species, the former called H. neanderthalensis and the latter H. sapiens. But there is ample genetic evidence for interbreeding of the two forms in Eurasia, and for fertility of the hybrids, for many people outside of Africa carry Neanderthal genes. I myself have a small fraction of Neanderthal genes in my DNA. This kind of data is prima facie evidence that when the two groups were in contact, that they produced hybrids and also that those hybrids were fertile, backcrossing to modern H. sapiens.
At best, then, I consider the two forms to be subspecies: somewhat differentiated populations that were members of the same species. Those who agree with this, like me and Hillis, see the subspecies as H. sapiens sapiens and H. sapiens neanderthalensis.
The Neanderthals evolved from a hominin population that came to Eurasia out of Africa about 400,000 years ago. They persisted until about 40,000 years ago, when they (identifiable by fossil features; see below) mysteriously disappeared.
There have been several hypotheses accounting for the disappearance of the Neanderthals. They include demographic fluctuations or inbreeding leading to disappearance of the species, extinction due to inability to survive changes in the environment (there was glaciation when Neanderthals lived), competition (including aggression) from “modern” H. sapiens who left Africa to populate the world about 60,000 years ago, and “extinction through interbreeding”, which is the hypothesis supported by a new paper discussed below in Nature Scientific Reports. I should add that “modern” H. sapiens arose in Africa about 300,000 years ago.
Click both screenshots to go to the comment and the paper.
This paper, by Swiss and Italian authors, shows that “extinction via interbreeding” is at least possible under reasonable and conservative conditions obtaining at the known times of extinction. Successive waves of H. sapiens coming out of Africa, they posit, interbred with Neanderthals and eventually, because the sapiens were far more numerous than Neanderthals, the Neanderthal genome became absorbed into the modern population of H. sapiens. Click to read; it’s highly mathematical and you should read just the abstract, introduction, and conclusions:
Here are a few of the assumptions:
- Neanderthals consisted of tribes of about 500-1000 individuals, subdivided into “bands” of 25-50 individuals.
- The total population of Neanderthals was about 5000-7000 individuals
- The population of H. sapiens was ten to a hundred times larger than this
- Repeated waves of H. sapiens came out of Africa, comprising an effectively infinite reservoir of individuals and genes
- The genetic data in modern H. sapiens shows repeated instances of introgression
The upshot: The “genetic dilution” or “extinction through interbreeding” hypothesis can be supported by a reasonable mathematical model that takes into account evidence of population sizes and times of migration, and accounts for the genetic data. This does not mean, of course, that this is why the Neanderthals disappeared. It could have been another scenario or perhaps a combination of this scenario with others. Mathematical models can only tell us what is possible, not what is true. But the paper does show that an interbreeding scenario cannot be ruled out.
And the genetic data we do have, including ancient DNA data from Neanderthal bones, shows that we and Neanderthals are brothers and sisters: members of the same species. There were other distinct biological species that arose during hominin evolution, but Neanderthals and modern humans were almost surely “conspecific.”
The paper’s conclusions:
The mathematical model we presented provides a plausible explanation for the gradual disappearance of Neanderthals, suggesting that their genetic assimilation with H. sapiens, a demographically much larger species, may have been a significant factor in their decline. Rather than sudden extinction, our model proposes that repeated cycles of H. sapiens immigration leading to the Neanderthal gene dilution, could account for the Neanderthals’ disappearance and the observed patterns of Neanderthal ancestry in modern human populations. Although this model provides a possible robust genetic explanation, it is important to note that it does not exclude other possible contributing factors that were not considered in our approach, such as environmental changes, competition, or demographic fluctuations. Evidence of interbreeding and genetic introgression supports the notion that H. sapiens and Neanderthals interacted extensively over thousands of years. Therefore, the interplay of genetic, demographic, and environmental factors likely contributed to the complex process of Neanderthal demise.
Future studies incorporating both genetic and archaeological data will be crucial in refining our understanding of this pivotal moment in human evolution.
Here’s a Neanderthal skull showing distinctive subspecific features (from Wikipedia):
Very interesting! If Neanderthals disappeared through hybridization, then H. sapiens sapiens and H. sapiens neanderthalensis are surely subspecies of the same biological species. But even if Neanderthals were extinguished by other means, the almost universal presence of Neanderthal genes in (modern) humans of European provenance almost surely points to the same thing: the two are subspecies.
I haven’t been up on this literature for some time, but this analysis is the best I’ve read regarding the possibility of extinction by hybridization—which perhaps we shouldn’t call “extinction” at all! Another positive is that the hybridization theory doesn’t require ad hoc explanations such as competition, predation, climate change, etc.
Interesting post. I haven’t read the Scientific Reports paper. Until then, I have a general question about speciation.
Humans are a relatively young species. We do carry a small fraction of Neanderthal DNA, but it isn’t the same cluster of genes that have been fixed (last I read, about 40% of the total Neanderthal genome is present in the human population). We also have some notable genomic deserts- eg, the X chromosome.
How can we tell if this is a slow process of eliminating all or most Neanderthal genes or if we are a stable, fertile hybrid?
I don’t know what you mean by stable, but we are certainly fertile. And even if some genes introgress in clusters because other introgressions were maladaptive, the data show clearly that fertile hybrids were produced between Neanderthals and “modern” H. sapiens.
Sorry, I’m not a geneticist, which showed in my phrasing of the original question. What I meant is: how can we tell whether sapiens and Neanderthals are the same species, or whether this is hybridogenesis in action, with initial fertility but eventual cleansing of the introgressed genetic material (eg, some European water frogs)?
The % of neanderthal DNA has decreased from the initial hybrids (which would be 50% neanderthal), simply because there is more DNA from our subspecies in the DNA pool. I don’t know what will happen with these %s in the future.
Besides selection for/against DNA, there is also genetic drift where frequencies of DNA change by chance alone. This can be stronger than selection under some circumstances.
Even if there is selection against some neanderthal DNA, we still are interpretable as being members of the same species.
You might find this paper of interest….Homo sapiens, Neanderthals and speciation complexity in palaeoanthropology https://academic.oup.com/evolinnean/article/3/1/kzae033/7900502
‘We begin by discussing evidence about species-level differentiation of H. sapiens and Neanderthals and analyse major sources of taxonomic disagreement, before illustrating the potential of this perspective in making progress on the earliest stages of H. sapiens speciation within Africa’
Some of the phenotypic differences between sapiens and neanderthalensis feel like they were pretty big (the brow ridge, the chin, the projecting face), but obviously their mate selection behaviour or preference was similar and the two genomes were compatible with each other in embryonic development. So I guess the tempo of evolution of some of those traits was quick. Would be fun to know how that tempo compares to the typical tempo of evolution of the same traits in other ape lineages.
Articles like this is why I read this site daily. Very intersting.
That all seems very reasonable. It has been clear that our subspecies considerably outnumbered the neanderthals, and I do favor arguments that we were ecological dominants when compared to our cousins.
There is no evidence that I am aware of, but consistent human nature from pre-enlightenment days suggests that this genetic absorption was accompanied by displacement, murder, and probably rape.
I asked ChatGPT if neandertals had recognizable different mDNA than humans. The answer was positive. If interbreeding was mainly male humans and female neandertals (as you suggest, probably forced) would that not show up in the mDNA of today’s humans?
This off-spring would very probably have lived with the mother and her clan or would mother and child have been integrated in the human group?
That is a clever angle of questioning! The Google AI says that neanderthal mtDNA has disappeared in both human and ancient samples of DNA from modern humans, suggesting either that the matings were primarily with neanderthal males X H. sapiens sapiens females, or it disappeared by genetic drift. Of course the ancient samples would be a very tiny fraction of the population, but still, my notion above could very well be wrong. 👍
You said that I support the Biological Species Concept, and that is not entirely accurate. I see the BSC as one of many ways of conceptualizing species, all of which rely to some degree (whether explicitly or not) on reproductive interactions. Reproductive compatibility is one important consideration in thinking about species and speciation, and people who are especially interested in the development of reproduction isolation tend to emphasize that aspect of species. There is nothing wrong with that, but I don’t see it as the whole story. To me, what matters about reproductive isolation is if it sufficient to keep the respective populations on separate, isolated evolutionary paths. If it is, then clearly we are looking at species. But if locally differentiated groups merge upon contact, as in this case, then they are part of the same evolutionary species.
I prefer a broader view of species as distinct evolutionary lineages that evolve independently of one another through time. I prefer not to use any of the formal concept names, but this is basically a lineage concept of species, which is compatible with virtually all of the named species concepts (and it is how many biologists have thought about species since the days of G. G. Simpson). Lineage independence does require some degree of reproductive isolation (for sexual species), but it does not require absolute reproductive isolation. Most widely distributed species exhibit geographic variation, as populations in different areas adapt to local conditions, but if these populations exhibit continuous connections, they often merge or exchange genes in a way that the various lineages are not evolving independently. That seems to be the case for H. s. sapiens and H. s. neanderthalensis: two groups of humans that diverged and adpated to local conditions, and then one expanded, and the two groups merged again.
Given that modern humans are mixtures of both (and other) ancestral groups, it makes no sense to me to call any one of those ancestral groups “modern humans,” and exclude the others as part of Homo sapiens.
I think it is clear that if we had any “pure” Neanderthals around today, everyone would consider it highly inappropriate and racist to say that that they were not human. Human Neanderthals were incorporated into lineages of humans that migrated out of Africa, and the two geographically differentiated groups are now merged in a single lineage, which we call a species. Yes, Homo sapiens once showed stronger levels of geographic differentiation than we do today, which is hardly surprising given our increased abilities to disperse rapidly and easily.
Many biologists seem to have thrown up their hands and given up thinking about species, and instead now give taxonomic “species names” to virtually any group of organisms that they can distinguish from other groups. This is a strangely non-evolutionary approach, and it fails badly in any species that exhibits local adaptation (geographic variation). In groups with low vagility, there can be striking geographic variation, although it is often distributed in a continuous manner. This is why there is a sudden “explosion” in the apparent number of species of many groups of amphibians, reptiles, mammals, fishes, plants, and many other groups with low vagility, as well-known geographic variation within species is subdivided into ever-smaller local populations and given new names.
In the mid-1900s, some taxonomists went to extremes in naming local populations of wide-ranging species as subspecies. In many cases, the subspecies were poorly differentiated and parts of continuous clines, so that practice fell into disfavor, and many of the named subspecies began to be ignored. Now, we are seeing a revival of that era, except that now some biologists are calling these locally adapted populations “species” this time around. I think this is even less helpful than all the named subspecies within species.
Biological concepts, as concepts in many areas of human thought, tend to act like pendulums, and swing widely at first in one direction, then in the other. In most cases, the pendulum eventually comes to rest somewhere in the middle. I think most biologists think of species as the distinct evolutionary lineages on the Tree of Life, whether they use that exact language or not. Clearly, those lineages remain distinct for many reasons, including some necessary degree of reproductive isolation.
Thanks for the clarification, David. I remembered your defenses of reproductive isolation but wasn’t aware of any publication in which you defend the lineage concept above all others.
In fact Allen and I did consider the “lineage concept” (we called it the “evolutionary species concept”) in our book, but found it deficient for two reasons. First, if a population is isolated on an island with no migration from, say, the mainland, it is according to that concept an “instant species”. since the lineages will remain evolutionarily distinct because of geography. Second, if there is some sympatry, it becomes equivalent to the biological species concept. Readers who are interested in our take can find it in the Appendix of our book, though it’s bloody expensive now that Oxford has taken it over.
All I can say is that every single paper trying to analyze how species actually arise deals with THE ORIGIN OF REPRODUCTIVE ISOLATION, not things like “how a new evolutionary lineage arises” (as I said, in all interesting cases they’re equivalent). This says that when the rubber meets the road, the process of speciation is taken by nearly all evolutionists as the origin of reproductive barriers. In Chapter 1 of our book, we show that the BSC is the only concept that actually gives one a research program to understand the “species problem”, which is “why does nature come in lumps–discrete units–rather than as a continuum.” The BSC answers that by saying, “because reproductive barriers create those lumps in sympatry”, and the lineage concept does not touch that question. I think you’re mistaken in saying that people like me adopt the BSC because we’re interested in reproductive isolation. Actually, I am interested in reproductive isolation because it’s the only explanation for why nature is lumpy–why species remain distinct in places where they coexist. I didn’t become interested in reproductive isolation because of some a priori fascination with it.
And in our book we note that you don’t have to have ABSOLUTE reproductive isolation to have biological species–you have to have it to have PURE biological species. There is a continuum between lots of hybridization, as in Neanderthals and modern H. sapiens, and species like us vs. chimps, where there is pure reproductive isolation. It is somewhat subjective where one draws the line between “same species” and “pure or good species.”
In purely allopatric taxa, most bets are off unless you can hybridize the allopatric populations artificially and show that hybrids are either inviable or sterile.
DAVID HILLIS
David couldn’t post this comment, so I’m doing it for him and will respond.
Reply from Jerry: In my book with Orr, we discuss that the BSC is a CONCEPT, not a definition: it is a way of encapsulating the discontinuities of nature in words. And we also note that the BSC does not apply to asexual organisms; indeed, it’s not clear that asexuals like bacteria even form discrete clusters. So the BSC is not universal, but it is for sexually -reproducing organisms.
David doesn’t address my main issues with the phylogenetic species concept. First, it is even less applicable than the BSC, and, as Allen and I show in the Appendix of our book, it has substantial problems depending on how you conceive of “evolutionarily independent lineages.” Most important: the BSC gives us a research program for understanding why nature is not a continuum: the answer is reproductive isolation. Thus, to understand the origin of species, the problem becomes one that is in principle tractable: you need to figure out the origin of reproductive isolating barriers.
In contrast, the lineage concept leads to no program at all about how to understand why nature is lumpy. The problem devolves to “how do evolutionarily independent lineages arise?” And the answer is twofold: if they arise in geographic isolation, then we have allopatric speciation, BUT ONLY IF THERE IS REPRODUCTIVE ISOLATION. If they become sympatric and can clearly be see as distinct lumps, then that is a result of REPRODUCTIVE ISOLATION. Thus, the lineage concept is not useful at all in answering the “species problem,” which is “why is nature lumpy?” I am not sure, indeed, what question the lineage concept answers!
It is interesting that even though we conceptualize species differently, in application we agree on what are species and what are not (at least for sexual species). I’d say that my research program on species includes all the questions and approaches that yours does (the evolution of reproductive isolation), but does not limit itself to that one aspect. Why asexual species are also discrete and non-continuous is also of interest to me, as are questions about species that are no longer extant.
David,
I hope you read my book with Allen, because we do discuss whether asexual species are discrete, and, if so, why. It’s in chapter 1. I too am interested in that question, but at least for bacteria the evidence for discrete species is mixed, even though they’re given names.
Thanks so much for the response. I was born 17 years after Mayr’s BDS was published. It has been around for about three human generations and seems to be working about as much harm as good. I guess l should resign myself to the reality that some people will always need a black and white world view. To me the universe is infinate shades of gray.
Umm. . . our whole book on Speciation, which was received quite well, is based on the BSD (not the BDS). As for the implication that people who use the BSD, like me, “need a black and white world view, well, that’s insulting. Although touting your superior wisdom, I think it’s misguided. As for “infinite shades of gray,” does that apply to biological sexes, too?
Is not the continual violence in human history evidence that homosapiens simply murdered neanderthals over time? Combined with inter breeding this was a genocide and science is too polite about this murderous aspect of the homosapien “personalty.”
As I said, aggression is a possibility, but they interbred, so they couldn’t have murdered them ALL! And no, scientists have studied agression in great detail.
I often wondered how neanderthal and sapiens could be different species; since I have about one and one half percent neanderthal dna, and my ancestors obviously had to be fertile. I really enjoyed this post. Thanks, Jerry, and all the other scientists who have commented.
This is so informative and interesting, thank you.
Neanderthal-
What happened to them?
In the animal kingdom different species within the same family of animals can mate and produce offspring. Lions and tigers produce Ligers. Horses and donkeys produce mules. But their offspring are always sterile.
It is a certainty that Neanderthal males and human females could produce fertile offspring since all non-Africans today have some Neanderthal DNA.
Conversely it is also likely that human males and Neanderthal
females would have produced sterile offspring, since no Neanderthal mtDNA has been passed down to humans alive today. Mt-DNA is passed to offspring through the mother, in this case a Neanderthal mother.
As it has been throughout the history of mankind, it probably was the same back then, that Humans and Neanderthals were enemies because they were different from each other. In battle the victors usually took the women as slaves and had forced sex with them. However this was unlikely with the strength and physicality of Neanderthal women. They could overpower a human male most likely. But, if sexual relations did happen between a human male and Neanderthal female the resulting offspring would be sterile. So it is unlikely that human or Neanderthal DNA was passed down in this way.
In the event a Neanderthal male mated with a human female, a fertile hybrid offspring would be produced with both human and Neanderthal DNA.
The Neanderthal men may have found the human women more desirable, and with every sex union of this kind, more and more half-human, half-Neanderthal offspring were produced. These mated with other human females which further diluted the Neanderthal line.
The only fertile offspring from a Neanderthal woman would have been 100% Neanderthal.
Human females could conceive and give birth through human/human sex, and also human/Neanderthal sex, and the offspring would all be fertile.
Over many, many generations this could have severely diluted the Neanderthal line until after many tens of thousand’s of years later there were no more pure Neanderthals left. They bred themselves to extinction.
You are making a lot o this up, and thee is a lot that is wrong, stating with the claim that ligers and mules are sterile. Not all of them are.
This is wrong, too:
You can puzzle out why you cannot make flat assertions like that.
The whole thing is a just an idea that occurred to me after reading that MtDNA has never been found in humans, which made me wonder if human male/neanderthal female offspring were sterile. This comment was not a scientific study, not tested, not proven. But if anyone wants to pursue it further, that is the scientific method. And mules are sterile!
Some mules are fertile. I have dealt with this question previouly. Most but not all are sterile. See below:
Thanks for the information. I suppose if all but 100 Neanderthal/human births by Neanderthal females didn’t succeed that would explain the lack of Neanderthal MtDNA in us.
As we know, what counts as a species and what doesn’t is arbitrary. These days, biologists are in a splitting phase, and certainly not just with fossil humans. Who knows–maybe the pendulum will swing back one of these days.
No it is not arbitrary. Is modern H sapiens a species? How about lions? What is arbitrary about the giant panda?
A lot of this is above my pay grade, but I always have wondered why there are so many different species that look so much alike, for instance, frogs. I do like to read about things that are hard for me to understand though.
I confess that I’m not a biologist but I don’t see the value of the concept “sub-species” – if fertile interbreeding is the criterion, then all organisms that meet that criterion are the same species. But what distinguishes sub-species from each other? If it’s simply phenotypic characteristics, how does that differ from other sub-groupings like “race”?
My theory, which is my own, is that there is a powerful admixture of Magyar genes in us all, which can be traced all the way back to Budapest Man , and the “we’re out of paprika” migration, after the final land-bridge linking Europe to the Spice Islands sank into the sea.
I seem to recall (but can’t recall where) that Svante Pääbo found that one human/neandertal difference was in some protein involved in sperm motility, and the inference was that that might have been a contributing factor to sapiens overtaking neandertals, but I never heard anything more along those lines.
Am I missing something? As a lay person the answer to this question is so obvious that makes me think I must.
If you dilute Coca-Cola with water, Coke molecules don’t magically disappear there would be fewer of them per volume of the liquid. Similarly, if dilution was the ONLY cause of Neanderthals disappearance, you should still be able to recover all Neanderthal alleles from the current human population. In other words, can you construct at least one complete Neanderthal individual from the current genetic pool. The logic suggests to me that if the answer is no to the above question, then dilution cannot be the only cause.
No, there could have been (and there’s evidence for) selection against some Neanderthal ALLELES in the human genome. This explains why some Neanderthal alleles are rarer than other. That said, although we know the whole Neanderthal genome from ancient DNA, only about 70% of it appears among modern humans. But this does not refute the idea that Neanderthals were “mated to death”. Yes, selection has to be added to the mix to account for what we see?