I’ve just been googling around about the Denisovans. Long story short, it seems that there were at least three different kinds of humans in the last 100k years, and it seems that there may have been a fourth (homo erectus?).
So, how different, really, were these early humans? If they could interbreed so successfully, it seems to me they were more like breeds of dogs than different species. I’m fascinated by the notion that there were many different kinds of humans roaming the earth, but were there really? Was the genetic diversity back then greater than the diversity among all modern humans? Was it a significant diversity? I.E., dogs today range from the chihuahua to the St. Bernard, but they’re all just dogs. Were the Denisovans, Neanderthals, homo, and Human Type X genetically more diverse than today’s native Americans, Australian aboriginals, and Celts?
Even just with in H. sapiens sapiens itself, the genetic diversity was much greater at some point in the past. We went through a pretty severe population bottleneck not all that long ago, and haven’t yet fully recovered from it.
And it’s worth noting that human-bred dogs aren’t more diverse than wild wolves: All of the genes that make St. Bernards or chihuahuas are found in wild wolf populations, just not grouped in those particular ways. A St. Bernard and a chihuahua are very different from each other, but the flip side of that is that all chihuahuas are very, very much like other chihuahuas.
It is a fascinating topic and more is being found out out it quite rapidly. However, you make a distinction that is taught in schools and yet is quite misleading. The concept of a ‘species’ is not a well-defined scientific idea and even falls short of logical biology.
The currently accepted definition of a species is “a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding. The species is the principal natural taxonomic unit, ranking below a genus and denoted by a Latin binomial, e.g., Homo sapiens.”
All of that is made up to some degree by people and the lines are not clear. Many animals are able to interbreed outside of their species, genus and sometimes even more. Most of those produce infertile hybrids but even that is not always the case (see Dog-coyote hybrids called coy-dogs for one example).
All modern humans are the same species by that convention just like all dogs and wolves are the same species by that definition but that doesn’t mean that all populations are exactly equal. The last 10 - 15 years have produced substantial evidence that there were at least four different modern human species at one time that overlapped across different ranges: homo sapiens, Neanderthals, Homo floresiensis (hobbit people who lived as recently as 9000 years ago in Indonesia and possibly much more recently) as well as the Desmovians in Asia. Not much is known about the Desmovians except that they have unique DNA sequences obtained through a few scant samples. Note that homo erectus was not a concurrent human species. That is an ancient ancestor that may or may not be a direct relative of the other known four modern human species.
Homo Sapiens as a whole all originated in Africa but the story doesn’t stop there. Sub-Saharan African tribes still have the most genetic diversity of all human groups in the world. Selected bands of them moved into the Middle-East and then into Europe where they met the Neanderthals and then they interbred. Contrary to popular belief the Neanderthals never died out. They simply got outbred. If you are a white or Asian person, you will about 4% Neanderthal DNA on average with some groups having slightly more. No one fully understands what the Desmovian DNA contribution to modern population is but it is presumed to be some as well.
The study of human migration patterns and interbreeding among similar competing human species is rapidly shedding light on how homo sapiens differentiated and adapted so rapidly to wildly different environments. Part of it was because of hybridization with other human species.
I am going to have to bump this thread once. I am shocked it only got two replies to a very interesting question. I am only an amature anthropologist that reads every article on the topic as they become available but I am interested in the topic. The OP got some fundamental things wrong but the question was a good one. The study of human origins is much more complex than previously believed even going back 15 years ago.
We have strong evidence of 4 known human species that competed for territory. Homo Sapiens in general won overall but that doesn’t mean that the others ever truly went extinct either especially for selected populations that hybridized with populations like the Neanderthals (proven through DNA evidence) or the Desmovians (too recent a discovery to know for sure).
This stuff is incredible. We have true evidence of four separate human species that overlapped in recent history in human evolutionary terms. It turns out that there were real Hobbit people on at least one island in Indonesia less than 9000 years ago that were a distinct species and some of the locals still swear that they were still around in living memory.
If you aren’t from sub-Saharan Africa, the chances are absolute that you are also part Neanderthal plus maybe part Desmovian and the genetic contribution is not trivial in the least.
What do you think of that? We are used to living with the assertion that all of us are homo sapiens but imagine a world where that was not the case. It turns out that genetic contributions to modern populations are not equal either. What does that mean? We will let the science figure that out.
Dog breeds are a bad analogy, since they are artificially constrained to not breed with other dogs. Whether two given populations are considered different species or not has little do with whether or not they can interbreed, but whether or not they do interbreed. Lions and tigers can interbreed, but they don’t do so in the wild. Wolves and coyotes can, but they also don’t very often in the wild.
So, if you go back 100k years, you have several population of humans (that is, members of the genus Homo)
H. neanderthalensis: Humans found in Europe and Western Asia.
H. denisovanensis (sp?): Humans found in North Eastern Asia, but we don’t really know their full range.
H. erectus: Possible remnants of early emigrant populations for Africa found mainly in South East Asi.
H. floresiensis: “The Hobbits”. A diminutive population of humans living on the island of Flores in Indonesia, possibly an offshoot of H. erectus, but we don’t know. Whether they lived elsewhere, we don’t know.
H. sapiens: Our ancestors who were living in Africa 100k years ago.
From what we know about modern mammalian populations, it would be astounding if any of these populations were unable to interbreed. The real question is whether they actually did. So far, it seems that there was interbreeding between Sapiens, Neanderthals and Denisovans. Maybe with Erectus. We have no indications of interbreeding between the Hobbits and other populations. But absence of evidence is not the same as evidence of absence.
It is a fascinating topic, and as DNA analysis techniques get better and better, we’re going to find out more. An article earlier this month highlights evidence for interbreeding between Neanderthals and Denisovans and a fourth group that hasn’t been identified yet. Unfortunately, Nature’s website isn’t working for me right now. But the article is:
Prüfer, Kay, et al. “The complete genome sequence of a Neanderthal from the Altai Mountains.” Nature (2013). link
One of many news releases here
I wonder if some supposed Neanderthal specimens are actually Denisovan or “other”.
It seems to me that part of what the OP is asking, and certainly something I would like to know, is why it is that Sapiens, Neanderthals (particularly), and Denisovians (perhaps) are considered separate species (rather than say, subspecies), given that it is now pretty certain that the first two, and probably the third too, interbred and that DNA derived from Neanderthals and probably Denisovians too, is still found in modern humans (who we surely consider to be a single species). I get it that the traditional conception of a species as an exclusively inbreeding group is not terribly satisfactory, but by what alternative conception are these types of human still considered separate species? Why would not the long-established concept of “subspecies” be more appropriate in this context, (or even a term implying even less differentiation, such as “race”)?
I suspect he means the Toba Catastrophe, but if I could metahijack, I’m curious why he thinks a population that rebounded to >7,000,000,000 hasn’t “fully recovered”.
I would venture a guess that what he means is that while we have very much recovered in numbers our genetic diversity is unusually small for such a numerous species.
Right. If we stay numerous for long enough, then we’ll gradually get our diversity back, through mutations, and the more numerous we are the quicker that’ll happen.
Is the issue of interbreeding with Neanderthals definitively settled? As recently as 5 to 10 years ago, I remember seeing articles on New Scientist within a week or two of each other, one saying we definitely interbred, and one saying it was ruled out.
I would summarize it for you but it is already a good summary of the current research and also a good start to understanding this question. It contains a lot of detail plus the things that are still unknown.
The main reason that Neanderthals and homo sapiens are considered separate species rather than subspecies is because their last common ancestor was about 700,000 years ago and they were separated into very different ranges for most of that time. They had different morphological traits (high forehead and larger brain for Neanderthals) and are presumed to have had different behavioral traits. It is unknown if Neanderthals could or did use vocal language for example. That is plenty of differentiation to be considered different species rather than subspecies even if they could interbreed when they met back up again after homo sapiens migrated into Europe and Asia. Much less is known about the Desmovians or the Hobbit people but they also have DNA sequences that are sufficiently differentiated from homo sapiens to be different species of humans as well.
“Neanderthal DNA:” what is it (“good for/do”)? Besides being DNA sequences we don’t have (right?) and which matches a certain consistent phenotype?
ETA: I guess that question is self-answered…
Again, it’s not a matter of whether they could interbreed or even if they did interbreed. Wolves and coyotes do both. It’s a matter of whether there was significant interbreeding.
Let’s just consider H. sapiens. They (or we) existed as a geographically isolated species for about 150K years. Then, for a few thousand years there appears to be some very limited interbreeding with other geographically dispersed populations. A short period of time after making contact with us, those populations went extinct. Or, in the case of H. floresiensis, maybe no contact was made at all.
Since there is no evidence that the different populations regularly interbred, forming a truly hybrid population, then the species designation seems more appropriate. A subspecies is generally prevented from interbreeding only by some physical barrier, and it is expected that once that barrier is broken, the populations would merge. That does not appear to have happened in our case.
n.b.: I’m not claiming that the issue is black and white. There have been lumpers around for decades wanted to call all these species one. But most of them based that opinion on the Multi-regional Origin hypothesis, which does not seem to be an accurate description of what happened.
What are typically termed “anatomically modern humans” showed up somewhere around 150-200kya. Subpopulations left africa, and upon arriving in what we now call eurasia, found archaic (non-anatomically-modern) human lineages who had made it out of africa much earlier (and were genetically separated by say, at least 700kya). Of these, the Neandertal and Denisovans are the most popular nicknames for some of the populations, and I would say based on the research of Svante Paabo and others, it’s reasonably well-accepted that modern eurasian lineages (but not modern african lineages) contain a few percent of genes from those archaic lineages. (Africans may have other admixtures with other archaic populations.) And in very recent times you have more gene flow across populations so that the geneset for an african american becomes even more complicated: their african source pool, plus eurasian genes plus neandertal genes riding in with the rest of the eurasian ones.
Mixing is nowhere near homogeneous for large groups, but I don’t think there’s enough of a gene representation within any given population–even source populations–to suggest speciation is a reasonable designation. The strongest argument you could make for sub-speciation, given how clinal we tend to be, would be to take a fairly ancient lineage (some !Kung L0d’s, maybe?) and compare them with some other group that’s been branched off for tens of thousands of years. It’s likely each of us is more similar to any other human on earth than any of us to neandertal or denisovan simply because the time separations are so much smaller. Anyone alive today can’t be more than 150k years–and probably quite a bit less–separated from anyone else.
Neandertal mixture does add an interesting twist because those who have neandertal genes do have genes with a lot more separation–maybe a half million years more. And actually, 2 or 3% is not a trivial amount. With genes, just one change in one gene can drive a huge difference in the kinds of outcomes that drive speciation. But we are pretty clueless right now about how practically significant any neandertal/denisonvan/other genes are in driving outcome differences. The first goal is just to identify the genes.
I might recommend these comments from John Hawks if you find the topic interesting.
According to current interpretations of the Biological Species Concept, hybridization that is restricted in space and/or time does not preclude two populations from being considered separate species. In the case of H. sapiens and Neanderthals, hybridization appears to have been limited to a narrow range of time in the Middle East. Where the two populations occurred over a larger area over a longer time in Europe they don’t seem to have interbred very much if at all. They never hybridized wholesale to form an entirely mixed population. For these reasons the two may be considered different species even though they did interbreed.
Some minor corrections. The 700k year figure is a maximum. More likely it was something like 500k years ago. The “high forehead” trait is for us, not Neanderthals-- they had rather low foreheads compared to us. There are lots of other morphological differences, but more importantly is that it appeared Neanderthals reached sexual maturity a bit sooner than Modern Humans do.
My mistakes especially on the forehead thing. I just know that Neanderthals had funny looking foreheads compared to modern people. I will have to learn the difference between the high and low terms for that. I think most people have an idea of what Neanderthals looked like however in general terms.
This is complete conjecture on my part but the Out of Africa hypothesis always seemed completely unsatisfactory in many ways to me at least (it is the older theory that says that all modern humans came from bands that migrated out of Africa just tens of thousands of years ago and never interbred with any other human species in the new territory; they just displaced them completely or killed them off).
The biggest concerns I had were how Caucasians and Asians could physically evolve morphological traits that were so different from their African ancestors in just a few tens of thousands of years. Those include hair, eye and skin color as well as differences in body shapes. Compare a stereotypical Swede to members of any African tribe to see the greatest range of those types of differences. That is just the blink of an eye in evolutionary terms and it doesn’t sound like enough time for them to evolve through genetic mutations alone because there weren’t enough generations to do it or at least that is my educated layman’s opinion.
Cross-breeding bands of Africans that moved out of Africa and into Europe with existing human species in those ranges could introduce new genes into those populations so that traits like white skin and blue eyes could be selected for even fairly quickly. I have never seen a depiction of a Neanderthal that looked anything less than like a brutish version of a white person. That makes sense because they lived in Europe, the Middle East and Asia for the hundreds of thousands of years needed for those traits to evolve on their own.
I don’t know if I am right or wrong on that guess but it makes more sense than the previous theory and we have the genetic tools starting to become available that can sort it out over the next few decades.
The Multi-regional Hypothesis is older than the Out of Africa hypothesis. That former hypothesis, as it was originally stated, has been pretty much thoroughly discredited by modern genetic analysis. That hypothesis said that Modern Humans were the descendants of whatever local populations existed in Europe, Africa and Asia a million years ago or so, and that there was continuous, though low frequency, gene flow between the continents. IOW, Europeans would be mostly Neanderthals, genetically. We know that isn’t true.
The OoA hypothesis does a better job predicting what the data actually tells us. There is no reason that lighter skin could not evolve in small populations over 10s of thousands of years. In fact, it appears to have done so twice, since the East Asian genetic mutations for lighter skin is different from the European one(s).