Mon
Apr 22 2013 2:00pm

Homo Sapiens Didn’t Evolve in a Vacuum: Scientific American’s Human Hybrids

Scientific American Human HybridsMy fondness for Neanderthals is documented, and one thing that has always been on the teeter-totter of scientific opinion was the mechanism of their extinction. Did they just get out-competed by Homo sapiens? Were they murdered, driven to extinction? Did they just die out, unrelatedly? Or were they absorbed, swallowed up through inter-breeding? For a while it seemed that there might not have been any gene-mingling—which seems odd, given humanity’s fairly active sexual appetite—which sort of stood against the notion of hybridization. Since the completion of the Neanderthal Genome Project in 2010, that tide has shifted, and Scientific American’s May issue has the most recent broadside in that volley, in an article by Michael F. Hammer called “Human Hybrids.”

I think it is smart to announce biases up front, and in this case in particular, it bears mentioning. The Out of Africa model’s ascendency over Multiregionalism—the idea that humans evolved in pockets, then intermingled, as opposed to Out of Africa’s single origin—was how I learned things in the first place, back in college. Recently, Out of Africa’s champion, Christopher Stringer, published Lone Survivors, which is the other thread in the genome of my bias, if you’ll forgive such an outrageous metaphor.

In Lone Survivors, Stringer talks about modifications to the Out of Africa model, so I was already fairly up to date on the armchair scholarship front for my read of the Scientific American article. Specifically, regarding the African Replacement model, which has been the standard for the past few decade: the idea that Homo sapiens evolved in Africa, while other Homo breeds like Neanderthal were already extant in Europe. Humans left Africa, then…well, killed or out-competed the other hominins.

Scientific American Human Hybrids

In contrast, voices like Günter Bräuer and Erik Trinkaus have argued for a more flexible version of this theory, the Hybridization model, which is largely and broadly sort of a version of the Out of Africa model that incorporates the possibilities of hybrids—of the archaic Homo sapiens leaving Africa and not just “replacing” the existing populations but also, well…mating with them, at least a little. The question then becomes—did Homo neanderthalis just breed into modern “Humans” completely, disappearing—the Assimilation model—or was the gene flow more sporadic, only occasionally bringing new traits into the sapiens pool?

For a while, the conversation was largely being held in an echo chamber; with fragmentary archeological evidence, it was hard to make a convincing argument, though it managed to be enough to put Multiregionalism into decline. Increasingly, however, genetic analysis has been transforming the debate. The first volley was from mitochondrial DNA—you know all about the tiny little guys swimming in our cells, I’m sure—and from the start, it was a mixed bag. When UC Berkeley’s “Mitochondrial Eve” analysis came out in ’87, it confirmed the Replacement model; there was a clear line back into Africa, across the board. It took another decade for the Max Planck Institute to put out its findings that there didn’t appear to be any interbreeding between sapiens and neanderthalis.

Ah, but then things started changing as the picture got crisper, as more information (not just mtDNA) started flooding in. Different bits and pieces of human DNA started showing outliers, pieces of the genome that seemed to come from Asia rather than Africa, with an age dating to before the Homo sapiens exodus from Africa. From there, the evidence took a decided turn toward the models with at least some elements of hybrization. Svante Pääbo at the Planck Institute’s Neanderthal Genome Project estimated that a full 1% to 4% of people of non-African descent’s genome is Neanderthal.

Scientific American Human Hybrids

Upsetting the primacy of the Replacement model even further was the fact that people of Oceanic descent—Aboriginal Australian, Melanesian, Polynesian, etc—shared 1% to 6% of their genome with Denisovians, while people of Eurasian and African decent did not. What exactly the Denisovians are is a bit of a mystery, for the moment—the evidence points to them as another species of human, like sapiens or neanderthalis, to my eyes, though there are voices that posit them as a sub-species or a break-away hybrid group—but the fact that humans of various species interbred with each other in a meaningful way is now becoming the consensus.

Michael Hammer, the author of the Scientific American article, with his team at the University of Arizona and Jeffrey D. Wall at UC San Francisco, dug deeper into the genetic soup, focusing on Africa. Though Neanderthal (and Denisovian) is the popular focus, Africa would have provided many opportunities for mating between groups, both on the basis of Africa as the home of the most hominin diversity and the longest periods of species existing along side each other. They concluded—based on computer analysis of three populations—that 2% of those groups’ genome came from interspecies sources. A mystery regarding a South Carolina man of African descent’s Y chromosome and fossils at Iwo Eleru in Nigeria and Ishango in the Congo (where the famous primitive lunar calendar is from) offer further evidence for admixture.

Did this mingling provide evolutionary advantages? Analysis of a few key regions seems to suggest that it does; some of the inherited sequences relate directly to the immune system, for instance, and the evidence seems to suggest that they spread at a higher rate because of pressures from natural selection. The picture painted is a complicated one: it supports the Out of Africa model…but. It appears that Homo sapiens didn’t evolve in a vacuum; rather, relatively isolated but significant interspecies breeding happened. Hammer suggests that the African Multiregional Evolution model, in which hominins of various species shared DNA as humans moved from archaic to modern and then tapered off, in combination with Hybridization. What the complete picture is, however, is elusive, but at least our understanding of it is deepening.


Mordicai Knode always conflates Neanderthals and Orcs, so he’s just happy to hear about the plausibility of Half-Orcs from respected scientific sources. You can find Mordicai on Twitter and Tumblr.

36 comments
Eugene R.
1. Eugene R.
I have been intrigued by the Hybridization hypothesis ever since reading Bjorn Kurten's Dance of the Tiger in 1980. So, of course, when early evidence turned up, like the Lapedo child skeleton from Lagar Velhos in Portugal that includes a cranium with sapien/neanderthalensis features, I was inspired enough to include a contemporary human/Neanderthal character in a role-playing scenario. Of course, the troop of surviving Neanderthals in central Asia that she met were completely fictionalized. But they did talk quite courteously.
William Carter
2. wcarter
One thing I'm curious about is whether interbreeding influenced live-birth/infant mortality rates or fertility.

Obviously not all offspring resulting from interspecies breeding today are viable (or in most cases fertile).

So presuming interbreeding did happen at least occcasionally, and that groups codeveloped in isolation, the question for me becomes: which groups were genetically close enough that their children could have children themselves?
Eugene R.
3. Shava Nerad
The whole notion of the "mule" in terms of non-fertile interspecies offspring is often misunderstood by laypeople. Science is sloppy in defining species taxonomies, historically. Without genome maps and genomic literacy, we have mostly judged what species boundaries were by phenotype (what animals look alike) and what we observe of their breeding patterns.

So, sometimes we have been spectacularly wrong.

Many animals have "plastic" genes, that readily adapt to breeding and interbreeding, and so can create a wide variance within a single species. We've used this characteristic to breed canines from chihuahuas to mastiffs, for example -- all within the same species. Recent experiments in Russia with foxes show they may have the same plasticity and potential for domestication.

We don't know how differentiated these species of hominids are just because our naming taxonomy calls them species. There is, I understand, less genetic drift between a homo sapiens and a bonobo than between a horse and a donkey (perhaps we shouldn't ponder that too deeply ;) so two sapient hominids would probably be even closer. They might exhibit more hybrid vigor than mules.
Mordicai Knode
4. mordicai
1. Eugene R.

I am glad to hear that that connection between Neanderthals & RPG "races" isn't just in my own head. "Wait, there actually were non-human people? Who like...met humans? So like, elves & orcs...but for real?"

2. wcarter

I assume you are talking about things like the human/neanderthal equivilent of "mules" or like, the various other kinds of hybrids? The whole "male/female, female/male" dichotomy of husbandry has always interested me; I seem to remember reading something about it in regards to Neanderthals recently, but I can't recall if it was fiction, science, or just speculation.
William Carter
5. wcarter
@Mordicai

I was thinking " proto-people mules." Post JordanCon fatigue probably kept me from being very coherent yesterday.

I'm sure science will be able to paint us a clear(er) picture on that at some point. For all I know there are already some decent studies on it and I just don't know where to look.
Mordicai Knode
6. mordicai
5. wcarter

If I come across the book/article/webpage where I saw the discussion, I'll pass it along. I've been wracking my brains-- was it Wikipedia? It might be this article on Neanderthals, in reference to the lack of shared mitochondrial DNA.
While modern humans share some nuclear DNA with the extinct Neanderthals, the two species do not share any mitochondrial DNA,which in primates is always maternally transmitted. This observation has prompted the hypothesis that whereas female humans interbreeding with male Neanderthals were able to generate fertile offspring, the progeny of female Neanderthals who mated with male humans were either rare, absent or sterile. However, some researchers have argued that there is evidence of possible interbreeding between female Neanderthals and male modern humans.
(Also I originally wrote "DND" instead of "DNA" & had to edit it. Nerrrrrd.)
Eugene R.
7. The Grey Drape
@6. mordicai

Female neandertals mating with male sapiens? Exclusively? That sounds like the female neandertals were trophies of raids on neandertal bands.

I've long considered genus homo to be a sort of cline rather than a group of genetic "islands"; I wrote that sort of thing into a group of stories about a cline of "original" humans and Genetically Engineered and Modified (herein GE/Med humans) humans set in Alpha Centauri (abducted by aliens, of course: I couldn't let that bit of modern mythopoesis pass me by! :). In this story and others of the same ilk as yet unpublished.

I mean, genus homo has only been around for about 3 or 4 million years, and ardi/australopithecines for up to 8 million years. Homo erectus and ergaster have been around for about 1.5 million of those years and they constitute the source tree from which the rest of homo branches sprang.

So I don't find it inconceivable that hybrids were conceivable, and fertile. I also don't expect that "to every neandertal a sapiens spouse" ... I expect I could with time find a neandertal or ergastine or erectine or denisovan girl attractive; but it wouldn't be likely as finding a sapiens girl attractive. Of course, all bets are off with the appropriate use of beer goggles, but that also applies to sapiens ... :) :(
Mordicai Knode
8. mordicai
7. The Grey Drape

Actually, vice-versa; male Neanderthals & female humans. Speculations aside-- yours are pretty reasonable!-- there is the fact of the mitochondrial DNA. Since you inherit it from your mother, & human mtDNA doesn't appear to cross with Neanderthal mtDNA, while simultaneously there does appear to be crossing over in the "regular" DNA, the evidence as it currently stands seems to support that. Whether it is social or biological, we can only...well, speculate. For now, at least.
Eugene R.
9. Eugene R.
mordicai (@8): You mean to say that when it came to Neandertals ... they wanted our women?? By Leakey! Grade-Z sci-fi films were right!
Mordicai Knode
10. mordicai
9. Eugene R.

Or Human women wanted them!

(or you know, some sort of biological issue resulting in male Humans & female Neanderthals being unable to concieve, or their offspring being infertile.)
Peter Erwin
11. PeterErwin
Mordicai @ 8:

human mtDNA doesn't appear to cross with Neanderthal mtDNA, while simultaneously there does appear to be crossing over in the "regular" DNA,

How would human mtDNA "cross", except in a statistical population sense? Nuclear DNA can "cross", because the mother/father contribution to offspring is about 50/50. But normally all of the mDNA comes from the mother, so if there were some Neanderthal mtDNA in humans, it would presumably be in an all-or-nothing pattern: some people would have pure Neanderthal mtDNA, while the rest would have pure human mtDNA.

Of course, you could still have some female Neanderthal/male human matings with fertile offspring contributing to the human (nuclear) genome, if all the offspring of a mating happened to be male, and thus incapable of contributing their (mother's) mtDNA to any of their offspring.

Or if any mother-to-daughter-to-daughter lines of descent died out.


Oh, and from the original post:

the Max Planck Institute

I'm going to be extra nit-picky and point out that the institute in question is the Max Planck Institute for Evolutionary Anthropology, in Leipzig. Saying "the Max Planck Institute" by itself is a bit like saying "University of California", as though it's a single place. (Except worse, since there are about 80 different Max Planck Institutes...)
Eugene R.
12. Alison Grey
Could our neanderthal mt dna, be mixed up in the x chrosomone of a down syndromed man? Could we might have evolved or mixed mostly in Monglia, Mongolian features being similar and Africa was just a place, a small tribe stayed for a bit.
Mordicai Knode
14. mordicai
12. Alison Grey

This is one of the reasons (among many, many) why racial terminology like "mongoloid" is so confusing. The relationship between the obsolete term mongoloid, in terms of race, & "mongoloid" as refering to Down Syndrome is not a real link; just some old medical terminology from the 1800s, largely due to the fact that one of the symptoms of Down Syndrom is epicanthic folds, & people of Asian descent tend to have epicanthic folds. It doesn't really go deeper than that, which is why it was abandoned (besides both terms being offensive, partially because the link is entirely specious). At least, that is my understanding.
Eugene R.
15. Alison Grey
Sorry, hope I didn't offend. I meant it purely as a geological and descriptive term. Is it just me or does the Deponia cave girl look like a mirror image of the neanderthal man?
Mordicai Knode
16. mordicai
15. Alison Grey

I didn't think you were using the terms as slurs, I just wanted to point out the problematic links & how they could be confusing, for purposes of clarity. Anyhow, I'm not familiar with Deponia; I guess from Googling it that it is a game? If it has a cave girl in it, I wouldn't at all be surprised if the authors used Neanderthals as their inspiration.
Eugene R.
17. Alison Grey
That's denisovain girl, as in the diagram above. I copied and pasted the term, to find out where the denisovain cave is.
Mordicai Knode
18. mordicai
17. Alison Grey

That is funny; there IS a game called Deponia about cave people! I don't know what reconstruction you saw, but I think all we have of the Denisovians is a finger, some teeth, & a toe. Reconstructions are well & good but they are implicitly biased towards whatever hypothesis the artist favors, I would argue, since there isn't that much to go on besides extrapolation.
Eugene R.
19. Alison Grey
I thought they could do amazing things with a tooth and little finger. The author did seem to back the cross breeding idea. How were the land masses connected back then? Was Australia connected to Africa, Were the Polyensia islands a mountain range?
Mordicai Knode
20. mordicai
19. Alison Grey

From a historical scale, the Paleolithic was a long, long time ago. From a geologic scale, it was like the blink of an eye; so the continents were pretty much as you see them today. Now, water levels rose & shrunk, so there were things like land-bridges & such, but Siberia was still...pretty much Siberia, I think, at least in terms of geography.
Mordicai Knode
21. mordicai
11. PeterErwin

Didn't see your comment! I meant "crossed" as your deduced entirely in a population sense. Because it is mother-only & because we don't see the same evidence of cross breeding in Neanderthals, it raises the question of why not. I'm not so much arguing for fertility/infertility as the deciding factor as brainstorming. As you pointed out, extinction is a hell of a thing.

& thanks for the tip about the Max Planck Institute!
Eugene R.
22. Alison Grey
Am I correct in thinking, that if you put land bridge back, put it on a globe Ireland is a short swim away from another neathandal inter breeding section? Chimps being unable to swim. There is geological time and errosion time, which takes us from the sea shore to the edge of the sea in 70 years.
Sanne Jense
23. Cassanne
Maybe it was behavioral, and neanderthal men had a strong tendency to kill other men's offspring - and the hybrids from neanderthal women were very obviously not their children.
While the human women with hybrid children most likely lived in neanderthal groups (is there any evidence pointing either way? were hybrids found in neanderthal or human sites?) and were quite capable of convincing the dominant man that he was the father (and he probably was).
This assumes of course that neanderthal women wouldn't choose to mate with human men, or at least to join their groups.
Mordicai Knode
24. mordicai
22. Alison Grey

I mean, Doggerland-- the landbridge between Europe & the UK, at least during the last Ice Age, I'm not sure if it is always called that-- would connect Ireland to the mainland, where the Neanderthals were. You are thinking about red hair, I'm guessing?

Anyhow, chimpanzees are fully another matter entirely; they are in Africa & unrelated to this conversation, & to this topic of human evolution-- they are a different branch, that branched off below the section where Humans & Neanderthals are.

23. Cassanne

Yeah, possible; I'm leerly of behavioral theories only because then you'd expect exceptions, but then, the population pool of Neanderthals was pretty small, like around 70,000 at the upper edge, I think. Speaking of exceptions, though, I just read an abstract that speculated that uncommon events of interbreeding could explain the Neanderthal mDNA in Humans...
Eugene R.
25. Alison Grey
I mentioned chimps being unable to swim, because that is a profound difference in our 2 branches. What the joke about a Welsh man being an Irish man unable to swim. Any way we are literally born for swimming, that's the one area we are not helpless as babies. The ability to swim would help us create a semi-species of swimming and none swimming apes until they get to the main land. Then pretty soon after we invent the boat or some floatation device which I am guessing pre-dates the wheel.
Mordicai Knode
26. mordicai
25. Alison Grey

Careful not to swallow the attractive but debunked Aquatic Ape quackery. Humans are, if anything, born to walk. They just, you know...have to finish cooking on the outside, first. The human infant is mostly adapted to...fit through a bipedal hip structure, while still having the biggest brain possible. That, & to be cute.
Eugene R.
27. Alison Grey
Why was it debunked? and does having a semi species / proto mule help iron out the problems? I'm thinking Ireland is on the right latitude for both sides of the equation and yet, neathandal never stepped foot in Ireland!? and there's the missing mt Eve chromonsone. I thinking female / male quadrant of the semi species gave their son an x breed with any species card, the feet not quite adapted for swimming at this point. Meanwhile the non swimming ape was making breeding in Ireland and got to Britian, and went to the 3 interbreeding spots, by passing some by boat.
Mordicai Knode
28. mordicai
27. Alison Grey

The problems with the Aquatic Ape Hypothesis are mostly that it is...speculative & unserious with no real evidence to back it up. It is convincing to lay people, but doesn't have any rigor whatsoever. The Wikipedia page can probably educate you better than I can on the subject, though. & I am still not sure what Ireland has to do with the conversation?
Eugene R.
29. Alison Grey
Maybe aquatic is too strong a word, maybe wading ape is a better description, giving us that waddling walk that allows us to have premature babies and their brains to cook. England was covered by trees, allowing an aboral ground dwelling hybrid to cross to Ireland covers that end point top to bottom. I am speculating that there are as many, human species as there are dogs, dogs literally having taken the same journey as us. The reason we can interbreed with each other is we are a mirror image hybrid where a male doesn't find his exact mirror match a female is born or a male with chromosone imbalance I think if you want to find the missing female dna follow it along the path of parents of a boy / girl twin. This probably explains are liking for the exotic. Who knows maybe aliens did diddle with our dna, most of our water comes from space and some drawings of aliens looks amphious looking but I'm being fanciful here.
Mordicai Knode
30. mordicai
29. Alison Grey

The polytypic genome of dogs isn't really the same as what humans have, as attractive as that hypothesis is. Red hair, light skin, epicanthic folds...sure, there is plenty of variation within the species, but the crux of speciation is that...well, dogs are different from wolves & foxes, & Humans are different from Neanderthals & Hobbits. Anyhow, I'd recommend to you The Wolf in the Parlor; I think you'd like it! It is pop science about dogs & humans co-evolving.

On the "big eyed" grey alien thing...I've always been fond of the notion that it is a residual memory that people have of being a baby, before their eyes have focused, when adults just looked like...well, big round undifferentiated faces with dark depressions where the eyes are.
Eugene R.
31. Alison Grey
I will definitely give that book a go. The inter breeding is happening with dingos now, it must of happened with the newly tamed dog, maybe causing a wolf with pups to be kicked out of its pack and man seeing it as natures gift. I am not thinking about the iconical grey but a frog like being I am not sure whether that was a did aliens visit us, but there's this myth from India where a man found out the cause of a ghost child's sickness and the man asked for fire, as eating raw meat twisted their stomachs and she gave him 2 clay pots and an iron bangle which when put on top of wood fire came from the bangle.
Eugene R.
32. Alison Grey
I am still confused about Ireland and not because a diddy bit of sea stops the spread according to the above diagram, but it also stops in Britian where Ireland starts.
Mordicai Knode
33. mordicai
32. Alison Grey

OH! Now I see what you are talking about, regarding Ireland. I think that map is populated by actual dig sites, but I could be mistaken-- maybe we just need to get behind Irish archeologists, see if they can't dig something up, literally!
Eugene R.
34. Alison Grey
I think I've figured out the plague or the lack of tramission to Scotland, Scotland having a population of wild cats. The plague only getting there when a band of Scots decided to visit England.
Eugene R.
35. Alison Grey
p:s I forgot to say, the plague being the biggie, the one that wiped out 2/3 of the population in the 10th century, not the seemingly weaker version in the 16th century. I'm thinking before then the plague hit dozens of generation in Europe. Is there any way we can transmit an e-mail address without it being on this site? I've got an uber long transcription, this conversation playing a large part!
Eugene R.
36. Alison Grey
Actually, forget about the transcription I think I've figured out something that will make me more famous than Edward Jennings! Toxoplasiosm, is an natural immunity to the plague. So the way to deal with lung worm from slugs in man's best friend, isn't to give the dog treatment it's to stop putting slug pellets down and to introduce hedgehogs to an area instead. Oh yes my real name Jess Jackson and I'm from Mexico
Eugene R.
37. Jean Patton
I recently sent in dna samples to find out what regions of Africa my slave ancestors were from, and along with finding out our family ties are Nigerian, I also found out that our family had some Native American and European links. What I found most shocking was some Neanderthal strands swimming around which I assume were from the European bloodline. The European dna linked to Ireland and Germany, so I am assuming this where the Neanderthal strands are coming from. Intriguing.

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