I thought this was an interesting article. Summarizing from my very-non-expert POV, he's dividing the world into three groups: a small number (less than 1 million) original African inhabitants; a larger number (about a billion) sub-Saharan Africans who broke off about 100,000 years ago; and then the rest-of-the-world (who went though a tight bottleneck less than 100,000 years ago) about the time they left Africa.
The surprising part is that measured genetic diversity within the small group of original Africans is greater than that of rest-of-the-world. Counter to intuition that skin tone is a strong indicator of relatedness, a Swede and an Andaman Islander probably have more common genetics than two members of different tribes who both live in the Kalahari desert in Namibia.
I'm confused though by his comments about "our species’ one-time amazing levels of genetic diversity." Overall, he seems to be espousing the idea that "diversity" is something we once had lots of, but have lost over time. Is this true? Did the the different tribes of African Bushmen start out as genetically distinct as they are today, or did they start from a common population and diverge over the last 200,000 years? Were those who left Africa always distinct from those who stayed, or did they change through selection (and interbreeding with Neanderthals and Denisovans) over time?
As a general rule, species start out with fairly high levels of diversity from natural processes of evolution. What happened with the out-of-Africa population is what's known as the founder effect. A small population leaves and becomes reproductively isolated. Whatever diversity can be found in their descendents is largely limited to the diversity that was present in the founding population.
One thing that should be made very clear is that ancient population structures generally don't map neatly to modern cultures or languages. It's best to treat modern terms as labels of convenience.
Do we have any idea about the size of the original population, the size of the group which left for Europe, the size of the group's who migrated to central/west Africa?
I had heard that there were several waves of migration towards Europe, do we have a better estimate about this? Wouldn't multiple waves over time provide more diversity?
you can infer sizes from genetic diversity ('effective pop')
for the out of africa group i've seen sizes from 100 to 10,000. i used 1,000 to 10,000 as a likely moderate number, and the range also points to the census size being larger
we can assume that conditional on the size of OoA the other groups were way larger (10x or more). otoh, the main thing i did not explore is that i am now believing that africans themselves especially agriculturalists are 'compounds' of various bottlenecked groups (why west africans are closer to eurasians than they are to khoisan)
It's worth noting that effective population size != actual population size. The former is simply a lower bound on it. If the population is structured enough and in the right ways, the actual size can be orders of magnitude higher. This has been an issue with several supposed bottlenecks within African populations.
I'm not sure that's super applicable to humans. Doesn't it also assume panmixia and non-overlapping generations?
Either way, my understanding is that all indications point to middle and late Pleistocene populations being highly structured and "unusual" as far as standard assumptions go.
> I'm confused though by his comments about "our species’ one-time amazing levels of genetic diversity." Overall, he seems to be espousing the idea that "diversity" is something we once had lots of, but have lost over time. Is this true?
"Diversity" can mean a lot of things. In this case, he's referring to the variety of individual alleles. Due to population bottlenecks, most of humanity has reduced variation: for example, at a certain spot in the genome everyone has a T, because everyone who survived the bottleneck happened to have a T there; but in the population that didn't go through the bottleneck, some people have a G instead.
On the other hand, for phenotypical diversity it's not just the set of individual alleles that matters, but their distribution and correlation. For example, let's say height is affected by thousands of genes, for each of which there is an allele that makes you slightly taller and one that makes you slightly shorter, and the combined effect of all these genes is approximately additive.
Then you may have a population A which has both variants for each gene, but they're independently distributed, so everyone is of medium height, with some variance. We also have a population B, which has only one variant left for several of these genes (they have lost diversity!). But B is split into two sub-populations, which differ in the distribution of the remaining height genes: B1 has more tall alleles, and B2 has more short alleles.
Now the average height of B1 is higher than that of A, and the average height of B2 is lower than that of A. You will find very tall people in B1 that you would never see in A, and very short people in B2 that you would also never see in A. So B has more diverse height phenotypes, even though B1 and B2 (even put together!) still have lower genetic diversity than A in terms of the available repertoire of alleles.
> Did the the different tribes of African Bushmen start out as genetically distinct as they are today, or did they start from a common population and diverge over the last 200,000 years?
While groups like the San have a great deal of diversity within the group, they are also distinct from other Hunter Gatherer groups as well. The San are separated from the Hadza by about 110K years, IIRC.
> Were those who left Africa always distinct from those who stayed, or did they change through selection (and interbreeding with Neanderthals and Denisovans) over time?
Yes, those who left Africa were the remnants of a distinct population, separated from the ancestors of sub-Saharan Africans by 40K years when they left Africa 60Kya.
Note that intermixing between those whom we call 'Sub-Saharan' Africans and the "rest of the world" is definitely not new or unique to the 'Western Hemisphere' as OP seems to assume: we know very clearly from Classical written sources and iconography that there were many peoples of 'Sub-Saharan' appearance well north of the Sahara desert, in close contact with Mediterranean and Levantine (thus, clearly "Out of Africa") humans. Even more than this, we know of such 'Sub-Saharan' folks who were very deeply integrated in ancient Mediterranean and Levantine societies, and while these folks were clearly a tiny minority and their distinctive appearance was often remarked upon, this did not hurt their social standing in any sense; there was even a well-known Egyptian dynasty of "Black" pharaohs.
> we know very clearly from Classical written sources and iconography that there were many peoples of 'Sub-Saharan' appearance well north of the Sahara desert, in close contact with Mediterranean and Levantine (thus, clearly "Out of Africa") humans.
We also know they had negligible genetic impact.
> while these folks were clearly a tiny minority and their distinctive appearance was often remarked upon, this did not hurt their social standing in any sense; there was even a well-known Egyptian dynasty of "Black" pharaohs.
You say that like social standing determines who gets to be king. The Nubian pharaohs got their position in the standard way, by conquering Egypt. They weren't native, they took very little interest in their new territory, and they didn't last very long.
There was a well-known Chinese dynasty of Mongol emperors. What do you think the social standing of Mongols in China was before and after that happened?
For me, it's easier to understand these ideas by adopting a "lumper" mentality to human history. IE, Neanderthals, Denisovans, late Erectus & other archaics were just populations of humans. Sections on a large map of human diversity. On another section were, for example, that small subpopulation accounting for most modern human genetics. Near that little section were other subpopulations that even a "splitter" would call Sapiens. Just because they're Sapiens and we're Sapiens doesn't mean that they left more genetic legacy than Neanderthal subpopulations.
I do have some questions though...
"If you don’t have Sub-Saharan African ancestry, all of your ancestry (aside from your differential rates of Neanderthal/Denisovan ancestry) goes back to the same 1,000-10,000 humans that split off from other Africans."
No introgression with other Sapien populations? We know that Sapiens existed outside of Sub-Saharan Africa long before that bottleneck. Are all additions to that genetic stock from other species?
Also, what's the relationship between genetically isolated and genetically diverse? Most of our ancestors went through single a bottleneck 60kya-120kya, with non africans going through at least one more. Theirs did not go through that bottleneck or a seperate one. Coincidence, or is it part of the story somehow?
Perhaps indirectly. As he addresses later in the text, it's possible that Denisovans in South East Asia encountered Homo erectus and interbred with them. The Denisovans appear to have more population structure as compared to Neanderthals, and introgression from H. erectus might explain some of it. Considering that South East Asia and Papua New Guinea has the highest Denisovan introgression in modern humans, it's possible that some of that H. erectus ancestry ended up in those of us from East of the Indus.
For what it's worth, I have seen other studies pop up from time to time that claim to have found introgression from some unknown ghost sapien population, but these outliers usually go away by tweaking the parameters.
Interesting. What I meant though was introgression with closer groups. IE there clearly were surviving populations that didn't go through the major sapien funnels. Their descendants are in the "very diverse" group. Obviously there was, is and will be "introgression" with them.
Were there other such groups, in or out of sub-saharan africa... whos genes we carry?
I'm not sure what you mean by the other closer groups. My understanding of the field is that when anatomically modern humans expanded out of Africa for the first time 60Kya, they encountered Neanderthals and eventually Denisovans. What other "closer" group was out there for them to encounter?
The other anatomically modern humans were all in Africa, and it looks like there's been no interbreeding with them for at least 100K years. There has been a small introgression back into Africa by non-Africans within the last few thousand years, but that's a separate issue.
Other Sapiens. People whos descendants would have been group 3, if a descendant population had survived to modern times.
90% of the DNA for groups 2 & 3 comes from those 1000-10,000 individuals. A small smattering from neanderthals denisovans and ghosts. What's the rest?
There were anatomically modern humans outside of sub-saharan africa at some point. They just don't have descendants. But, my question applies either way. What matters is that they descend from groups outside of the funnel group, not where they lived.
> 90% of the DNA for groups 2 & 3 comes from those 1000-10,000 individuals. A small smattering from neanderthals denisovans and ghosts. What's the rest?
That's not my understanding of the text. Other than the 2-5% Neanderthal & Denisovan contributions, the rest of our (ie, for non-Africans) ancestry arises entirely from this single group of 1-10k, not 90%. Perhaps you're confusing it with this statement?
> ... only 1,000-10,000 lucky humans made it through the impossibly narrow neck. Our best (always provisional) models suggest they dwindled from an earlier population maybe 10 times bigger. So this round of cuts diminished the breeding pool by 90%.
I followed you on Twitter for several years and also heard your interview on the New Liberals podcast. Would love to see you around here more often as well.
I subscribe to Razib’s substack. His writing style is a little weird and scattered, but he writes a lot about how this area of science is rapidly changing, and has good coverage of new hominid fossil discoveries.
I've been reading Razib for close to 20 years, and he's never been a great, or even a very good writer. He more than makes up for it with how prolific he is, and I appreciate how frank he is with his theories.
Yep, my take was that the content of this article is brilliant but there is something annoying about the writing at the paragraph level that needs work.
I'm curious if this genetic diversity is just something that shows up as a number on a computer, or does it translate into something phenotypically* visible?
Also, how much of, say, our medical/biological knowledge actually only applies to the descendants of the bottleneck mentioned in the post? I understand the "very diverse" in the article have better things to do than be subjected to researchers, but reading the article I can't help but be curious what this genetic diversity means in practice?
On an extremely tangential note: I wish we would stop using the term, "Sub-Saharan." The semantic quandaries should be obvious, but it also subscribes to a curious linguistic glitch of putting the North and South into a hierarchy that only exists if you are rather incurious about the ambiguity of geographic perspective (see: the infamous "upside down" Gall-Peters projection for some mind-blowey fun). I would personally prefer something like "SOS (South-of-Sahara) Africa," or perhaps even something as provocative as "Africa Major" (riffing on Asia Minor and referring to the fact that that part of the continent is, in fact, a majority of its landmass).
There is no top of the map since there is no objective frame of reference in space. The axis on which the Earth spins relative to its orbit is a point of orientation but which side is the "top" is arbitrary.
Sure, but at the end of the day you've got to pick a frame of reference and people have settled on North being the 'top'. You can draw a map that ignores this convention and still have it be valid but you're going to confuse a lot of people.
For what it's worth there's more landmass and population in the Northern Hemisphere so make of that what you will.
Orienting with terms like "top" and "bottom" inherently denotes hierarchy. Whether you ascribe meaning to that hierarchy (and I think people do) is another matter.
Explicitly stating direction rather than something that only refers to it with the aid of outside knowledge (the common orientation of maps) would be better. Otherwise, why don't we spin the map 180 degrees? Now the "correct" term would be "Super-Saharan Africa." Which is just as silly.
People seem really upset at the notion. Even you can't seem to engage with it as anything but a farce. I have to wonder why.
I'm just going to start calling it Super-Saharan Africa. When people point out the issue on their map, I'll just flip it 180°. Google Maps conveniently reorients text.
The real fun begins when you decide to make longitude the horizontal aspect. Is it Dexterous Africa or Sinister Africa?
>"Even you can't seem to engage with it as anything but a farce. I have to wonder why."
No need to wonder, it's because I see it as a farce. The idea that the top / bottom aspect is something that needs to be re-evaluated because it implies hierarchy is something so inconsequential that I just can't bring myself to take it seriously. And, if you really wanted to flip the hierarchy then you've got the same 'problem' just in reverse.
I know you really want to change the way people think but adopting the term "Super-Saharan Africa" is just going to make you look strange. I guarantee you no one will think "oh, this person means the southern part of Africa, but from the reference point of the equator to the South Pole." They're going to think "Wait what? A bigger Sahara? Desertification? What?"
"Super-Saharan Africa" was a joke, meant to draw attention to the farce of this associative naming convention in general. Hence "Left Africa" and "Right Africa". :)
Now, I'll ask you to take it seriously. If it's not a matter of concern for you, but it is for me, it would seem most fair to address my concern (since it's apparenty beneath your care). But if you do care, I'm happy to have that conversation. As stated, I would like to remove hierarchy (or the connotation of hierarchy) from the terminology altogether, and more explicitly refer to directionality. If you disagree with this course, please say why (beyond, "It shouldn't matter," because at this point we'd have agreed that it does).
I'm going to be honest and say that I'm reluctant to get into a debate with you on this. Simply put, I already know we aren't going to change anyone's mind on this subject.
I will say this, it is perfectly fine to worry about hierarchical structures and the impacts they have on people. But the mere existence or implication of hierarchy is not a problem in-and-of itself. Especially in this context, where I am certain the impact of North being considered 'up' and South being considered 'down' has not materially harmed anyone living down under. I also see no evidence that this paradigm/convention is actually harmful. Only speculation that it could be, in an indirect way, and for reasons I don't find compelling.
I'm also concerned that we're going to slip into one of the least productive kinds of argument, trying to define, redefine, and shift the meaning of words. So I'd like to avoid all of this and let bygones be bygones.
>I also see no evidence that this paradigm/convention is actually harmful.
And that's fine. But you're putting a lot of energy into trying to convince me of that.
Do you see the contradiction? You claim that the nomenclature isn't a big deal, but then you go on to defend it - and even in this defense, you make an effort to wash your hands of the entire matter as quickly as possible. It's a weird appeal to the status quo, which I don't understand. Either the terminology matters to you or it doesn't. Does it? Do you prefer to refer to a "Sub-Saharan Africa"? If you do, why? And if not, why can't we just call it something else?
I am really curious. I think this sort of discourse is perfectly reasonable because it unpacks the meanings behind our initial reactions.
> I'm just going to start calling it Super-Saharan Africa. When people point out the issue on their map, I'll just flip it 180°. Google Maps conveniently reorients text.
You'll just confuse people and look like a joker when you explain to them that you've decided to change universally accepted geographic nomenclature on your lonesome because you took a whiff of maybe possibly who knows racism.
> Is it absurd? Do they understand? Can you show me how you know this?
It is absurd, they do understand, and I know it because I'm a native English speaker and I've never been in a conversation where anyone was confused by the term nor even in which someone bothered to clarify that they didn't mean to imply a hierarchy. I'm sure you're going to understand that this is satisfactory evidence and not resort to some sort of "BuT Do YoU kNoW eVeRy SiNgLe PeRsOn On ThE PlAnEt?!" nonsense.
> Sorry, I can see where I was unclear. I was referring to Africa.
Because there's no indication that it causes confusion for anyone except, perhaps, you. There is a lot more ambiguity in English than this and we don't put aside all of our other problems to deal with this. Anyway, these kind of semantic debates are the most boring, dull, tedious things I can think of (see also the people who think we need to replace "latinos" with "latinxs") so I'm going to entertain myself with anything else besides this thread.
I never held that it causes confusion. The duplicity of meaning is probably, in fact, a bonus for some. I don't like that duplicity. I would rather the terminology be unambiguous and direct, if it is possible (and, in this case, it is).
The surprising part is that measured genetic diversity within the small group of original Africans is greater than that of rest-of-the-world. Counter to intuition that skin tone is a strong indicator of relatedness, a Swede and an Andaman Islander probably have more common genetics than two members of different tribes who both live in the Kalahari desert in Namibia.
I'm confused though by his comments about "our species’ one-time amazing levels of genetic diversity." Overall, he seems to be espousing the idea that "diversity" is something we once had lots of, but have lost over time. Is this true? Did the the different tribes of African Bushmen start out as genetically distinct as they are today, or did they start from a common population and diverge over the last 200,000 years? Were those who left Africa always distinct from those who stayed, or did they change through selection (and interbreeding with Neanderthals and Denisovans) over time?