What does your forehead look like?
Frowny.
Ah ha!
The “(sub-saharan) african/non-african dichotomy” occurs because of the evidence that it has largely been a one way population migration, with backflow admixture from out-of-africa populations back into africa via much smaller groups (or even individuals) bringing genes back into areas with already established populations.
In simple terms, the small population(s) that made it out of africa (permanently) about 60K years ago were the ancestral population for the non-african world. For populations traced by mtDNA lines, this is roughly at the L3-N/M haplogroup split. Some N/M descendant populations did make it back to sub-saharan africa. For example, at least one population made it to the central Sahel across the sahara (maybe when the sahara was greener); others to eastern africa and some of the Lemba in southern africa might be a handful of the Jewish diaspora. Plus, of course, there’s been a fair amount of individual gene donation by sailers and traders and the like. But for the most part, there’s a pretty broad divide between the pre-L3 lines and the post L3 lines that diverged from the L3-N/M divergence point.
This is an entirely artificial point to make a split, of course. You could take any divergence point you want and split descendant populations into two groups. “Africa” and “non-africa” is simply one of many dividing points.
However, from a genes standpoint, it is an actual split. N-M descendant lines are going to have gene prevalences quite different from african lines for any genes developed right around the time of the split. This is because a positively selected gene can penetrate a descendant population much more readily than it can penetrate established populations by geneflow spread across geographic divides (especially when those divides were less cross-able in the past). Migrating populations often stay relatively isolated for cultural or geographic reasons, and although the species at large tends to be clinal by virtue of geographically proximate populations admixing more than geographically separated populations, you don’t get anything close to admixture homogeneity for genes across populations separated by language and culture.
Back to the africa/non-africa divide: Suppose that an MCPH1 gene variant arises in an early L3 population that has left africa, and suppose that it has a significant positive selection pressure. You’d see such a gene variant widely distributed in N-M descendant populations, because they get it free from their moms (I’m using mtDNA groups, all the same idea is true of Y-DNA groups). You wouldn’t see it widely distributed in sub-saharan populations because the back-to-africa diaspora is relatively limited and even where it occurs, the gene pools tend to stay in the migrating populations since migrating populations don’t necessarily hook up with established local populations. (Again, as an example, the Lembas hook up with the Lembas, so their gene pools don’t get very admixed with other tribes).
At some level, we’re all mutts. We all have bits and pieces from all over. But they are bits and pieces superimposed upon gene pools established by migration patterns and evolutionary history. The haplogroup D MCPH1 variant has been found in 70% of non-african populations and not in african populations. These numbers will change as more groups get their genes parsed out, but it won’t ever be the case that we all have the same genes, and one big divide is africa/non-africa because of those migration patterns.
ywtf: Take a look at this article, which is a blog by one of the scientists working on the 1,000 Genome project. I think it’ll answer you question about “bright lines” because it shows the data as histograms. It’s not that there aren’t any Neanderthal genes in Africans, but the frequency is significantly lower.
This whole field is just in its infancy, so expect lots of updates and corrections as more data comes out.
Thanks John. I found the visuals most helpful.
Yeah, that’s probably the best article I’ve seen on this subject. I poked around on his blog, and he’s got some interesting stuff there. I bookmarked that for definite review every couple weeks.
Me too. Thanks John
It seems like they’re using the average African number of alleles as the “baseline” and then saying that any excess similarity with Neanderthals is due to interbreeding by early humans and Neanderthals.
I can see why they do that, obviously all humans are going to have a huge overlap with Neanderthals just because both species are in the same genus. So you need to separate out that from the effects of interbreeding. And since no Neanderthal remains have been found in Africa, there’s a good argument there was no interbreeding there.
But it also kinda presumes what the OP says was “discovered”, that Africans have no genetic input from Neanderthals. Since Africans are used as the baseline, the average African is going to have zero Neanderthal input by definition. Any effects from back-migration will be invisible.
I see what you’re saying, but I think we must be missing something because that would be too much of a silly mistake to make. Keep in mind that we’re jumping into the middle of a discussion on that blog, and there may be more detail in earlier pages. I’m guessing that the African “baseline” had already been established and they are now using it to differentiate between different non-African populations.
I’m not sure its really a “mistake” so much as just a limitation of the technique that has been glossed over in popular press accounts (as is often the case). The end of the article you linked to actually starts going into differences in “Neaderthalness” in between different African populations. So its not like scientists are ignoring the issue.
The same blogger mentions a somewhat similar problem in that post and at greater lenght here. The technique used can’t really distinguish between Neanderthal+human breeding vs the possibility that the original “out of Africa” population was just carrying more DNA from our shared Neanderthal human ancestor due to a quirk in the geography of ancient African populations. And there’s at least some reason to favour the latter hypothesis.
That latter article also mentions other methods that may distinguish between the two possibilities, so again, its not like its really a “mistake” in that its a possibility that’s been overlooked or ignored. Its just an alternative hypothesis that hasn’t been ruled out yet.
You say “remember” like I already knew that. 
This is hardly my field, and some of the posts in this thread make my head hurt.
You say “remember” like I already knew that. This is hardly my field, and some of the posts in this thread make my head hurt.
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Well, you already knew you had 2 parents, right? So you knew about having 4 grandparents, and 8 great-grandparents; 16, 32, 64, 128, 256, hut, hut, hike.
I am still plowing through John Hawks’s previous posts.
But I think the “baseline,” so to speak, reflects archaic genes from an original common ancestor to all anatomically modern humans and Neandertals, and now the increase in Neandertal genes for non-africans therefore represents recent (w/in 40K years ago or so, if I remember the data correctly from other sources) introgression of Neandertal genes from cross-breeding of anatomically modern humans w/ Neandertals.
Neat blog, by the way. Thanks for the link.
See his discussion here.
“David Reich and colleagues came up with an appealingly simple test of introgression, which they applied to both the Neandertal and Denisovan genomes. Eric Durand, Reich, Nick Patterson and Monty Slatkin described the method formally this year [8], which they call the D-statistic. Informally, this has become known as the ABBA-BABA test, after their labels for the discordant genealogies that the test compares. By and large, across the genome, humans living today share many more new mutations with each other than they do with an archaic human like a Neandertal. But sometimes two genomes are different from each other, and one of them shares a new mutation with the Neandertal.
A human might share a mutation with a Neandertal because it actually isn’t very new, and both inherited the mutation from some much more ancient population of humans. This scenario is called “incomplete lineage sorting”, because humans today have multiple gene lineages that existed within some very ancient population, instead of these having been “sorted” cleanly into the different human and Neandertal populations. Incomplete lineage sorting does happen a lot between humans, Neandertals, and Denisovans. ILS is the normal mode of variation among recent human populations, who trace their genealogical histories back much further than the earliest “modern” humans. So if one human has a Neandertal allele, and another human has a different allele, it’s probably no big deal. They both just inherited gene variants that already existed in our distant common ancestors.”
You have that many roles or positions in your ancestry tree, but not that many individual ancestors. After a certain point it becomes inevitable that a number of the same people are occupying multiple slots.
Let’s have Heather Locklear explain it. And if she starts talking politics, turn your ears off and just look at her.
Heck; if siblings procreate, the pedigree collapses at the grandparent level. But I like the idea of it expanding again back when grandma hooked up with Neandertals and Denisovians.
We know that there were differences between Neanderthals and us on the biochemical level. It is possible that there were differences in the uterine environment which meant that a fetus could only survive in one of the species.
That didn’t even occur to me.
Possible, but from what we know about inter-species fertility and even enter-Genus fertility, I think it unlikely.
There are no living mammal populations with as recent common origins that are not inter-fertile, even when you ignore the unusually long generational span of humans. For instance, our closest extant relatives, chimps and bonobos, diverged 2M years ago, but are inter-fertile. That’s about 4x the time from the Modern/Neanderthal split.