Bantus, Pygmies, and Ghosts

 

dads
Aka Pygmy Father and child, from “Why the Aka Pygmy People of Africa have the ‘best dads in the world‘”.

A paper has just been released on the first ancient DNA recovered from central African burials: Ancient West African Foragers in the Context of African Population History, by Lipson, Ribot, Reich, et al. This is exciting news because our ancient genetic coverage of central Africa has been, until now, completely nonexistent. The local climate tends to degrade human remains quickly, making it difficult to recover DNA, and most genetics researchers don’t live in Africa.

Researchers have recovered the remains of four people, two from about 8,000 years ago and two from 3,000 years ago, buried in Shum Laka, Cameroon. (Cameroon is the country right in the big turn in the curve of Africa’s coast; Shum Laka has been inhabited by humans for about 30,000 years.) The really interesting part is the “ghost population,” which we’ll get to soon.

The burials turned out to be Pygmy people, not Bantus, despite the belief among linguists that Cameroon is the Bantu homeland. From the paper:

One individual carried the deeply divergent Y chromosome haplogroup A00, which today is found almost exclusively in the same region12,13. However, the genome-wide ancestry profiles of all four individuals are most similar to those of present-day hunter-gatherers from western Central Africa, which implies that populations in western Cameroon today—as well as speakers of Bantu languages from across the continent—are not descended substantially from the population represented by these four people. We infer an Africa-wide phylogeny that features widespread admixture and three prominent radiations, including one that gave rise to at least four major lineages deep in the history of modern humans.

And then we hit the paywall. Thankfully, Science Magazine has a summary.

The Bantu language group is a branch of the larger Niger-Congo language family, one of the biggest (along with Indo-European and Sino-Tibetan) language families in the world. Niger-Congo contains about 1,740 languages (depending on how you count) with 700 million speakers. The Bantu branch accounts for half of them, or 350 million people.

The Bantu branch has clearly undergone a massive expansion over the past 3,000 years. 4,000 years ago, central Africa belonged to the Pygmies, Bushmen, and their relations. Today, those populations are tattered remnants of their former empires; the Bantus are dominant. The Bantu expansion is thus one of the great conquering events of recent history, comparable to the Indo-European expansion. The size of the Pygmy and Bushman population has consequently collapsed, though at what speed, we don’t know.

The presence of a significant Pygmy population in the supposed Bantu homeland back when the Bantu speakers were gearing up to conquer a huge chunk of the Earth’s surface indicates that Cameroon might not actually be the Bantu homeland. Of course there are easy fixes to this theory, like “the region just to the west of Cameroon is the Bantu homeland” or “there are still Pygmies in Cameroon today; researchers just happened to find some Pygmies,” but I propose a different possibility: the Bantu homeland is in the Sahara.

megatschad_gis
Lake Megachad and its tributaries in blue; modern day Lake Chad in green

Yes, the Sahara is an enormous desert–today. Before 3,500 BC, the Sahara was significantly wetter. The Niger-Congo speakers started as agriculturalists who farm along the edge of the Sahara. During the African Humid Period, 3,500 years ago and before, much of the Sahara was green, full of plants and animals, flowing rivers and giant lakes. I propose that the Bantu homeland was in the vicinity of lake “Megachad,” which aside from having a great name, was an enormous lake in central Chad, overlapping the borders of Nigeria, Cameroon, and Niger, fed by a suitably extensive network of tributaries. Today, only remnants of the lake remain.

The drying of the Sahara and Lake Megachad turned the Bantu’s homeland to dust; just based on the African topography and modern rivers, they probably headed into northern Cameroon, eastern Nigeria, and the Central African Republic. The area of Cameroon where these pygmy skeletons have been found looks a little harder to get to, cut off from the north/east by highlands. This area may have therefore been a bit of a refugia during the Bantu expansion.

I think it is common for people to think of African populations as relatively homogeneous because it is the origin point from which humanity spread to Asia, Europe, Australia, the Americas, etc. But Africa isn’t a point. It’s big, and people spread out and wandered around Africa for thousands of years before some of them headed north.

722px-homo_sapiens_lineage-svg

The oldest extant human splits aren’t between Africans and non-Africans, but between Pygmies/Bushmen and everyone else. This split happened around 250,000 years ago. This was followed by more splits within Africa, like the one between West and East Africans about 150,000 years ago, and the out-of-Africa event about 70,000 years ago. Here’s a mostly-accurate tree diagram, Bushmen and Pygmies on the right:

(The big inaccuracy in this diagram is the yellow line representing Eurasian back-migration leaving genetic traces in modern Bushmen/Khoisan peoples. That never happened; the results turned out to be a computer error.)

Before the Bantu expansion, Pygmies and Bushmen were among the biggest ethnic groups on the planet. The extremely high Baka Pygmy population on this graph is probably partially due to high genetic diversity due to the merger of multiple long-separated groups rather than a massive Pygmy boom and then genocide, but I think it is still fair to conclude that relatives of today’s Pygmies and Bushmen once controlled most of central and southern Africa.

Populationsize
source

From Whole-genome sequence analysis of a Pan African set of samples reveals archaic gene flow from an extinct basal population of modern humans into sub-Saharan populations, by Lorente-Galdos et al.

(Today, the biggest ethnic group is the Han Chinese.)

From the Science article:

In the new study, geneticists and archaeologists took samples from the DNA-rich inner ear bones of the four children, who were buried 3000 and 8000 years ago at the famous archaeological site of Shum Laka. The researchers were able to sequence high-quality full genomes from two of the children and partial genomes from the other two. Comparing the sequences to those of living Africans, they found that the four children were distant cousins, and that all had inherited about one-third of their DNA from ancestors most closely related to the hunter-gatherers of western Central Africa. Another two-thirds of children’s DNA came from an ancient “basal” source in West Africa, including some from a “long lost ghost population of modern humans that we didn’t know about before,” says population geneticist David Reich of Harvard University, leader of the study.

I spent a while trying to figure out what this is saying, because it isn’t clear. First, I doubt they found that the 8,000 year olds were cousins to the 3,000 year olds. The notes in the extended data section of the paper claim to have found a nephew/aunt or niece/uncle relationship between two of the children; the other two were less closely related–possibly cousins.

This doesn’t tell us which skeletons they got the DNA from, but it turns out that one of the good ones was 8,000 years old.

The article claims that 1/3 of their DNA came from ancestors related to the [Aka] Pygmies and 2/3s from “basal West Africans”, who are also closely related to the modern Bantus.

This is confusing because it makes it sound like these children were a cross between Aka Pygmies and Bantus, and that the’re 2/3s Bantu, in which case they’d be more Bantu than Pygmy and this really wouldn’t upset the idea of Bantus in Cameroon.

ShamLaka
From Extended Data 6: Deep Ancestry Correlation  “An allele-sharing statistic sensitive to ancestry that splits more deeply than southern African hunter-gatherers … is shown as a function of ancestry related to the West African clade (from admixture graph results; the Mota individual, Yoruba and Lemande are shifted slightly away from the boundaries for legibility).

The thing they didn’t say–and I only figured out from looking at the extended data–is that the modern Aka are not 100% “ancestral pygmy.” They are also part “basal West African.” (41% pygmy ancestor and 59% BWA, to be exact.) This is actually quite similar to the 1/3 and 2/3s found in the burials in Shum Laka. So there probably was an event where people related to modern Bantus mixed with an ancient Pygmy population, and their descendants include both the modern Aka Pygmies and the Shum Laka children. 

The Aka Pygmies now live near the border between Cameroon and the DRC. (The “ba-” suffix, found in names like Bantu, Baka, and Batswana, means “people,” so “Baka” just means “Aka People.” Batswana means “Tswana people;” “bo-” means land, so Botswana is “Land of the Tswana.”)

The Mbuti Pygmies, whom you have probably also heard of, live much further from the Cameroonian border and have less Bantu DNA. The Mbuti Pygmies average only 4’6″, while the Aka Pygmies average a couple more inches. The average Aka man stands about 4’11”; the women a little less. Don’t fall into the trap of thinking that the Aka aren’t “real pygmies”–they’re still very short by modern standards, and as this paper shows, the mixing that created them occurred over eight thousand years ago. The Aka have been a distinct ethnic group for an extremely long time.

People always ask why the Pygmies are so small, but I think this question is backward. Bushmen are also short (compared to Dinka and Norwegians); I think our common human ancestors were only about 5′ tall. The pygmies got a little shorter, yes, but not by much; the rest of us got taller.

humantree
Schematic of first alternative admixture graph, (the primary admixture graph is here; it is just a more complicated image)

Now, the really interesting thing in this paper is the identification of three “ghost” populations.

First we have the Ghost Archaic:

The Ghost Archaic was a species similar to Neanderthals, but in Africa. We don’t have any skeletal remains from this species because of the aforementioned climate difficulties, but its DNA shows up in groups like the Mende and Yoruba of West Africa. Here’s the relevant paper, Recovering Signals of Ghost Archaic Introgression in African Populations:

Using 405 whole-genome sequences from four sub-Saharan African populations, we provide complementary lines of evidence for archaic introgression into these populations. Our analyses of site frequency spectra indicate that these populations derive 2-19% of their genetic ancestry from an archaic population 15 that diverged prior to the split of Neanderthals and modern humans.

That’s a lot of archaic! Since the populations with the highest rates of ghost archaic live in far West Africa, I assume the Ghost Archaic did, too.

Next we have the Ghost Modern, which I regretfully did not realize is different from the Ghost Archaic when I first wrote about it.

From Whole-genome sequence analysis of a Pan African set of samples reveals archaic gene flow from an extinct basal population of modern humans into sub-Saharan populations:

Here, we examine 15 African populations covering all major continental linguistic groups, ecosystems, and lifestyles within Africa through analysis of whole-genome sequence data of 21 individuals sequenced at deep coverage. … Regarding archaic gene flow, we test six complex demographic models that consider recent admixture as well as archaic introgression. We identify the fingerprint of an archaic introgression event in the sub-Saharan populations included in the models (~ 4.0% in Khoisan, ~ 4.3% in Mbuti Pygmies, and ~ 5.8% in Mandenka) from an early divergent and currently extinct ghost modern human lineage.

The Ghost Archaics were in the genus Homo, just like Homo erectus, Homo Neanderthalis, but they were not Homo sapiens. The Ghost Moderns were Homo sapiens. They split off from the rest at about the same time the Pygmies, Bushmen, and everyone else went their separate ways.

The Ghost Moderns later contributed to the ancestors of the Niger-Congo people of West Africa and the Mota burial, a 4,000 year old burial from Ethiopia. A branch later split from the Niger-Congo people, creating the “Basal West Africans” and carrying the Ghost Modern DNA (and a bit of the Ghost Archaic) with it. That branch eventually contributed to the Aka Pygmies, including the children found at Shum Laka.

The third ghost population is the Ghost North African.

GNA split from the West Africans well before the Ghost Moderns, shortly after they had split with the East Africans. They appear to be related to the folks buried at Toforalt, Morocco.

I don’t know anything about the Ghost North Africans, but apparently they also contributed to the Shum Laka people. We’ll have to leave that question open for later.

Perhaps it is this infusion of “ghost” DNA into the ancestors of the Aka Pygmies that that accounts for their apparent enormous population size around 20,000 years ago. In this case, their population probably wasn’t actually enormous so much as it had a lot of genetic variation, caused by the merger of several different groups.

All of these Ghost populations used to be full ethnic groups (or species) in their own right, but today they exist only as a trace of DNA in modern people; they no longer exist as a group. These ghost populations were most likely killed off by other human groups or completely absorbed into them. The Ghost Moderns, for example, were probably finished off during the Bantu expansion.

(Let’s remember that all of these numbers are estimates based on the genetic data we have so far, which is not very much. The numbers could change quite a bit as we uncover more information.)

The final interesting thing was the “deeply divergent Y chromosome haplogroup A00,” found in one of the children. The authors did not look into mtDNA (passed down from mothers to children,) but did investigate local Y-chromosome diversity. A00 is estimated to be around 270,000 years old and is relatively common in Cameroon and, as far as I know, nowhere else. (The relevant Wikipedia page unfortunately contains an error, claiming that the Shum Laka children are most closely related to the Mbuti. They are, as the paper actually says, most closely related to the Aka.)

That’s all for now, but here are a few related things if you want to read more:

Whole-genome sequence analyses of Western Central African Pygmy hunter-gatherers reveal a complex demographic history and identify candidate genes under positive natural selection:

African Pygmies practicing a mobile hunter-gatherer lifestyle are phenotypically and genetically diverged from other anatomically modern humans, and they likely experienced strong selective pressures due to their unique lifestyle in the Central African rainforest. To identify genomic targets of adaptation, we sequenced the genomes of four Biaka Pygmies from the Central African Republic and jointly analyzed these data with the genome sequences of three Baka Pygmies from Cameroon and nine Yoruba famers. … Our two best-fit models both suggest ancient divergence between the ancestors of the farmers and Pygmies, 90,000 or 150,000 yr ago. We also find that bidirectional asymmetric gene flow is statistically better supported than a single pulse of unidirectional gene flow from farmers to Pygmies, as previously suggested. … We found that genes and gene sets involved in muscle development, bone synthesis, immunity, reproduction, cell signaling and development, and energy metabolism are likely to be targets of positive natural selection in Western African Pygmies or their recent ancestors.

Insights into the Demographic History of African Pygmies from Complete Mitochondrial Genomes:

To investigate the demographic history of Pygmy groups, a population approach was applied to the analysis of 205 complete mitochondrial DNA (mtDNA) sequences from ten central African populations. No sharing of maternal lineages was observed between the two Pygmy groups, with haplogroup L1c being characteristic of the Western group but most of Eastern Pygmy lineages falling into subclades of L0a, L2a, and L5. Demographic inferences based on Bayesian coalescent simulations point to an early split among the maternal ancestors of Pygmies and those of Bantu-speaking farmers (∼70,000 years ago [ya]). Evidence for population growth in the ancestors of Bantu-speaking farmers has been observed, starting ∼65,000 ya, well before the diffusion of Bantu languages. Subsequently, the effective population size of the ancestors of Pygmies remained constant over time and ∼27,000 ya, coincident with the Last Glacial Maximum, Eastern and Western Pygmies diverged, with evidence of subsequent migration only among the Western group and the Bantu-speaking farmers. Western Pygmies show signs of a recent bottleneck 4,000–650 ya, coincident with the diffusion of Bantu languages, whereas Eastern Pygmies seem to have experienced a more ancient decrease in population size (20,000–4,000 ya).

Western Pygmies, ie the Mbuti, were killed by the Bantus during the Bantu expansion of the past 3,000 years.

Eastern Pygmies, ie the Aka, probably experienced a genetic diversification event about 20,000 years ago that makes it look like their population was much bigger back then than it is now. Their population probably has dropped over the years, but probably not as precipitously as the data shows.

Steve Sailor’s summary:

As I mentioned yesterday, Carl Zimmer’s article in the New York Times on the new ancient DNA paper with its ho-hum title, Ancient DNA from West Africa Adds to Picture of Humans’ Rise, is a model of how to construct articles upside down to bore complacent NYT subscribers with the opening paragraphs before revealing the unsettling details toward the end. Carl doesn’t mention the word “pygmy” until his 18th paragraph and the word “ghost” until the 24th paragraph.

 

 

Book Club: Chinua Achebe’s Things Fall Apart, pt 3/3

 

Chinua_Achebe_-_Buffalo_25Sep2008_crop
Chinua Achebe, Author and Nobel Prize winner

Welcome back to our discussion of Chinua Achebe’s Things Fall Apart. Today I wanted to take a closer look at some of the aspects of traditional Igbo society mentioned in the book.

If you are a regular reader of this blog, you know by now that just as early modern humans (Homo sapiens) mated with Neanderthals and Denisovans somewhere over in Eurasia, some sapiens mated with archaic humans in Africa.

Unfortunately, the state of knowledge about African genomes and especially archaic African genomes is very primitive. Not only does ancient DNA not preserve terribly well in many parts of Africa, but the continent is also rather poor and so people there don’t send their spit to 23 and Me very often to get DNA tested. Thus, sadly, I do not have archaic DNA percents for the Igbo.

WAfricaAdmixture
“Approximate Bayesian computation estimates for the introgressing population across four African populations (Yoruba from Ibadan (YRI), Esan in Nigeria [ESN], Gambian in Western Divisions in the Gambia [GWD], and Mende in Sierra Leone [MSL]),” from Recovering Signals of Ghost Archaic Introgression in African Populations
However, we do have data for their neighbors, the Yoruba, Esan, Mende, and Gambians.

Keep in mind that so far, Eurasians measure about 1-4% Neanderthal and Melanesians about 6% Denisovan, so 10% Ghost in west Africans is a pretty big deal (if you’re into archaic DNA.) The authors of the study estimate that the admixture occurred about 50,000 years ago, which is coincidentally about the same time as the admixture in non-Africans–suggesting that whatever triggered the Out of Africa migration may have also simultaneously triggered an Into Africa migration. 

If you’re not familiar with some of these groups (I only know a little about the Yoruba,) the Esan, Mende, Gambians, and Yoruba are all speakers of languages from the Niger-Congo family (of which the Bantu languages are a sub-set.) The Niger-Congo family is one of the world’s largest, with 1,540 languages and 700 million speakers. It spread within the past 3,000 years from a homeland somewhere in west Africa (possibly Nigeria) to dominate sub-Saharan Africa. As far as I can tell, the Igbo are quite similar genetically to the Yoruba, and the admixture event happened tens of thousands of years before these groups spread and split, so there’s a good chance that the Igbo have similarly high levels of ghost-pop admixture.

Interestingly, a population related to the Bushmen and Pygmies used to dominate central and southern Africa, before the Bantu expansion. While the Bantu expansion and the admixture event are separated by a good 40 or 50 thousand years, this still suggests the possibility of human hybrid vigor.

Edit: A new paper just came out! Whole-genome sequence analysis of a Pan African set of samples reveals archaic gene flow from an extinct basal population of modern humans into sub-Saharan populations:

Here, we examine 15 African populations covering all major continental linguistic groups, ecosystems, and lifestyles within Africa through analysis of whole-genome sequence data of 21 individuals sequenced at deep coverage. We observe a remarkable correlation among genetic diversity and geographic distance, with the hunter-gatherer groups being more genetically differentiated and having larger effective population sizes throughout most modern-human history. Admixture signals are found between neighbor populations from both hunter-gatherer and agriculturalists groups, whereas North African individuals are closely related to Eurasian populations. Regarding archaic gene flow, we test six complex demographic models that consider recent admixture as well as archaic introgression. We identify the fingerprint of an archaic introgression event in the sub-Saharan populations included in the models (~ 4.0% in Khoisan, ~ 4.3% in Mbuti Pygmies, and ~ 5.8% in Mandenka) from an early divergent and currently extinct ghost modern human lineage.

So the ghost population that shows up in the Pygmies the same ghost population as shows up in the Mende? Looks like it.

There’s a lot of interesting stuff in this paper, but I’d just like to highlight this one graph:

Populationsize

I don’t really understand how they compute these things, much less if this is accurate (though their present estimate for the size of the Han looks pretty good,) but assuming it is, we can say a few things: One, before 100,000 years ago, all of the groups–except the Laal of Chad–tracked closely together in size because they were one group. Most of the groups then got smaller simply because they split up. But there seems to have been some kind of really big population bottleneck a bit over a million years ago.

The other really interesting thing is the absolute Pygmy dominance of the mid-10,000-100,000 year range. The authors note:

It is noteworthy that we observed by PSMC a sudden Ne increase in Baka Pygmy around 30 kya. A similar increase was observed in another study that analyzed several Baka and Biaka samples [25]. In addition, this individual presents the highest average genome-wide heterozygosity compared to the rest of samples (Fig. 1b). Nevertheless, such abrupt Ne increase can be attributed to either a population expansion or episodes of separation and admixture [60]. Further analyses at population level are needed to distinguish between these two scenarios.

Until we get more information on the possible Pygmy-cide, let’s get back to Things Fall Apart, for the Igbo of 1890 aren’t their ancestors of 50,000 BC nor the conquerors of central Africa. Here’s an interesting page with information about some of the rituals Achebe wrote about, like the Feast of the New Yams and the Egwugwu ceremony:

 The egwugwu ceremony takes place in order to dispute the guilty side of a crime taken place, similar to our court trials… Nine egwugwu represented a village of the clan, their leader known as Evil Forest; exit the huts with their masks on.

Short page; fast read.

The egwugwu ceremony I found particularly interesting. Of course everyone knows the guys in masks are just guys in masks (well, I assume everyone knows that. It seems obvious,) yet in taking on the masks, they adopt a kind of veil of anonymity. In real life, they are people, with all of the biases of ordinary people; under the mask, they take on the identity of a spirit, free from the biases of ordinary people. It is similar to the official garb worn by judges in other countries, which often look quite silly (wigs on English barristers, for example,) but effectively demarcate a line between normal life and official pronouncements. By putting on the costume of the office, the judge becomes more than an individual.

I have long been fascinated by masks, masquerades, and the power of anonymity. Many famous writers, from Benjamin Franklin to Samuel Clemens, published under pseudonyms. The mask implies falseness–on Halloween, we dress up as things that we are not–but it also allows honesty by freeing us from the threat of retribution.

It is interesting that a small, tightly-knit society where everyone knows everyone and social relations are of paramount importance, like the Igbo, developed a norm of anonymizing judges in order to remove judicial decisions from normal social relations and obligations (as much as possible, anyway). Since most Igbo villages did not have kings or other aristocrats to dictate laws, rule was conducted by notable community members who had effectively purchased or earned noble titles. These nobles got to wear the masks and costumes of the egwgwu.

Ok, so it’s getting late and I need to wrap this up. This moment comes in every post.

I know I haven’t said much about the book itself. The plot, narrative, pacing, structure, writing style, etc. To be honest, that’s because I didn’t enjoy it very much. It was interesting for its content, along with a sense of “I’ve been trying to tell people this and I could have saved myself a lot of time by just pointing them to the book. And if this is a book taught in schools (we didn’t read it in my highschool, but I have heard that many people did,) then why aren’t people more aware of the contents?

What was tribal life like before the Europeans got there? Well, women got beaten a lot. Children were murdered to avenge tribal conflicts. Infant mortality was high. In other words, many things were pretty unpleasant.

And yet, interestingly, much of what we think was unpleasant about them was, in its own way, keeping the peace. As Will (Evolving Moloch) quotes from The Social Structure of Right and Wrong on Twitter:

“Much of the conduct described by anthropologists as conflict management, social control, or even law in tribal and other traditional societies is regarded as crime in modern [nation state] societies.” This is especially clear in the case of violent modes of redress such as assassination, feuding, fighting, maiming, and beating, but it also applies to the confiscation and destruction of property and to other forms of deprivation and humiliation. Such actions typically express a grievance by one person or group against another.

See, for example, when the village burned down Okonkwo’s house for accidentally killing a villager, when they burned down the church for “killing” a deity, or when they took a little girl and killed a little boy in revenge for someone in another village killing one of their women. To the villagers, these were all legal punishments, and the logic of burning down a person’s house if they have killed someone is rather similar to the logic of charging someone a fine for committing manslaughter. Even though Okonkwo didn’t mean to kill anyone, he should have been more careful with his gun, which he knew was dangerous and could kill someone.

Unlike penalties imposed by the state, however, private executions of this kind often result in revenge or even a feud—Moreover, the person killed in retaliation may not be himself or herself a killer, for in these societies violent conflicts between nonkin are virtually always handled in a framework of collective responsibility–or more precisely, collective liability–whereby all members of a social category (such as a family or lineage) are held accountable for the conduct of their fellows.

And, of course, penalties so meted out can be incredibly violent, arbitrary, and selfish, but ignoring that, there’s clearly a conflict when traditional, tribal ways of dealing with problems clash with state-based ways of dealing with problems. Even if everyone eventually agrees that the state-based system is more effective (and I don’t expect everyone to agree) the transition is liable to be difficult for some people, especially if, as in the book, they are punished by the state for enforcing punishments prescribed by their own traditional laws. The state is effectively punishing them for punishing law-breakers, creating what must seem to them a state of anarcho-tyranny.

As for polygamy, Achebe seems to gloss over some of its downsides. From Christakis’s Blueprint: The Origins of a Good Society (h/t Rob Henderson), we have:

Co-wife conflict is ubiquitous in polygynous households… Because the Turkana often choose wives from different families in order to broaden their safety net, they typically do not practice sororal [sister-wives] polygyny… When co-wives are relatives, they can more easily share a household and cooperate… But while sororal polygyny is especially common in cultures in the Americas, general polygyny tends to be the usual pattern in Africa. An examination of ethnographic data from 69 nonsororal polygynous cultures fails to turn up a single society where co-wife relations could be described as harmonious. Detailed ethnographic studies highlight the stresses and fears present in polygynous families, including, for example, wives’ concern that other wives might try to poison their children so that their own children might inherit land or property.

Anyway, let’s wrap this up with a little article on human pacification:

There is a well-entrenched schism on the frequency (how often), intensity (deaths per 100,000/year), and evolutionary significance of warfare among hunter-gatherers compared with large-scale societies. To simplify, Rousseauians argue that warfare among prehistoric and contemporary hunter-gatherers was nearly absent and, if present, was a late cultural invention. In contrast, so-called Hobbesians argue that violence was relatively common but variable among hunter-gatherers. … Furthermore, Hobbesians with empirical data have already established that the frequency and intensity of hunter-gatherer warfare is greater compared with large-scale societies even though horticultural societies engage in warfare more intensively than hunter-gatherers. In the end I argue that although war is a primitive trait we may share with chimpanzees and/or our last common ancestor, the ability of hunter-gatherer bands to live peaceably with their neighbors, even though war may occur, is a derived trait that fundamentally distinguishes us socially and politically from chimpanzee societies. It is a point often lost in these debates.

I think we should read Legal Systems Very Different from Ours for our next book. Any other ideas?

Cracks in the Out of Africa Theory?

The most exciting finding of the past two decades in biological anthropology has been, without a doubt, evidence for interbreeding between Homo sapiens, Neanderthals, Denisovans, etc. and the sheer multiplicity of new hominin species being uncovered.

Exactly where the lines between species lie is a bit of a matter of semantic debate–where exactly did our ancestors end and Homo sapiens begin? Should we classify Neanderthals and Sapiens as one species if we interbred? etc–but if we accept the current classifications as decent approximations, we have:

Capture
Source: Wikipedia

Sorry, I realized it would be much more efficient if I just grabbed the family tree off Wikipedia instead of copying it over bit by bit. The top part of the tree got cut off, so I’ll note that Homini (6.3 million years ago) includes us + chimps, while Hominina (5.7 million years ago) has no chimps, but includes australopithecines. Gorillas are way back in Homininae, with an e. Homo, our genus, includes all of the “human” species, but usually doesn’t include australopithecines.

There is further debate on exactly who descended from whom. We’re finding new fossils all the time, which is quite exciting, but our current record is not nearly as complete as we’d like it to be. So sometimes branches get moved around or re-categorized as more data comes to light.

The most recent notable additions to our genus are the Denisovans, Homo floresiensis, Homo luzonensis, Homo Naledi, Homo doesn’t have a name yet, and more Denisovans.

You have probably heard of the “hobbit,” Homo floresiensis. The remains we have uncovered of this diminutive hominin are remarkably good, including a skull in great condition (despite some damage caused after excavation.) They lived on the island of Flores from about 200,000 to 50,000 years ago (though their arrival may get pushed back considerably because there are stone tools on Flores that are much older–700,000 years old–we just don’t know yet who used them.)

The hobbits are remarkable in multiple ways. First, they lived in an area that was not connected to the mainland by any landbridges–that is, they had to swim, boat, or otherwise be carried to their island. I am skeptical of the idea of anyone surviving a tsunami as a means of populating an island, but they arrived in an era when, as far as we know, humans had yet to build boats. So perhaps their ancestors were among the first humans to build boats, and we just haven’t found the remains of their crafts (wood being a material that degenerates very quickly.)

Second, the Hobbits are most likely descended from Homo erectus, who lived nearby on mainland Indonesia (at the time, connected to the rest of Asia via a landbridge), but are morphologically very different. They are tiny–shorter than pygmies, Homo erectus, or even australopithecines.

There is much debate about whether they are descended directly from erectus, or part of a sister-clade to erectus that descended from a common ancestor. It was previously believed that erectus was the first hominin to leave Africa, but if Floresiensis is not descended from erectus, Flores could be the first.

Now a similarly diminutive hominin has turned up in the Philippines, also past a significant water barrier that would require some effort to cross. It has been dubbed Homo luzonensis. Not much is known, yet, about luzonensis, (we haven’t found as many of its bones), but what we do know is tantalizing:

It was soon apparent to Détroit that the remains featured a puzzling mosaic of traits both modern and ancient. “Each of the features [of Homo luzonensis] corresponds to some hominin or another,” he says. “But the combination makes for something really unique. There’s no known species with this same suite of features.”

They’re small, possibly even smaller than the Hobbits. Their feet resemble australopithecines, but australopithecines supposedly died out a couple million years before Luzonensis arrived on the scene. And their teeth were “remarkably uniform,” which probably sounds boring to anyone who isn’t a dentist, but provides strong evidence of them being a different species.

Two island dwelling species in the same area supports the idea that their ancestors either developed boats or were remarkably skilled at surviving tsunamis, and that southeast Asia was a remarkable hotspot of hominin diversity.

And then there are the Denisovans!

Denisovans are mysterious because we have so few of their bones–a chunk of skull was recently uncovered, but we have no jaws, no faces, no ribs, etc–so we don’t have a good idea of what they looked like. What we do have are Denisovan DNA (extracted from those fragments of skeletons) and traces of Denisovan DNA in modern humans.

Oddly, those Denisovan bones turned up in Siberia (a good place for preserving old bones, but not such a great place for humans adapted to warm climates) while the humans with Denisovan DNA live in modern Papua New Guinea and nearby areas.

The obvious answer to this puzzle is that both the Denisovans had a much broader range than one cave in Siberia and the ancestors of modern folks from PNG used to live in different areas than they do now.

A more detailed analysis of PNG DNA was recently released, which reveals three separate, significant groups of Denisovans who interbred with sapiens:

… modern Papuans carry hundreds of gene variants from two deeply divergent Denisovan lineages that separated [from each other] over 350 thousand years ago. Spatial and temporal structure among these lineages suggest that introgression from one of these Denisovan groups predominantly took place east of the Wallace line and continued until near the end of the Pleistocene. A third Denisovan lineage occurs in modern East Asians. This regional mosaic suggests considerable complexity in archaic contact, with modern humans interbreeding with multiple Denisovan groups that were geographically isolated from each other over deep evolutionary time.

The Wallace line is a place that’s too deep for a landbridge, and thus the area to the east was an island even during the Ice Age. In other words, it looks like Denisovans could use boats. (Or survive tsunamis, pfft.)

Next, we sought to retrieve dates of divergence between D1, D2, and the Altai Denisovan genome … to encompass two deeply divergent Denisovan-related components, our best fitting model indicates that D1 and D2 split from the Altai Denisovan approximately 283 kya … respectively (Figure 4B). While clearly branching off the Denisovan line, D2 diverged so closely to the Neanderthal-Denisovan split that it is perhaps better considered as a third sister group… For context, even the youngest of these divergence times is similar to the evolutionary age of anatomically modern humans … Our model implies substantial reproductive separation of multiple Denisovan-like populations over a period of hundreds of thousands of years. … 

The genetic diversity within the Denisovan clade is consistent with their deep divergence and separation into at least three geographically disparate branches, with one contributing an introgression signal in Oceania and to a lesser extent across Asia (D2), another apparently restricted to New Guinea and nearby islands (D1), and a third in East Asia and Siberia (D0). This suggests that Denisovans were capable of crossing major geographical barriers, including the persistent sea lanes that separated Asia from Wallacea and New Guinea. They therefore spanned an incredible diversity of environments, from temperate continental steppes to tropical equatorial islands.

(We will probably reclassify some of the older fossils from Asia as Denisovans once we figure out what they looked like.)

Then we have Homo naledi, from South Africa. Naledi lived around 250,000 years ago, about the same time as Homo sapiens were differentiating from their ancestors. We have a wonderful array of Homo naledi fossils, preserved in the bottom of a cave pit. If they were placed here intentionally, this was pretty advanced behavior, though I wonder if perhaps they just got lost in the cave from time to time and then died in the pit.

At about 5 ft tall, (male height) naledi was short, but not nearly as short as Floresiensis, and taller than some groups of sapiens. Its skull was significantly smaller than a modern skull, however, and I find it odd that, out of the thousands of bones and fragments discovered, we have not yet recovered much of the front of their faces. Perhaps their faces were shattered when they fell into the cave?

Naledi, like Floresiensis and Luzonensis, shares some more modern traits with other members of the homo genus, and some traits with the older australopithecines. Unlike them, we have yet to uncover evidence that Naledi used tools.

The we have a couple of unnamed hominins

These so-called “ghost populations” are known entirely from their presence in the DNA of modern humans. We don’t have any fossils from them, either because they lived in areas where the weather didn’t favor preservation, or the modern political climate makes searching for fossils difficult.

The pygmies and Bushmen derive about 2% of their DNA from an archaic population or two that we estimate split off from the rest of us about 700,000 years ago. They met and mated with these other hominins around 35,000 years ago.

More interesting is another ghost population that shows up in the genomes of west African groups like the Mende. Now, the average non-African has about 1-4% Neanderthal DNA, and Melanesians have about 4-6% Denisovan, but some tribes in west Africa, such as the Yoruba, Mende, Gambians, and Esan, may derive about 10% of their DNA from an otherwise unknown ghost population that split off before the Neanderthals! (Razib’s very nice article summarizing the paper.)

 

Oh, jeeze, it’s three am, let me finish this in the next post…