early modern

Laurent Excoffier and colleagues' work has investigated how range expansions may have affected human genetic diversity. I've commented on this work several times ("One model, hold the extra parameters", "The Neandertal mtDNA story, 2004 edition, "Surfing and recent selection). They have applied a "geographically explicit" model to questions of human population history, modeling how populations expand and interact in the face of a simulated model of the Old World.

In the past, I've found some things I like in this work, and other points where I disagree with the models' assumptions. Personally, I like to examine analytical models first, because the assumptions are often much more explicit, so we can see more easily how the results follow from them.

This week, a new paper by Mathias Currat and Excoffier in PNAS claims to find evidence for some degree of reproductive incompatibility between Neandertals and modern humans. This is another case where I think the approach is very clever but I disagree with the model's assumptions. I just don't believe that today's distribution of genetic variation can tell us about "reproductive incompatibility" with Neandertals or other archaic people.

Today, if you take a large random sample of people within the continental U.S. and look for a DNA legacy from Precolumbian American people, you will find it in your sample at a level somewhat under 2 percent. This percentage results from the differential growth of European and African-derived peoples during the last 500 years of American history. Whatever else it may be, the current percentage is notevidence for hybrid incompatibility of the world's populations before 500 years ago.

It's not a perfect analogy. Today, Native American ancestry is heterogeneous in the continental U.S., with some people carrying very high fractions. After 30,000 years, such heterogeneity would likely have balanced out. With the Neandertals, we are looking at a much longer history, and different events.

I would contend that the events that have affected today's representation of Neandertal-derived genes were demographically larger than those leading to the European colonization of the Americas. The contraction of the European population during the Last Glacial Maximum, the subsequent movements of Late Upper Paleolithic and settlement of Mesolithic peoples, followed by the introduction of agriculture and waves of population growth and invasions, have partially erased the genetic patterns of the initial Upper Paleolithic. We know that the mtDNA complement of Europe changed markedly both before and after the Neolithic. Today's Europeans are not the people who encountered the Neandertals 35,000 years ago. The genes of those initial Upper Paleolithic people may be almost as rare today as Neandertal genes.

Range expansions and surfing

Nevertheless, I think the analysis in this paper gives us some valuable information about how populations may have interacted at that final stage of population mixture among archaic populations.

A range expansion occurs when a population that is initially limited to some small area begins to expand outward across a larger area. The expansion may include interbreeding with other populations who already occupy those areas, for example, the movement of Neolithic agriculturalists into Europe. Or the range expansion may go into territory where nobody lives, like the initial habitation of the Americas some 14,000 years ago.

Range expansions can distort allele frequencies beyond the pattern expected in a random-mating population. As the population pushes its boundary outward, individuals at the frontier carry with them a slightly skewed sample of the alleles in the population as a whole. Pushing further and further along, this skewed sample gives rise to a founder effect. This phenomenon has been called "allele surfing", by analogy with a spreading wave of population expansion.

When a population expands its range into that of another population, the invaders usually mate with the natives. As the "wave" of migration continues to spread, more and more of the natives' genes are picked up into the expanding population. As a result, you expect to see a gradient of genetic contribution from the original native population, higher and higher as you look farther from the invaders' point of origin.

Currat and Excoffier [1] assume that a group of 50 people originated in Northeast Africa 50,000 years ago and then began to spread throughout the Old World. This population (moderns) expands into the range of another human population (Neandertals) by virtue of a higher carrying capacity: in fact they assume that modern humans existed at four times the density of Neandertal populations. The modern human value is set at 1 person per 10 square kilometers, which is very low compared to ethnographically described hunter-gatherers. The population as a whole is made up of demes that occupy an area 100 km on a side (in some trials, four times as many demes 50 km on a side). The outcome is inevitable: the higher carrying capacity leads modern humans to replace Neandertals, while incorporating some amount of Neandertal ancestry.

Any model is unrealistic to some extent. An unrealistic model generally leads to results that are very different from reality. In modeling, there's a common strategy to deal with this problem: Leave one free parameter and change it until the results fit reality. In this case, the free parameter is migration rate, the probability that an individual will move to an adjoining deme. Currat and Excoffier used values for this parameter that caused their "modern" population to displace Neandertals in Europe over a span of 6000 years. The value that made this dispersal speed was 20 percent per generation for the dispersing modern human population.

I'm a little concerned that a whole literature of geographically explicit population models has emerged in human genetics without any apparent reference to the anthropological literature on human demography. If you know ethnography, a migration rate of 20 percent per generation over 100 km distances seems very high. It's more than double the observed rate of intertribe marriages among precontact Aboriginal Australian people, for example. The value of one person per 10 square kilometers for population density is near the low end ever observed for hunter-gatherers. If it's a stretch to make a model fit with parameters found in known hunter-gatherers, that's when I go back to the drawing board. But then, my philosophy about this is different from most human geneticists. I'm an anthropologist.

Anyway, with these values the result is foreordained: modern humans will replace Neandertals, and fast. What Currat and Excoffier observe in their simulated populations is that the modern humans tend to pick up a larger fraction of Neandertal genes, especially in Europe. How can we explain why our population today has a relatively small fraction of Neandertal genes? In particular, how can we explain why Europeans have no more Neandertal genes than any other population? They conclude that some kind of reproductive incompatibility must have existed.

Where I think the method falls short

I think this paper would be perfectly reasonable if I was willing to assume that the range expansion of modern humans was the last major event in our evolution. If this were true, then echoes of this range expansion would be the most highly visible today — just as astronomers can still find echoes of the Big Bang in the cosmic microwave background.

But I would offer that our genetic diversity today is not the result of a single Big Bang of movement out of Africa. Many population movements of comparable or even larger scale have happened during the last 30,000 years.

The paper presents the hypothesis of reproductive incompatibility as an attempt to solve two problems: First, Chinese, New World peoples, Southeast Asians and Europeans today have approximately the same amount of Neandertal ancestry. Second, the amount of Neandertal ancestry in Europe is only around 2-4 percent. A 6000-year wave of population growth and mixture as modern humans entered Europe might have left more Neandertal genes, and a higher proportion in this Neandertal-rich area of the world than in East Asia.

Here's how I currently see those problems. Europeans today are not the Europeans of the past. They have undergone massive population movements and replacements since the initial Upper Paleolithic people encountered Neandertals. That's not only the result of archaeology, it's also clear from the paleogenetics. If we recognize this subsequent history, then we will find it easy to explain why the rest of the population outside Africa has basically the same small amount of Neandertal ancestry: they received a massive influx of genes from some West Asian population with Neandertal mixture. Europe also got these genes, mostly long after the initial Upper Paleolithic.

So I don't think the present fraction of Neandertal genes tells us anything about sex between Neandertals and humans, except that it happened. Many times. Hooba-hooba.

There is some irony in the timing of this publication, since only last week PNAS published a paper claiming that today's African populations derive some of their DNA from a population fully twice as different from non-Africans as Neandertals were.

I don't fully believe that, either.

References

When I last wrote about the Neandertal genome, I showed that across the X chromosome, Europe and China have different Neandertal genes. There is overlap between the two, but as a generalization few Neandertal haplotypes that are common in Europe are also common in China, and vice-versa. I described the basic method for finding Neandertal haplotypes in recent people last month ("Neandertal segments of X chromosomes").

Almost all of the Neandertal haplotypes found in the X chromosomes of recent people are relatively rare, occurring in fewer than 10 percent of individuals. The largest fraction of Neandertal haplotypes occur in only a single person in the HapMap samples.

But is this a pattern that occurs on the autosomes, or does it reflect X chromosome dynamics in some way?

That's not a hard question to answer, and I went looking first at chromosome 19. The number of haplotypes is fewer, because chromosome 19 is shorter than the X. The overall pattern is the same. Most Neandertal haplotypes are rare in the HapMap samples, and relatively few are common in both the CEU and CHD samples.

I put the origin at the rear; CEU (European ancestry in Utah) number of copies goes toward the left, CHD (Chinese immigrants in Denver) toward the right. You can see that most of the cases are clumped on the extreme edge of both axes. There are not higher counts in CHD; the two axes are at different scales because of one extremely common region in Europeans, as noted below.

I've received a few comments on the 3-d histograms. I don't like them much, either, and I'm looking for an alternative. This one in particular is miserable; because it's out of scale. I'd like to plot these in 2-d using shading to denote bin counts. Unfortunately I haven't found a quick and dirty program that will do this in 2-d, and I've got too wide a range of bin counts for a bubble plot to do it without a lot of tweaking. So I'm stuck with these for now. I can either write about them and share them or spend my time finding a better graphing solution.

I've done a few more comparisons. When we look for Neandertal 10-SNP haplotypes in CEU versus TSI (the sample from Tuscany), we find mostly the same haplotypes in both samples. A haplotype in 10 copies in CEU is certain to be in TSI, and vice-versa.

Number of copies in CEU goes across the bottom, TSI back into the picture. This is such a striking difference from the CEU-CHD comparison. It's very comforting to me, because this is totally the expected pattern -- CEU and TSI should have the same things, because they share most of their population history! I will mention that for the X chromosome, CHB and JPT have a similar pattern, they mostly share the same stuff. This helps lend some significance on the finding below that GIH is also pretty different from all these other samples.

You can see that there is one locus where CEU has more than 100 copies (the little cluster there indicates that this haplotype extends over more than 10 SNPs, in fact it's 13 SNPs with possibly 2-3 flanking SNPs forming a decay pattern on either side; the total length is around 150 kb. There are more than 80 copies in Tuscans, and more than 40 in Gujaratis, but only a single copy in the Chinese sample. Three genes lie in this interval but none point to any obvious hypothesis (to me, at least), about why the Neandertal haplotype would be especially common in western Eurasia. I note it because this is the first Neandertal haplotype I've found with a frequency up over 20 percent or so; this one is about 60 percent in CEU and 50 percent in TSI.

The Gujarati (GIH) sample adds its own distinct twist. There is some overlap between GIH and CEU, and some overlap between GIH and CHD. But by and large the same pattern obtains as between Europe and China: India has its own Neandertal common variants, not widely shared with either CEU or CHD. For example, here's the CHD comparison; CHD going toward left, GIH toward right. The basic pattern is that most cases are clusted on the edge of the graph, few are scattered across most of the area, and there's no consistent pattern among them. Still, the highest-frequency GIH case is the same as the high-frequency haplotype noted in CEU and TSI above.

These examples should demonstrate pretty clearly that this is not solely an X chromosome phenomenon; basically we're looking at the effects of drift in small ancient populations after they mixed with Neandertals.

I did have an excellent question today after my talk where I discussed this pattern -- how do we know that this isn't separate mixture events giving rise to different Neandertal-derived variants in different recent humans?

That's not a trivial question to answer, and I don't think we could easily rule out the hypothesis in the abstract. But the fact that these populations have very similar fractions of Neandertal contribution overall does suggest a single history of mixing. I'll give this some more consideration as I look across the rest of the genome.

Earlier in the week, I pointed to a news story about upcoming research that substantiates some amount of gene flow among Pleistocene groups, persisting into living populations ("Multiregional evolution lives!"). That scenario made a bit of a splash in the news, but the result is not unprecedented.

Last year around this time, I noted a study by Wall, Lohmueller and Plagnol that came to a similar result -- estimating that around 5 percent of the gene pool of today's people outside Africa derived from ancestral non-Africans. That followed on earlier work by Plagnol and Wall from 2006 with essentially the same result.

Looking back at the blog, this has been a recurring topic since the beginning. For example, back in 2005 when I was still doing a meetings update, I posted about several conference presentations on the topic ("Genetics and multiregional evolution, meetings 2005"). At that time the new results were from Alan Rogers, Jody Hey, and Mike Hammer's lab, all suggesting that ancestral diversity either suggested some ancestral population structure outside Africa, or at least didn't reject it. A key finding was published by Garrigan and colleagues (2005), who found a pseudogene on the X chromosome with an unusually deep gene tree in East Asia ("Modern human origins: X marks the spot?").

But there's a lot of literature out there that contradicts this line of research. Some of it is still being published. A case in point is the paper released in PLoS ONE this week by Guillaume Laval and colleagues (2010). The key part of the abstract:

Our results support a model in which modern humans left Africa through a single major dispersal event occurring ~60,000 years ago, corresponding to a drastic reduction of ~5 times the effective population size of the ancestral African population of ~13,800 individuals. Subsequently, the ancestors of modern Europeans and East Asians diverged much later, ~22,500 years ago, from the population of ancestral migrants. This late diversification of Eurasians after the African exodus points to the occurrence of a long maturation phase in which the ancestral Eurasian population was not yet diversified.

That seems to directly contradict all the research suggesting some component of intermixture among pre-modern populations outside Africa. So what's the deal?

I have a good idea what's going on now with these apparent contradictions, thanks to a recent paper by Alan Templeton (2010). I'll discuss that paper in detail later this week, as I'm covering these issues now in my graduate seminar.

In the meantime, I want to give a little thought to the new paper by Laval and colleagues. Following through their simulation methods may give us some ideas about how we can resolve the discrepancies among these tests of human population history. I also want to explore some of the ways that paleontology and paleogenomics may help to inform our tests about these issues.

Here's a nice paragraph from the results section of the paper that outlines many of the simulation methods. I'd like to have seen some issues laid out more clearly, as some of the parameter combinations are hidden in the supplements and not clearly explained there. It means that as I discuss these, I may not quite have understood everything correctly.

First, we determined the evolutionary scenario that took place in the ancestral lineage that culminated in the emergence of modern humans (for a complete list of parameter symbols used along the manuscript, see Tables 2 and S4). We tested different evolutionary models [2], [5], [19], [22], [51]–[56] that allow different levels of introgression of archaic hominids to modern human populations. We assumed an early diffusion of archaic hominids (Homo erectus) out of Africa ~1.25 and ~2.25 million years ago [57], various ancestral migration rate intensities (m₀, ancestral migration rate is the proportion of migrants before the Out-of-Africa exodus) and an African exodus of modern humans between ~40,000–100,000 years ago [38]. By tuning the replacement rate δ, we then simulated scenarios that consider different levels of replacement of archaic hominids by modern humans (i.e. different levels of introgression of archaic material into the modern gene pool), including the most extreme cases of complete (δ = 1) and no replacement (δ = 0) as well as several scenarios with varying intermediate levels of replacement (Figures 3A and S2, Table S4). The summary statistics were calculated by merging all population samples (except for global F_ST) in order to minimize the effects of recent demographic events related to the continental populations. We thus considered in all models a constant size for the three modern human populations. The model with residual ancestral migration rate (m₀~10⁻¹⁰) and full replacement (δ = 1) clearly better fitted our data than any other model (Figure 3A, highest ψ1, the ψ1 of this model is significantly higher after correction for multiple testing when compared with the other ψ1 values, P

For a long time, I've been bothered in the back of my mind about the outcomes of these analyses based on simulations and "approximate Bayesian computation" (ABC). I learned a long time ago never to argue statistics with a Bayesian. It's not that the approach is infallible, it's just that if someone is clever enough to use Bayesian statistics, it's going to turn into a long argument.

By my count the model has 18 parameters, and we could reshuffle them in lots of ways. Can it really be that no combination of the parameters provides a better fit than zero admixture? Intuitively, it seems wrong -- because it would mean that one combination of the other 17 parameters gave a near-perfect fit to the data. Perfect fits don't happen, not with genetic data as messy as in humans.

The conceptual scheme of the paper is an island model with migration, growth, and replacement as possibilities, between the three populations -- Africa, Europe and East Asia. Since any of the parameters could in concept vary from zero to infinity, each specific model ought to be a proper subset of the general island model, and they ought to grade continuously into each other. In other words, an out-of-Africa replacement is just one extreme of the general multiregional/admixture/introgression model.

That has some predictable consequences. A nonzero migration rate before a complete replacement ought to behave very much like a structured population within Africa before a replacement. An analysis that prefers the second really shouldn't be distinguishable from the first. Likewise, a very small population outside Africa before a slight replacement should look very much like a larger population with a more complete replacement. These options aren't identical, but they ought to grade continuously into each other.

But in the paper, the authors bounded the parameters in ways that make these different specific models discontinuous. For example, consider the parameter that defines the time of the initial population spread out of Africa:

We assumed an early diffusion of archaic hominids (Homo erectus) out of Africa ~1.25 and ~2.25 million years ago

If the time of founding of these populations could vary down toward the time of possible replacement (in the last 100,000 years), the continuity and replacement models would grade continuously. This would be the logical connection implied by the island model, but by limiting the range of times, the authors have generated an artificial difference between the models.

In principle, we could have a good reason for separating the models in this way. For example, we know that Europe and Asia were occupied by hominids before 1.25 million years ago, and we might specifically be interested in those people.

But in reality, we know that a 1.25-million-year old split between European and African populations is in conflict with paleogenomics. The Neandertal genome shows that we shared a small common ancestral population with Neandertal ancestors at most around 300,000 years ago, and possibly much more recently. If the human-Neandertal ancestral population was dispersed across Eurasia and Africa at that time, it must have had relatively high gene flow, enough so to behave as a single population with a small effective size.

So this is a possible problem. None of the "admixture" models included in the paper have a feature which the paleogenomic evidence says is necessary. My point is that the authors have generated a "rugged landscape" of models by eliminating the continuity between them. It may not be surprising that one choice of parameters fits the data much more strongly than alternatives, because the intermediates have not been examined.

There are other parameters for which uncertainty might be substantially reduced by using other evidence. For example, the authors consider a range of migration rates varying over three orders of magnitude between recent (modern) populations on different continents. The actual value of this rate has a large effect on the appearance of admixture, because it determines the likelihood that ancestral African variation has recently dispersed out of Africa into Eurasia. But we can probably obtain a good estimate of this rate of recent gene flow from other data, since we have large datasets of genome-wide SNP and microsatellite polymorphisms from these regions. These data could also provide a better test of the proposed Europe-Asia "split". Why are these aspects important? Because recent events will disturb or cover up the evidence for earlier dispersals and interactions. If we can constrain the recent events using other evidence, we can increase our power to test hypotheses about earlier events.

Although many possible intermediate models are excluded in the study, the authors in the end find an overlap in results between two apparently very different parameter combinations:

Among the 24 models tested, the model assuming a complete replacement rate of archaic hominids (δ = 1) and a residual ancestral migration (m₀~10⁻¹⁰) exhibited the significantly highest ψ1 except when compared with the model assuming an almost complete replacement rate of archaic hominids (δ≥0.99).

That result might seem paradoxical. At face value, it means that ancestral humans outside Africa did contribute genes to living populations, but only by means of very rare gene flow from Eurasia back into Africa before a subsequent replacement. In other words, the first modern humans in Africa would have been descendants of Africans and of Eurasian people. It's not surprising that the outcome is close to a model with very slight survival of Eurasian populations.

As we think of ways to improve these tests, I think we need to introduce independent tests of each parameter. This won't always be possible, and there will be cases where changing one parameter will impose a trade-off with one or more others. But that's the nature of these models -- each parameter is a dial, and twisting one of them may be corrected by turning others. The important point is that we can already falsify many of the conceivable possibilities. Until we have a model in which paleontology, archaeology, paleogenomics and the genetics of living people all form a single consistent picture, our work isn't done.

(see also, Gene Expression)

References:

Garrigan D, Kingan SB. 2006. Archaic human admixture: A view from the genome. Curr Anthropol 48:895-902. doi:10.1086/523014

Garrigan, D., Mobasher, Z., Severson, T., Wilder, J. A., Hammer, M. F. 2005. Evidence for archaic Asian ancestry on the human X chromosome. Mol. Biol. Evol. 22:189-192. doi:10.1093/molbev/msi013

Hawks J, Cochran G. 2006. Dynamics of adaptive introgression from archaic to modern humans. PaleoAnthropology 2006:101-115. Open access

Hawks J, Cochran G, Harpending HC, Lahn BT. 2007. A genetic legacy from archaic Homo. Trends Genet doi:10.1016/j.tig.2007.10.003

Laval G, Patin E, Barreiro LB, Quintana-Murci L (2010) Formulating a Historical and Demographic Model of Recent Human Evolution Based on Resequencing Data from Noncoding Regions. PLoS ONE 5(4): e10284. doi:10.1371/journal.pone.0010284

Plagnol, V., Wall, J. D. 2006. Possible ancestral structure in human populations. PLoS Genet. 2:e105. doi:10.1371/journal.pgen.0020105

Templeton AR. 2010. Coherent and incoherent inference in phylogeography and human evolution. Proc Nat Acad Sci USA 107:6376-6381. doi:www.pnas.org/cgi/doi/10.1073/pnas.0910647107

Wall JD, Lohmueller KE, Plagnol V. 2009. Detecting ancient admixture and estimating demographic parameters in multiple human populations. Mol Biol Evol (early online) doi:10.1093/molbev/msp096

I got to sit down and watch most of the first episode of "The Human Spark" on PBS tonight (my earlier post). Our local station shows these things later than the national release dates, and I missed out on the first ten minutes or so as I was putting the kids to bed. The host is Alan Alda, and here are my live-blogging thoughts after I sat down to watch:

8:16: Svante Pääbo interview. Alda watches Adrian Briggs drilling into ancient bones. Explains the problems with contamination.

"But that small difference between us could be crucial, couldn't it?"

8:18: Now, on to protein extraction from Neandertal bones to do isotopic analysis. Alda sits down in the cafeteria with Michael Richards, explaining the high proportion of animal protein in the Neandertal diet.

8:19: On to Grenoble. Nice shot from an Alp. The European synchotron. Tanya Smith is here beaming X-rays into them to get micro-CT data from inside the teeth. The skull here is from Roc de Marsal.

Some interesting animations of human versus chimpanzee cranial growth. Human brains develop slowly, etc.

"Neandertal children ... seem to have grown up more quickly..."

We're in the archaeological site of Roc de Marsal, with Harold Dibble and Shannon McPherron. How many Neandertals were there at any one time. They banter about 20,000, decide that's too many.

8:23: Dan Lieberman is showing Alda the original Skhul 5 skull. We've got a graphic of modern humans evolving in Africa, like little campfires from a night view of the Earth. And then they spread out to light little campfires in Europe. It's like the George Bush version of human evolution -- "a thousand points of light!"

Close shots of archaeological levels with Randall White.

8:27: "Even if Grandma kept her teeth in a glass..." Pierced human molars, being worn as ornaments. They go through the little museum near the site. "Microscopic analysis that we've been doing shows that they were sewn on, like to articles of clothing." This is a nice conversation they're having.

It's a little unfortunate that the film pushes the "no Neandertal ornaments" angle, particularly since this week's paper with the pierced shells.

"Here's what I don't get: The Neandertals survived, but didn't change. They came from the same people that we came from, and at some point we started changing; we became able to change.... Having come from the same background, why were we able to change and they weren't?"

White's answer -- Neandertals have a generalized technological approach; modern humans invent new technologies to address every problem that comes along. You can't separate society from technology (as a response to a followup about social organization). Population numbers may have limited lines of communication among Neandertals. With moderns, "once somebody invents something, everybody knows about it."

8:35: John Shea is teaching undergraduates how to knap. Explaining the value of projectile technology. Ooooh -- time to hit a deer decoy with an atlatl dart. "A hunter who's using this kind of thing would have to work with a group...it takes planning, cooperation...I can't imagine this functioning without the prior existence of language."

I find myself thinking wondering why this wouldn't have been true of Neandertals hunting the same animals? And didn't we hear a little while ago that it was the small animals and fish that set modern humans apart? There is a problem with the presentation here -- these seem contradictory.

8:40: Now, we're in Nairobi with Veronica Waweru. Looking at arrows with reusable shafts. Alda is narrating -- did modern humans start using poison?

8:43: Olorgesailie. Alison Brooks and (an unnamed) Rick Potts are there. Brooks has points that are 150,000 years old that may be arrow points (although the one they handle on camera is bigger than Shea's atlatl point...). Three different excavations, each representing a different age. Another small point "has just been unearthed". This one looks a likelier arrow point than the other. Then, 320,000 years old, they have left a bunch of small stone flakes on pedestals for the film crew. The stone raw material is taken from at least 45 to 50 km distance. Alda: "These people were choosy about their materials...quite unlike the Neandertals."

This is unfortunate, too -- there are some clear instances of Neandertals transporting raw materials over 250 km.

Now they're looking at a possible anthropogenic accumulation of pigment minerals. Brooks stresses human "inventiveness" as a cause of the success of modern humans.

8:50: Back to Ian Tattersall. I didn't see his earlier appearance. "When did people who would fit into human society now first appear?" Tattersall puts it down to 50,000 years ago or so. He suggests that the biological ability to behave in modern society might date back to 150,000 years ago, but lay latent until culture developed much later to bring out the modernity.

Whoa -- the points of light again. People are swarming like tiny sparking ants, and all the yellowish Neandertal fires are going out.

Not a bad program. Alda was a great host for this. You can tell he's genuinely interested in this stuff, and he really put the scientists at ease in the interviews. It's great that they got usable material again and again just having him talking with the archaeologists. And having one host actually travel to these field sites was great -- much better than the usual disembodied narration.

I was really liking it until around halfway through, but as the film went on, it started to raise contradictions that bothered me. Very one-sided about Neandertal behavior, too simplistic.

I don't think the interviewees were the problem here, I think in particular Shea and White were making fairly nuanced statements about Neandertals. I can guess that if either had given any black-and-white quotes, the editors would have included them. My impression was that the choice of topics dictated the result -- ornaments, pigment, and projectiles were chosen to emphasize the "behavioral modernity".

Where I think that approach fails in in the specifics. Projectiles may have been technically more difficult than large-point weapons, but they should have been socially easier. Does it take less cooperation to bring down large animals with close-contact weapons? I think it's the opposite -- I think Neandertals must have been under more pressure to cooperate in their hunts. The transport across long distances is important in MSA contexts, but it's also present in Mousterian France. Neandertals didn't spend hours and hours making beads, but they did wear ornaments and use pigments. If there's a distinction, it's the frequency of these behaviors -- which is a lot harder to measure or estimate.

It's too bad in a way -- it really wasn't necessary to talk about the "human spark" as a human versus Neandertal comparison. This didn't have to be a "modern human origins" program. The DNA segment was interesting, but it didn't really contribute anything to the show's theme -- the narration concluded the segment by saying that the genes don't tell us about the "spark" yet.

I'd have emphasized some older stuff, which is new science that actually does tell us about the emergence of humanness. The Brooks segment would fit into that theme, with the much earlier material from Olorgesailie (and this week we have 500,000-year-old blades from the Kapthurin Formation...). I'd have emphasized the new stuff from Atapuerca, especially the evidence about language. An earlier focus would bring a little more credible use of genetics, either FOXP2 (which I really don't need to see again...) or some human-accelerated genes.

It's curious to compare this program with the NOVA series last fall. The themes were very different (NOVA emphasized climate, this one technology). There was very little overlap of scientist lists -- although it never hurts to be based in New York. I think the programs go well with each other, but it sort of forces the casual viewer to notice that the same evidence can be read almost at cross-purposes, depending on what the scientist assumes is fundamental.