This week I've been at the Vienna Natural History Museum to do some work. It's one of the great museums of the world, and they have a new human evolution exhibition with several great reconstructions of fossil specimens by Elizabeth Daynès. Several original hominin skeletal specimens are here on loan for a short time, including these:
Those are Qafzeh 11 and Qafzeh 9 from the Middle Paleolithic of the Levant, around 90,000 years old.
Emily Sohn reports on a drilling project that is bringing to light ancient drying episodes in the Dead Sea basin: "A dry Dead Sea before biblical times".
At a level corresponding with 120,000 years ago, during a warm period between ice ages, the researchers found a layer of small round pebbles sitting on top of 45 meters (nearly 150 feet) of thick salt deposits. Those pebbles, they announced this week at a meeting of the American Geophysical Union in San Francisco, look just like the rocks that normally appear on the lake's beaches -- suggesting that one of the deepest parts of the lake was once dry.
Information about human occupation of the Levant and Arabian peninsula is getting crowded between 120,000 and 100,000 years ago. A total drying of the Jordan basin around the last interglacial would make things very interesting. Imagine the ancient artifacts on those beaches encased in meters of salt under the brine.
In Science this week, Nira Alperson-Afil and colleagues report on recent excavations at Gesher Benot Ya'aqov, Israel. I saw some of this research presented at a conference, and I thought it was quite amazing to see the preservation of organic materials at this site. The Science paper is a good summary of the high points, presented in a very readable way.
About the site:
Gesher Benot Ya’aqov is located on the shores of the paleo–Lake Hula in the northern Jordan Valley in the Dead Sea Rift (7). The Early to Middle Pleistocene sediments document an oscillating freshwater lake and represent some 100,000 years of hominin occupation (Oxygen Isotope Stages 18–20) dating to 790,000 years ago (8, 9). Fourteen archaeological horizons indicate that Acheulian hominins repeatedly occupied the lake margins, where they skillfully produced stone tools, systematically butchered and exploited animals, gathered plant food, and controlled fire (7, 10–15).
The current paper reports on a single occupation level, characterized by a hearth feature and associated plant, animal, and artifactual remains. Some interesting things:
1. The plant remains:
Although most taxa indicate wet habitats (e.g., lakes, lake margins, swamps, and near streams), the abundant fruit remains of woodland species such as olive, oak, and officinal storax (Styrax officinalis) imply human involvement, as their habitat was likely located some distance from the lake shore. Edible plants include oak acorns, prickly water lily (Euryale ferox) seeds, and water chestnut (Trapa natans) fruits; these were probably staple foods because of the nutritive value of their starchy nuts. Through roasting, the inedible shell of the nuts can easily be peeled and the tannin content of the acorns reduced. The fruits of the wild grapevine (Vitis sylvestris) and olive, and the leaves of the white beet (Beta vulgaris) and holy thistle (Silybum marianum), may also have been consumed.
2. Crabs:
The 17 crab specimens [minimum number of individuals (MNI) = 4 (22)], identified as the extant Potamon potamios, include pieces of the two asymmetric chelipeds, each with a distinctive form of the movable (upper) and fixed (lower) pincer....Of the seven pincers of the large cheliped present in Level 2, six occur around the hearth. These are the only crab remains in this area (fig. S4) (23).
What's not to like about people eating crabs?
3. Spatial patterning. There are two distinct areas of the horizon with anthropogenic activities -- the hearth and a second cluster of tools and stone waste flakes, I'm not very excited about the spatial distribution of activities. The story in the news is about how ancient humans knew how to "keep house" -- they're selling it as a major breakthrough in cognitive evolution.
But the reason why we rarely have archaeological evidence about spatial patterning is that an archaeological horizon doesn't have very good temporal resolution. Here's an alternative scenario to account for the spatial pattern of remains in this horizon: One day, some people came, made tools and ate some fish. Three weeks later, some other people were in the same area, and they stayed for a few days, made a fire, did a bunch of other stuff.
That's pretty much the spatial pattern that I would find if I went back home to Kansas and checked out campsites around the shore of the local reservoir. Few campsites are occupied for very long, and different people use them over time, sometimes with a fire, often not. Sure, we're cognitively advanced. I'm just not convinced that the spatial distribution of our campsite trash is very good evidence about it.
Here's what the paper includes about the spatial patterning:
The evidence from Gesher Benot Ya’aqov suggests that early Middle Pleistocene hominins carried out different activities at discrete locations. The designation of different areas for different activities indicates a formalized conceptualization of living space, often considered to reflect sophisticated cognition and thought to be unique to Homo sapiens (3). Modern use of space requires social organization and communication between group members, and is thought to involve kinship, gender, age, status, and skill (2).
I think this is weak on two grounds -- first because the archaeology is poor evidence about the formal conception of living space, and second because it's not obvious that there's anything very unique about it.
Why not unique? Any animal that can make a structure must have some capacity to pattern spatial activities -- if they don't, there's going to be poop everywhere. Conditioned on the fact that a human social group is sharing a single space, and group members are doing more than one activity, I don't see how you would ever expect to find a uniform scatter of evidence of these activities. There will always be some kind of spatial pattern, from the mere fact that two people can't occupy the same space at the same time.
Remember that Gesher Benot Ya'aqov provides the earliest good evidence of human-controlled fire. It's no coincidence that "spatial patterning" should be found with a fire -- anything that people did anywhere other than by the fire is automatically evidence of a pattern.
4. Fish. Now if there is one big reason why the spatial patterning is useful, it's the interpretation of the fish remains. It's not in the least bit surprising that there would be a lot of fish remains on an ancient lakeshore. But the remains are clustered into two distinct parts of the site, which happen to be the very two locations that humans were clearly using.
In other words, once you accept that the archaeology gives you some evidence of where the people were within the site, you can test for association of the fauna and plant remains with the people. The crabs aren't all around the fire because of a failed attempt to stay warm at night; the people brought them there and ate them. The fish remains are clustered around the fire and flintknapping areas because people were eating them.
Here's a good moral of the Gesher Benot Ya'aqov story: It's now past time to stop talking about whether "pre-modern" humans used aquatic resources. They did, sometimes intensively. I never understood why this argument about seafood and modern humans ever got any traction. We've known for twenty years that coastal Neandertals ate shellfish. We also have known from the numbers in caves near the coast that people never seem to have transported them very far inland. So there was a good reason why you didn't see more evidence of seafood; there just weren't that many sites very near the coast.
So why was it news when a bunch of coastal African sites started producing evidence of shellfish consumption? Evidence that we already had for coastal Neandertals? I don't understand. Well, here we have people eating crabs and lots and lots of fish, 800,000 years ago. We can add the paper by Jose Joordens and colleagues earlier this year about Trinil (I reviewed it in "The shells of Trinil"), a million years ago or more.
Another reason why Gesher Benot Ya'aqov is interesting: outside Africa, Middle Pleistocene sites (and Late Pleistocene sites, for that matter) have a fairly extreme bias toward caves and rock shelters. Caves can preserve evidence of within-site spatial patterns, and certainly offer some exceptional opportunities to track human activity over long periods of time. However, humans aren't very likely to have schlepped hundreds of fish from a lakeshore into some remote cave.
UPDATE (2009-12-18) Thanks to a reader who pointed out a hanging omission; I corrected the text.
Alperson-Afil N, Sharon G, Kislev M, Melamed Y, Zohar I, Ashkenazi S, Rabinovich R, Biton R, Werker E, Hartman G, Feibel C, Goren-Inbar N. 2009. Spatial Organization of Hominin Activities at Gesher Benot Ya’aqov, Israel. Science 326:1677-1680. doi:10.1126/science.1180695
Debbie Guatelli-Steinberg and Donald Reid report on the perikymata spacing of a sample of fourteen anterior teeth from Qafzeh. These are "early modern humans", among the earliest to be located outside of Africa, but their anatomical position relative to Neandertals and other groups has been subject to frequent dispute.
As I've emphasized several times ("Neandertal teeth: the other shoe", "How modern is "modern tooth development"?"), this growth characteristic of teeth is variable among living human populations. What remains totally unclear is why it varies.
Neandertals are at the low end of the human range of variation for perikymata counts on their anterior teeth, and the patterning of packing across the tooth is somewhat different. In particular, Neandertals have fewer perikymata nearer the roots of these teeth (for details, I suggest Guatelli-Steinberg's 2009 review article).
The current paper follows up on earlier work by Janet Monge and colleagues (2006). They emphasized that the Qafzeh anterior teeth fit within the overall human range of variation, but observed that two individuals were very close to Neandertals in their packing patterns. Here, Guatelli-Steinberg and Reid include more specimens in the sample, confirming this similarity.
From their conclusion:
The purpose of this study was to investigate whether Qafzeh teeth are different from those of modern humans in the percentage of perikymata present in their cervical [sic] appear to fall in the lower 50% of the modern human distribution, and a few fall within the lowest 5% of the distribution. Thus, this sample of Qafzeh teeth appears to differ from those of modern humans in the same direction that Neandertals do: with generally lower percentages of perikymata in their cervical regions. As can be seen in the SEM montages in Figure 2, perikymata become much more closely spaced in the cervical relative to incisal halves of the Inupiaq LI2 than they do in either the Neandertal or Qafzeh LI2s. Although sample sizes precluded a similar test between the Qafzeh and Neandertal teeth, plots of the averages for these teeth (Fig. 1a,b) reveal the similarity of the Qafzeh and Neandertal teeth, particularly for the UI2, LC, LI2, and LC. Values for two of the Qafzeh UI1s and a single UC are closer to the modern human than Neandertal means for these tooth types, revealing overlap in the ranges of values, as is also true for Neandertals and modern humans (Guatelli-Steinberg et al., 2007).
It may be worth pointing out that the perikymata packing pattern was a key part of Ramirez-Rossi and colleagues' conclusion that the Les Rois B mandible as well as several other Les Rois dental specimens show affinities to Neandertals.
I think Monge and colleagues are correct in asserting that this packing pattern is not a taxonomic diagnosis. Notwithstanding that the precise Neandertal-like pattern, present at Qafzeh, does not occur in the known human samples, we still don't know why human patterns differ from each other. In their discussion, Guatelli-Steinberg and Reid suggest alternatives for the mechanism forming the straiae, but I'd like to have some kind of genetic answer -- what developmental processes changed, carrying this feature along with them?
Anyway, another contrary observation to the idea of "modern human dental development", I guess.
Guatelli-Steinberg D, Reid DJ. 2010. Distribution of Perikymata
on Qafzeh Anterior Teeth. Am J Phys Anthropol (in press). doi:10.1002/ajpa.21158
Guatelli-Steinberg D, Reid DJ, Bishop TA, Larsen CS. 2005. Anterior tooth growth periods in Neandertals were comparable to those of modern humans. Proc Nat Acad Sci USA 102:14197-14202. doi:10.1073/pnas.0503108102
Guatelli-Steinberg D. 2009. Recent studies of dental development in Neandertals: Implications for Neandertal life histories. Evol Anthropol 18:9-20. doi:10.1002/evan.20190
Monge JM, Tillier A-M, Mann AE. 2006. Perikymata number and spacing on early modern human teeth : Evidence from Qafzeh cave, Israel. Bull Mem Soc Anthropol Paris 18:25-33.
Ramirez Rozzi FV, d'Errico F, Vanhaeren M, Grootes PM, Kerautret B, Dujardin V. 2009. Cutmarked human remains bearing Neandertal features and modern human remains associated with the Aurignacian at Les Rois. J Anthropol Sci 87:153-185.
I just read the new paper by Philipp Gunz and colleagues, titled, "Early modern human diversity suggests subdivided population structure and a complex out-of-Africa scenario". That's a mouthful.
The late Middle Pleistocene population of Africa was genetically variable, and that genetic variability is probably the biggest component of genetic variation still remaining in living humans. Moreover, the phenotypic variability of the Levantine sample has been recognized since its initial description by McCown and Keith (1939). So to read this is not surprising:
Seemingly ancient contributions to the modern human gene pool (36) have been explained by admixture with archaic forms of Homo, e.g., Neanderthals. Although we cannot rule out such admixture (37), the clear morphological distinction between AMH and archaic forms of Homo in the light of the proposed ancestral population structure of early AMH to us suggests another underestimated possibility: the genetic exchange between subdivided populations of early AMH as a potential source for ‘‘ancient’’ contributions to the modern human gene pool (9, 36).
I've stressed the importance of African population structure before (e.g., Hawks et al. 2008). So I agree completely with this part of the interpretation in the paper: African variation was larger than in other regions, and it was important.
But that being said, these morphometric comparisons are not very straightforward. Some comments:
1. Phenotypic variance is not a measure of genetic variance. If we see a population that has a large measure of phenotypic variability, it does not mean that the population had much genetic variability. Perversely, genetic variability can sometimes be lower in a population that has greater phenotypic variance -- often because genetic drift can cause a loss of epistases that once constrained the phenotype. In some cases environmental variance may actually increase when the additive genetic variance declines, because of a loss of developmental robusticity. In any event, we can't just go from a variable phenotype and infer that there's variation in genotypes.
2. There's no evidence for subdivision here. They measure a high phenotypic variance within the sample they refer to early modern humans. But that variance is expressed not mainly between geographic locations in the sample, but within them. Qafzeh 6 and 9 are far apart; Jebel Irhoud 2 and Skhul 5 are close together. The East African fossils Omo 2 and LH 18 are far apart. This isn't subdivision, it's just high within-population variance.
3. Weird sample composition. The early modern human sample includes the African and Levantine crania complete enough for analysis. But why lump these? Why is the South African Fish Hoek skull lumped with Upper Paleolithic Europeans?
4. Temporal range. There are two samples here that have a high average distance between nearest neighbors in the sample: "archaic" humans and early modern ones. What these two samples have in common is that they each cover a much larger range of time than the other samples. The early modern sample spans more than 100,000 years by current dates. That's more 80,000 years longer than the Upper Paleolithic sample, 50,000 years longer than the Neandertal sample -- a huge component of variance that is uncontrolled in the other samples.
5. Principal components. PC axes are those that account for the largest covariances in the sample. If two samples are lumped together, there is a within-population component of variance and a between-population component. These may be partly independent in their effects on the total variance, or they may not be. In any event, if we derive the PC structure from the total sample, or even from the individual samples pooled together, the larger samples will weight the PC structure more toward the factors that explain their within-sample covariances. In this case, we have many more recent humans than fossil ones, and many more archaic humans and Neandertals than "early modern" humans. It's hard to have an intuitive idea about the biases that can result from sample composition, and that's a big reason for caution.
Those are all reasons for re-examining the results in different ways. In particular, if I were doing this kind of analysis, I would repeat it for subsets of the cranium, where I could include a larger number of fragmentary fossils. If the African-Levantine sample is really unusually variable, that should hold up strongly when we examine parts as well as the whole cranium.
Well, although I listed several reasons for caution, we can ask how to interpret the study's conclusion:
Any model consistent with our data requires a more dynamic scenario and a more complex population structure than the one implied by the classic Out-of-Africa model.
If we take the high variance of their "early modern" sample at face value, what we have to conclude is that later humans evolved substantially less phenotypic variance than African-West Asian people who lived between 200,000 and 90,000 years ago. Genetics tells us that there was no massive genetic drift during the time span after 90,000 years ago within Africa. Thus we must conclude that some other force resulted in a significant restriction of the phenotypic variation of recent humans, including people who lived as long as 40,000 years ago.
My hypothesis would be natural selection on some significant subset of phenotypic characters, which reduced the phenotypic variance of most of the cranium by pleiotropy. An out-of-Africa migration is not sufficient to explain the reduction in variance, because all modern humans are limited in phenotypic variance, not only non-Africans. Selection on some significant set of genes would help to explain why the ancestral African population predominated within the last 100,000 years. This selection would have predated most of the recent acceleration we observe in the genomic variation of current populations -- indeed, whatever set of genes was strongly selected before 50,000 years ago might have been fixed long ago.
A wave of selection can promote dispersal and demographic growth without the necessity of complete population replacement (cf. Eswaran 2002). A substantial transition in the genetic background would alter the phenotypic effects of any genes that remained in non-Africans from their local ancestors. In other words, the answer about what happened to fossil humans outside of Africa depends on the kind of events that happened inside Africa. So from that perspective, this research is very interesting.
Gunz P, Bookstein FL, Mitteroecker P, Stadlmayr A, Seidler H, Weber GW. 2009. Early modern human diversity suggests subdivided population structure and a complex out-of-Africa scenario. Proc Nat Acad Sci USA (early online) doi:10.1073/pnas.0909160106
Eswaran V. 2002. A diffusion wave out of Africa: the mechanism of the modern human revolution? Curr Anthropol 43:749-774.
McCown TD, Keith A. 1939. The Stone Age Man of Mount Carmel: The fossil human remains from the Levalloiso-Mousterian. Clarendon Press, Oxford.
