South Africa

Today we finally get to learn about the exceptional discovery of four partial hominin skeletons from Malapa Cave, South Africa. Two of the fossil skeletons are described by Lee Berger and colleagues in the current issue of Science, descriptions of two more are still forthcoming.

A kind journalist sent me a copy of the research papers a few days ago, so my graduate students and I have had a chance to think about them a little bit and compare them with other material.

Berger and colleagues have named a new species to contain the fossils, Australopithecus sediba. For anybody who follows paleoanthropology, the new species won't be surprising -- if I found a fossil, I'd surely make up a new name for it, even if I thought it was my great-great-grandmother. In this case, the morphological reasons for naming a new species aren't trivial, but I'll begin by approaching them skeptically, especially in comparison with the large samples of South African fossils both earlier and later than Malapa. I'll conclude that a new species within Australopithecus was probably the right call, but not an easy one.

The press is running with a "new fossils provoke debate" storyline -- are they possible ancestors of Homo or not?

The simple answer to that question is that the Malapa skeletons are too late to be ancestors of Homo. After all, we have early Homo nearly a half-million years earlier.

A more complicated answer is that it depends what we mean by Homo. My feeling is that these skeletons don't comport with what most of us mean when we say "Homo". Most of us have in mind an adaptive shift from Australopithecus to Homo that included larger brain size as a significant element, and the MH1 skeleton has a small endocranial volume.

But if we accept that model of Homo, we have to accept its consequences, as the Malapa skeletons now make clear. One important consequence is that, if we assume that MH1 isn't Homo, we can no longer say have any skeletal evidence of Homo from before 1.95 million years ago. Because the Malapa specimens are more like Homo in their dental and mandibular features than are earlier specimens that have usually been called Homo.

And if we throw out all those earlier Homo specimens...well, then suddenly Malapa isn't too old to be an ancestor of Homo after all.

How old are they?

The fossils lay above a flowstone with a U-series and paleomagnetic date consistent with an age just around 2 million years ago. That's a maximum age for the fossils; they must be younger than that.

The hominins are in water-deposited sediments, which are inferred to represent ancient washes of subterranean water flows through the cave system. Two elements above the flowstone contain the hominin specimens, called facies D and E, and both have normal magnetic polarity. The most likely interpretation is that they belong to the Olduvai paleomagnetic subchron, which occurred between 1.95 and 1.78 million years ago. A specimen of the sabertooth cat Megantereon in one of these facies has a last appearance elsewhere in Africa at 1.5 million years ago. So it appears that 1.78 million years is a very likely minimum age for the fossils.

That's about as good as dating gets in South Africa, where we're used to seeing very wide age brackets on hominin-bearing localities. It means that the Malapa hominins lived at around the same time as KNM-ER 1470 in the Turkana basin, or OH 24 at Olduvai Gorge. Until today, I think we could justly claim that the only australopithecines still known to occur in this time interval were the robust species A. boisei and A. robustus -- although the first appearance of A. robustus might (might) be later than Malapa.

Why aren't they A. africanus?

To me, this is the hardest question to answer.

The Sterkfontein Member 4 sample of A. africanus is tremendously variable. The postcrania of both Malapa skeletons are tremendously informative, but fall within the range of variation at Sterkfontein for almost every feature that the authors reported. The few exceptions (such as humeral torsion and femur neck/shaft angle) are right at the edge of the Sterkfontein range.

In other words, it's my impression that the postcrania of the Malapa skeletons fit within A. africanus. The limits of my impression are that there are a whole lot of observations here, and the paper generally does not report metrics for the postcrania. Maybe the sequel will give us some more surprises.

I would have added a comparison with the Swartkrans A. robustus sample, which overlaps nearly totally in body size with Sterkfontein and contains elements that are in some cases more comparable to the Malapa skeletons. In particular, the os coxa of MH1 looks a lot like SK 3155, and the proximal femur looks like SK 82 to me, at least in the tiny picture provided with the paper. On the whole, I don't think that the Malapa hominins are particularly like A. robustus, I just think that if you put together a reasonably large sample of australopithecine postcrania, these two skeletons don't stand out.

I'll take up the discussion of proportions of the different elements below. My feeling is that the proportions aren't exceptional for Australopithecus, either, but we have to temper that against the observation that really only AL 288-1 (Lucy) is comparable, and it's more than a million years older.

What about the teeth? Generally speaking, the teeth of MH1 and MH2 are both at the small end of the A. africanus range. In a couple of cases (the lower canine of MH1, the lower second molar of MH2), the teeth are absolutely smaller than any Sterkfontein individual. The canines are within the range of A. robustus (remember that the robust australopithecines have small anterior teeth), but the premolars are nothing like the large, molarized Swarktrans sample of premolars.

They're a little small but within the range of those known for Homo habilis at Olduvai Gorge. For example, OH 7 -- the type specimen of Homo habilis has molars that are 1.5 mm larger than MH1 in both dimensions.

But then, Homo habilis really doesn't differ much in tooth size from Sterkfontein.

In size, the Malapa teeth are exactly what you would expect for Homo erectus. The first molars are smaller than those of Dmanisi D2700/D2735, for example. But unlike H. erectus dentitions, the molars of the Malapa hominins get bigger toward the back -- M3>M2>M1.

The Malapa mandibles are strikingly gracile. The MH1 mandible has a relatively vertical symphysis with a small cross-section. The long, parallel upper and lower corpus borders really strike me like a mandible of Homo erectus, something like KNM-ER 993 or OH 22 -- but this impression may be exaggerated considering the M₃ of MH1 has yet to erupt. Metrically, the corpus breadth and height are most like OH 13. There are small australopithecine specimens that compare to this, such as AL 277-1, and it is worth remembering that MH1 is a juvenile mandible. I can't compare the ramus heights with those of other samples because the authors don't report those measurements.

An interesting question: If these mandibles had been found in isolation, would we call them Australopithecus? The Olduvai H. habilis mandibles OH 7 and OH 13 have M3>M2>M1, while OH 16 has M2>M3>M1. The Malapa mandibles look much more like later Homo than do early Turkana basin mandibles like KNM-ER 1801, KNM-ER 1802, or KNM-ER 1482, all of which are much more robust and have larger, more molar-shaped premolars than MH1, and all of which have M3>M2>M1 except KNM-ER 1802 which lacks M₃. This is a quick comparison on my part, but I think the Malapa mandibles look more like Homo than does the existing hypodigm of Homo habilis. It's hard to imagine that the mandibles in isolation would have been referred to Australopithecus. More on that below.

Compared to the mandibles, the cranium of MH1 looks more like its counterparts from Sterkfontein. To be sure, it is an 11-13-year-old juvenile and more gracile in some respects than any of the Sterkfontein crania. But take a look at it next to Sts 71:

MH1 (left) next to Sts 71 (right)

They're not identical, naturally. Sts 71 has higher temporal lines, a slightly smaller vault, and more prominent cheeks. It also has more postorbital constriction compared to MH1, though that isn't obvious from this angle. MH1 has a true superorbital torus, Sts 71 has at best a shade of one. But you can see the similarities -- the angle of the zygomatic process of the maxilla, the narrow and concave interorbital region, the tall and narrow orbits. MH1 has no prominent anterior pillars (bony swellings on either side of the nasal aperture), but Sts 71 is not very different in this region. Sts 71 has bigger teeth.

Consider also Sts 52:

MH1 (left) next to Sts 52 (right)

Again, Sts 52 has anterior pillars and bigger teeth, but the shape of the face is very comparable between these two. The nasal bones in particular are similar in this pair, almost "pinched" at the midline, with a lateral expansion both superiorly and inferiorly.

We can do a similar exercise for most of the features of the MH1 cranium. What is exceptional, in the context of the Sterkfontein sample, is the overall gracility of the masticatory apparatus.

One important thing that is not in the least bit exceptional: Its brain. An endocranial volume estimate of 420 ml (from CT reconstruction) puts MH1 at the bottom of the range of variation at Sterkfontein -- the best-known skull from Sterkfontein, Sts 5, has a volume of 485 ml, while STW 505 has a volume larger than 550 ml. Before MH1, the smallest of the South African crania were estimated to have volumes of 428 ml. This one seems to be smaller mainly by being flatter -- a shape that it shares with early Homo, but I wouldn't say it was without parallel in Australopithecus.

But the smallest endocranial volume known for early Homo is KNM-ER 1813, at 510 ml. That specimen is extreme: the next smallest is 585.

The vault fits in A. africanus, most of the facial features have comparable specimens in the Sterkfontein sample, with some exceptions, and the postcranial skeleton is unexceptional. The teeth are mostly within the range at Sterkfontein with some exceptions. But the mandible -- like those few facial characters -- stands out.

Australopithecus sediba -- a new species within Australopithecus -- then seems like a fair diagnosis. The craniodental derived features are of the sort that would usually justify a new species. Heck, in the case of Kenyanthropus, even more minor differences in the face and size of teeth from contemporary A. afarensis caused Leakey and colleagues (2001) to name a new genus.

Is MH1 really a male?

Berger and colleagues (2010) infer that the MH1 skeleton (the one with the skull) is a male. It is large and more robust than the MH2 skeleton: Its teeth are bigger than the MH 2 skeleton, its mandible is more robust with a taller ramus, the articular ends of its limb bones are a bit larger. In addition, the greater sciatic notch on its preserved os coxa is narrower than other australopithecines like Lucy and Sts 14, and the pelvic inlet may (based on the anterior position of the auricular surface) have been smaller.

But the skeleton isn't really very big. Its endocranial volume is small, its long bones are not nearly so robust as some australopithecines. There are large male australopithecine skeletons -- STW 431, for example -- and MH1 doesn't seem so large as these. Again, it's hard to tell without postcranial measurements, but the sex of this specimen isn't a clear call either way.

The sex of the specimen is important to the way we interpret it, because the features that make it stand out from A. africanus concern masticatory gracility. If it's a female, it doesn't seem quite so different from A. africanus as if it's a male.

Are they Homo?

Let's start with the brain size, which at 420 ml seems to be the most obvious thing separating MH1 from our genus. Well, except for Liang Bua 1 -- with its endocranial volume of, um, 420 ml. Is brain size fundamental to Homo? Maybe. Maybe not.

Alan Boyle's report on the fossils ("Fossils shake up our family tree") has an excellent letter from Don Johanson, who makes the argument that the Malapa fossils should be assigned to Homo. Of course, Johanson and Bill Kimbel in 1996 described a 2.33-million-year-old fossil from Hadar as the earliest clear maxilla of Homo. That maxilla, AL 666-1, resembles Homo in having a more vertical subnasal profile, a parabolic dental arcade, molars that are long relative to their breadth, and a "squared-off" jaw that is relatively straight across the anterior dentition. In other words, basically the dental features seen in the MH1 maxilla.

We've got two choices. Maybe these are genuine shared derived features with these specimens and Homo -- in which case, we should probably name them Homo, as Kimbel and colleagues did for AL 666-1.

Or, there were several australopithecines after 2.5 million years ago with these dental and maxillary (and for the Malapa hominins, we can add mandibular) characters. In which case, they're not signs of Homo at all. They may reflect parallel dental reduction in several australopithecine lineages, all of which faced niche differentiation from the emerging robust australopithecines. One of those lineages may have given rise to Homo, but we don't know which. Maybe it was South African, but it need not have been. It could even have been Asian.

The question is just how important we think brain evolution was to the origin of our genus. If the brain was the key adaptation, then Malapa shows that the dental features are irrelevant to the brain -- because these skeletons have more dental reduction than most of the East African Homo habilis sample, but MH1 has a much smaller brain.

What about tool manufacture?

Part of the logic of pre-2-million-year-old Homo is the emergence of stone tool manufacture 2.6 million years ago. It stands to reason that this major shift in behavior and diet might have given rise to a new adaptive plateau for early hominins, and that would have been tied to the evolution of larger brains. The problem is that we don't have larger brains in any fossil remains until after 2 million years ago -- KNM-ER 1470 remains the earliest hominin with a brain larger than 600 ml. Up to now, people have conjectured that large-brained hominins may have existed earlier, even to the point of arguing about the brain size reflected by the otherwise-robust temporal bone from Chemeron. But it's worth pointing out that none of these pretenders to the Homo throne have smaller teeth than A. africanus. The diet shift that should have been made possible by a meat-eating stone tool economy didn't lead to smaller teeth until much later.

And now we know that at least one small-toothed hominin also was a small-brained one.

We don't know whether the Malapa hominins would have been toolmakers. The fact that they weren't found with tools isn't really evidence either way. Dirks and colleagues (2010) suggest that the skeletons were deposited by water washing them from an initial death trap into a secondary location. If true, it would be a miracle beyond belief for stone artifacts to have made the trip with them.

We do know that stone tools are present in Sterkfontein Member 5 and Swartkrans Member 1, and cutmarked fauna are in the latter. Both these may be roughly contemporaneous with the Malapa hominins, depending on their date. So toolmaking hominins were in the immediate area, around the time that the Malapa hominins lived.

SK 847 is from Member 1 of Swartkrans, and could be as old as the Malapa skeletons. Its endocranial volume isn't known, but facially it looks even more like Homo erectus than does MH1. It seems plausible that this skull represents the local toolmaking population, but even so, this skull does resemble MH1 in several respects, and again we don't know its volume. STW 53, probably a bit older than Sterkfontein Member 5, has also often been referred to Homo but it definitely doesn't have a substantially larger endocranial volume than MH1.

So again, we seem to be faced with two choices: Broaden the definition of Homo to include this very australopithecine-like sample, or restrict it to later large-brained hominins. In either case, brain size and tool manufacture do not necessarily go together.

What's the single most obvious thing that the paper doesn't describe?

Which brings me to the fingertip. MH2 has a distal phalanx. The paper doesn't describe it, even though this bone element has taken on such importance in the evolution of Homo compared to Australopithecus. Big fingertips are supposed to be adaptations to force transfer through the fingertip grip used in tool manufacture.

The picture of the thing is so tiny -- I mean, literally we're talking about two pixels of finger -- that I can't make anything out of it. Does it have a large apical tuft, like OH 7? Or is it like the Hadar distal phalanges, with narrow, apelike apical tufts?

If one was wondering about whether the thing was Homo or not, I would think this is one of the first things you would check....

What about those limb proportions?

For fifteen years, a bunch of otherwise sensible paleoanthropologists have been engaged in a debate about the limb proportions of A. africanus and H. habilis. The reason why this particular question may not be sensible is because the debate is about the length of the arm relative to the leg, but there's no specimen of A. africanus that preserves both the length of the arm and the length of the leg.

What there are: OH 62, a skeleton apparently of H. habilis that has a complete humerus and more than half the length of one femur, STW 431, which has an acetabulum and mostly complete humerus, and Sts 14, which has a partial femur, an acetabulum, and a piece of distal radius. On the basis of these fossils, we've seen some intense debate about the reconstruction of the OH 62 femur length, and a lot of discussion about whether the sizes of articular surfaces are relevant to the function of the limbs. Indirectly, it has appeared that A. africanus and H. habilis shared longer arms than were present in AL 288-1 (Lucy).

Well, now we have two fossil skeletons with both hindlimb and forelimb elements. The paper doesn't address the issue directly, nor does it provide raw measuremnets that would lead to a quick answer. But the humerus is short relative to the size of the femur head, compared to earlier hominins, while a bit long relative to Homo by the same comparison. So it looks like the Malapa skeletons may be somewhere in between.

The authors do argue that OH 62 is an odd skeleton in one respect: They consider the "diaphysial strength" of the humerus and femur. This is a cross-sectional measure of the area of cortical bone, and reflects the robusticity of both forelimb and hindlimb elements. In their estimation, OH 62 has a much stronger arm relative to its leg strength than the Malapa skeletons.

It's not obvious how to interpret this observation. Is OH 62 more apelike in its locomotor pattern than Malapa? Or does the strength ratio vary allometrically with body size, and OH 62 is just at the smallest end of the comparison? Hard to tell without the length measurements.

OK, what's the bottom line?

Here's the important thing. From today forward, there are a bevy of features of the face, teeth and jaw that are no longer "derived characters" of Homo.

Some people will want to fix this by broadening the definition to Homo to include the Malapa skeletons. Others will want to narrow the definition of Homo to include only large-brained specimens.

Every specimen attributed to Homo before 2 million years ago is now up for grabs. Maybe they're Homo, or maybe their resemblances to Homo are just masticatory parallelism. We already know that parallelism explains many of the derived locomotor and masticatory resemblances of great apes, and many strongly suspect that parallelism explains the derived masticatory resemblances of robust australopithecines. If the dental reduction that once was a marker of Homo joins this list, it will hardly be surprising.

If we follow the logic that connects tool use to dental reduction, however slowly and indirectly, then I think we have to conclude that A. sediba was likely a toolmaker and meat-eater. This hypothesis is testable, both by bone chemistry and dental morphology and wear.

Malapa suggests the hypothesis that brain evolution followed the appearance of stone tool manufacture by a considerable delay. If so, I wonder what exactly caused the brain to expand. Did the diet shift to higher-quality foods unfold over a long time, allowing brains to expand only after 3/4 million year delay? Or was brain evolution caused mostly by non-dietary factors, such as social dynamics or climate instability?

Or did the evolution of our genus happen somewhere else, far from the places where we currently have fossil samples? The Rift Valley and South African cave systems may have been wonderful for preserving fossils, but who's to say they weren't relative backwaters when it came to the evolution of Homo?

Well, I'm running out of gas for this installment. More later....

References:

Berger LR, de Ruiter DJ, Churchill SE, Schmid P, Carlson KJ, Dirks PHGM, Kibii JM. 2010. Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa. Science 328:195. doi:10.1126/science.1184944

Dirks PHGM, Kibii JM, Kuhn BF, Steininger C, Churchill SE, Kramers JD, Pickering R, Farber DL, Mériaux A-S, Herries AIR, King GCP, Berger LR. 2010. Geological Setting and Age of Australopithecus sediba from Southern Africa. Science 328:205. doi:10.1126/science.1184950

Today I was looking through the online data files for the South African genome. Those online files are available from the Data Libraries entry of the Galaxy bioinformatics tool website.

I noted last week that some of the most interesting data -- in particular, the genotypes for new SNPs -- are not yet available to download ("Online toolkits -- the good and the frustrating"). But in the meantime there are some very interesting things there. In particular, the sequencing team has made available a list of amino-acid-coding mutations present in one or more of the five individuals (four Bushmen and Desmond Tutu) for whom the team obtained exome sequence.

If you look at the summary information for this list, it gives the position of amino-acid-coding mutations against the human reference genome (hg18), the position and identity of the amino acid change. It then gives a "prediction" of whether the mutation is damaging to gene function.

This kind of prediction can be very misleading. The categories of effects include "tolerated" and "damaging", but these are based on whether the site tends to be conserved in other mammal lineages, and whether the new amino acid is very different in affinity (and possible conformation) compared to the reference. There's no "beneficial" -- even though some fraction of these polymorphisms are probably retained because of selection on the mutant allele.

I say that because one of the five individuals (TK1) has an amino-acid-coding mutation in FOXP2.

Yeah, that surprised me when I found it.

As you'll remember the coding sequence of FOXP2 is pretty strongly conserved in other mammals. Two amino-acid-coding substitutions in humans separate us from other primates, an additional one separates primates from the mouse genome (Enard et al. 2002). This area of the genome looks like it had undergone a recent sweep in human populations, with relatively little variation and a strong excess of rare mutations surrounding the gene. Coop and colleagues (2008) gave a point estimate of the time of a sweep in humans as 42,000 years ago, which I wrote about at the time ("FOXP2 is really recent, it really did introgress (if it's not contamination)"). That estimate has to be massively too young -- it's not plausible that a sweep could be that recent and fixed worldwide.

Meanwhile, last year, Ptak and colleagues (2009) followed up on my suggestion that there might really have been a recent sweep, but one near FOXP2, instead of involving one of the two human amino acid substitutions. They found statistical linkage between flanking sites immediately around the gene, which would be unlikely after a fixed sweep of FOXP2 itself. That linkage is quite likely if the human-specific substitutions were already fixed, and much later another nearby site underwent a partial sweep. It remains to be demonstrated, however, what nearby site is a plausible candidate for a recent partial sweep.

So, finding variations near FOXP2 is very relevant to the history of this gene region. If there is an ongoing sweep involving some site near the gene, we should expect that some human populations haven't undergone the sweep yet, or have the selected haplotype at a lower frequency than others. The existing datasets from Africa -- mainly HapMap and HGDP sets -- are insufficient to test the hypothesis because they include only common SNP variants at low density. But sequence data from South Africa can give us a direct estimate of the nucleotide diversity around FOXP2, thereby letting us test for the presence of a recent sweep.

The amino acid coding variant in one of these Bushman genomes came to me as a total surprise. Using the alignment with hg18, the location of the mutation is at position 114089380 on chromosome 7. The mutation changes a leucine in the wild-type sequence to a proline in the mutant, and the algorithm classifies it as "damaging" -- probably because the two residues are very different in their hydropathy. This position is not one of the two human-specific amino acid substitution sites. In fact it is in the forkhead box domain of the protein itself, which is the DNA-binding motif. Without going further into the biochemistry, I really can't guess what the effect of the mutation would be. I'm not really sure it's relevant -- after all, if it is a singleton in the population it might well be a recessive with no effect on the carrier phenotype.

Still, the mutation could be common in the Bushman population. Our point estimate of the mutation's frequency is one in eight. Maybe it's a new variant that confers some advantage; maybe it's a result of a founder effect tens of thousands of years ago. It could even be widespread within Africa. We won't know until we have more genomes.

The mutation is not in any of the regions sequenced by Krause and colleagues (2007) in the Neandertals from El Sidrón. I wouldn't expect it to be there -- as a derived variant, it would be unlikely to evolve in parallel in Neandertals and southern African populations. But who knows what else we'll find?

References:

Coop G, Bullaughey K, Luca F, Przeworski M. 2008. The timing of selection at the human FOXP2 gene. Mol Biol Evol 25:1257. doi:10.1093/molbev/msn091

Ptak S, Enard W, Wiebe V, Hellmann I, Krause J, Lachmann M, P&aauml;&aauml;bo S. 2009. Linkage disequilibrium extends across putative selected sites in FOXP2. Mol Biol Evol 26:2181-2184. doi:10.1093/molbev/msp143

Krause J, Lalueza-Fox C, Orlando L, Enard W, Green RE, Burbano HA, Hublin J-J, Bertranpetit J, Hänni C, Fortea J, de la Rasilla M, Rosas A, Pääbo S. 2007. The derived FoxP2 variant of modern humans was shared with Neandertals. Curr Biol 17:1-5. doi:10.1016/j.cub.2007.10.008

Enard W, Przeworski M, Fisher SE, Lai CSL, Wiebe V, Kitano T, Monaco AP, P&aauml;&aauml;bo S. 2002. Molecular evolution of FOXP2, a gene involved in speech and language. Nature 418:869-872. doi:10.1038/nature01025

Schuster SC and many others. 2010. Complete Khoisan and Bantu genomes from southern Africa. Nature 463:943-947. doi:10.1038/nature08795

Oh, good grief!

[post UPDATED]

I have had an unusual number of hits the past few days, so I went through my logs looking for the source. Turns out people are reading my 2008 review of the "Boskops race"("The 'amazing' Boskops").

Over 10,000 people have read that post since the New Year began. That post has always gotten a recurring readership, because of a 2008 book by Gary Lynch and Richard Granger, Big Brain: The Origins and Future of Human Intelligence.

Evidently the book is about to come out in paperback. And Discover magazine, which gave the book a fairly positive review on its release, has now reprinted an excerpt detailing the wondrous features of the Boskops race ("What Happened to the Hominids Who Were Smarter Than Us?"). Someone copied the whole thing to Richard Dawkins' website. And people reading the excerpt are trying to find out more about this fantastic story, and finding my blog.

Well, to all those seeking the light of paleoanthropology, welcome!

To those who have linked the post: I want to let you all know that your links have directed more than 10,000 people to find some actual true information about the "Boskop race". Good work out there!

What can I do to update people, now that this story is spreading once again? My original post gives a short history, but was not based on a real review of the book. I was just trying to get some accurate information out there.

Now I have read the excerpt, and much (but not all) of the Boskop-related text in the book (courtesy of Amazon).

It's worse than I feared. The excerpt actually presents 1920's-era anthropology as if it were the state of our knowledge about Boskop and the "Boskop race" today. I have not found any passages in the book or chapter notes that contradict the excerpt's portrayal. I cannot find references or citations of post-1940 research on skeletal remains or archaeology from southern Africa. There's no hint of what happened after archaeologists began to use radiocarbon dating, nor do we hear even the identity of any specimens, except for the original (and fragmented) Boskop skull itself.

How can this be? From the book's notes, it appears that the authors didn't find any information on these topics:

One of the oddities in the Boskop story is the disconnect between the rich trove of references from the early twentieth century, and the paucity of references after that time (Lynch and Granger 2008: 218).

I find that very sad, because there is a much richer trove of references after 1958. Archaeologists have developed a deep understanding of the chronology and material culture of LSA and later hunter-gatherers around the Cape and northward. Skeletal biologists have studied the health status, demography, and morphology of Holocene and earlier peoples. Some have even examined the endocranial volumes of southern African skeletal samples, and have tested the hypothesis of trends in brain size over time.

All this work shows a very different picture than that sketched by Lynch and Granger.

I'm going to be very measured, because while I am often snarky, I rarely come straight out and write that something is bunk. The portrayal of "Boskops" in the Discover excerpt is so out of line with anthropology of the last forty years, that I am amazed the magazine printed it. I am unaware of any credible biological anthropologist or archaeologist who would confirm their description of the "Boskopoids," except as an obsolete category from the history of anthropology.

[UPDATE (2010-01-04): I have heard from Amos Zeeberg, the Web editor at Discover. He writes that the excerpt was intended to run identified as a "controversial idea, but that context didn't come across as intended." The web page has been changed to make that context clear, and to link to my discussion here. I think it's great that he responded so quickly, although I think that this case is not controversial, it's non-science. ]

Besides that, the authors make several questionable statements about the relative sizes of parts of the brain and their relation to cognition and behavior in ancient hunter-gatherers.

IQ of fossils

We have no credible way of estimating the IQ of a fossil skull. The excerpt claims:

Even if brain size accounts for just 10 to 20 percent of an IQ test score, it is possible to conjecture what kind of average scores would be made by a group of people with 30 percent larger brains. We can readily calculate that a population with a mean brain size of 1,750 cc would be expected to have an average IQ of 149.

First of all, there never was any human population with a 1750 cc average brain size.

Now, taking the counterfactual: A regression equation within a population can predict an expected value for an individual within that population. But in population genetics, the average IQ that we would predict for a population with a 1750 cc average, depends on how the brain got to be that size. Natural selection on intelligence or brain size would have altered the relation that holds within humans. Nor do we know whether the present-day correlation would have characterized any ancient population -- or indeed most living human populations. The current value in Europeans may be an artifact of Holocene genetic changes.

The authors do not list the specific regression that they use, or its source. The correlation relates to the proportion of variance explained by the relation of brain size and intelligence is irrelevant to this prediction. What we want to know is the slope of the regression. The prediction here would require a slope of 0.14, assuming it had been derived from a population with a mean male volume of 1400 cc and an average IQ of 100. That's a higher slope than I've seen reported in any analysis of the brain size - IQ relationship.

The "inconceivable" prefrontal cortex

We know little about the relative sizes of cortical areas in fossil hominins. The excerpt claims that the prefrontal area of a Boskop must have been "inconceivably large"

Going from human to Boskop, these association zones are even more disproportionately expanded. Boskop’s brain size is about 30 percent larger than our own—that is, a 1,750-cc brain to our average of 1,350 cc. And that leads to an increase in the prefrontal cortex of a staggering 53 percent. If these principled relations among brain parts hold true, then Boskops would have had not only an impressively large brain but an inconceivably large prefrontal cortex.

First of all, there was never any human population with an 1750 cc average brain size.

Again, the example is a misapplication of regression, in this case an among-species regression. The excerpt appears to assume that the evolution of relative prefrontal area among human populations must have followed the same disproportionate pattern of increase as that between humans and chimpanzees. Prefrontal cortex volume is larger, relative to brain size, in humans compared to other primates. But this relation is not very much larger in humans -- recent estimates range from less than 10 to 30 percent compared to chimpanzees (Holloway 2002, Schoenemann et al. 2005). Even if some ancient humans had a second burst of expansion, again as great as that on the hominin lineage leading from apes to us, their prefrontal volume would hardly be "inconceivably large".

And there's no reason at all to assert such a second, bonus expansion of prefrontal area in ancient humans. The prefrontal area ought to scale close to the total brain size, as it does within living people.

Science fiction

The authors actually cite and discuss Loren Eiseley's Immense Journey, which I discussed in my earlier post. Eiseley was a naturalist/anthropologist/science writer, and a very popular essayist -- he's the kind of person we could use more of today. But his reflections on the "Boskop people" were a fictional trope -- and were already, in 1958. He was a great writer, but relying on Eiseley for up-to-date information on anthropology is like relying on Truman Capote as an authority on crime.

Suppose that we take the "Boskops" story just as a science fiction fairy tale -- a story showing that evolution is not synonymous with progress, as the authors imply. I still conclude that much of the other information about brain size in the excerpt is questionable or false.

The authors speculate:

Our big brains give us such powers of extrapolation that we may extrapolate straight out of reality, into worlds that are possible but that never actually happened.

That's Boskop, all right. Extrapolated straight from worlds that never happened!

References:

Broom R. 1918. The evidence afforded by the Boskop skull of a new species of primitive man (Homo capensis). Anthropol Pap Am Mus Nat Hist 23 (2):63-79.

Brothwell DR. 1963. Evidence of early population change in central and southern Africa: Doubts and problems. Man 63:101-104.

Dart R. 1923. Boskop remains from the south-east African coast. Nature 112:623-625.

Dubow S. 1996. Human origins, race typology and the other Raymond Dart. African Studies 55:1-30.

Henneberg M, Steyn M. 1993. Trends in cranial capacity and cranial index in Subsaharan Africa during the Holocene. Am J Hum Biol 5:473-479.

Holloway RL. 2002. How much larger is the relative volume of area 10 of the prefrontal cortex in humans? Am J Phys Anthropol 118:399-401. doi:10.1002/ajpa.10090

Pycraft WP. 1925. On the calvaria found at Boskop, Transvaal, in 1913, and its relationship to Cromagnard and Negroid skulls. J Roy Anthropol Inst 55:179-198.

Schauder DE. 1963. The anthropological work of F. W. FitzSimons in the Eastern Cape. S Afr Archaeol Bull 18:52-59.

Semendeferi K, Armstrong E, Schleicher A, Zilles K, Van Hoesen GW. 2001. Prefrontal cortex in humans and apes: a comparative study of Area 10. Am J Phys Anthropol 114:224-241.

Singer R. The Boskop "race" problem. Man 58:173-178.

Singer R. 1962. Presidential Address 1962: The South African Archaeological Society: The future of physical anthropology in South Africa. S Afr Archaeol Bull 17:205-211.

Stynder DD, Ackermann RR, Sealy JC. 2007. Craniofacial variation and population continuity in the South African Holocene. Am J Phys Anthropol 134:489-500. doi:10.1002/ajpa.20696

On occasion, I point out interesting findings from archaeological chemistry and microscopic study of site formation processes. Last month, I pointed to the ability to distinguish animal and plant fat residues on ancient artifacts. Before that, there was the discovery of flax fibers from the Upper Paleolithic of Dzudzuana Cave, Georgia.

In July, a paper by Paul Goldberg and colleagues described the "micromorphology" of the sediments from Middle Stone Age levels of Sibudu Cave, South Africa. The excavations at Sibudu have been able to distinguish many distinct stratigraphic units with distinctive spatial locations and compositions. Micromorphology involves looking at these sediments in microscopic detail, picking out small grains of crushed bone, charcoal, plant fibers, phytoliths, and other materials.

Goldberg and his colleagues were able to make some very cool observations. For one thing, they have charred drips of broiled grease:

Two types of amorphous organic combustion remains were identified in samples from Sibudu: a type with a typically vesicular texture and a type with a cracked texture. The first type was found as isolated bodies, subrounded with a diameter of 10 µm to 1 mm, and they exhibited no evidence of cell structure. Bubbles or vesicles give the bodies a highly porous nature, and they are often thin walled. The microstructure of these homogenous or finely heterogeneous isotropic particles and their droplet-shaped occurrence suggest that these bodies were originally fluid and that they underwent a degassing process but have since hardened. These bodies resemble char and are probably derived from the burning of flesh or animal fat (104-105).

Mmmmm...MSA barbecue. The other type of "amorphous organic combustion remains" are charred resins, from trees or seeds. Mmmm...MSA smokehouse.

Second, they have beds.

Because of its long fibrous nature, it seems that this material consists of herbaceous plants, possibly some type of sedge, reed, or grass. There is no evidence to suggest that this plant would have grown naturally in the rock shelter, and the presence of clay aggregates derived from the river valley found in association with the laminated plant fibers implies that the grass or reed was transported to the cave from the nearby Tongati River by the shelter’s inhabitants.

The compact and laminated structure of the organic fibers in this microfacies also suggests that, once brought to the cave, the grass or reed was subjected to compaction, most likely through trampling. Further evidence supporting the interpretation of trampling is seen in the stringers of charcoal, clay aggregates, and burnt bone that define horizontal and subhorizontal surfaces on top of and within the laminated organic fibrous material. Pieces of éboulis and lithic fragments also define surfaces wi thin the microfacies. The fact that this grass or reed was transported to the cave by humans and that once there it was influenced by human trampling suggests that this microfacies represents a type of constructed bedding. If this is the case, then Sibudu contains the oldest evidence for constructed bedding, significantly older than that reported at the open-air site of Ohalo in Israel (Nadel et al. 2004).

The bedding material was in some instances burned, in some instances swept or trampled in such a way that the regular alignment of the phytoliths was jumbled and disrupted. They interpret this as efforts to "maintain" the site -- in other words, housekeeping:

What seems a likely and reasonable scenario is that the original organic matter of this laminated layer of sedges, grass, or reeds was completely combusted, resulting in total ashing of the organic material. The calcitic ash in this microfacies was transformed through phosphatization, as evidenced by the presence of a few remnant pockets of phosphate in this microfacies. The fact that large crystals of gypsum often form directly below these phytolith layers provides suggestive evidence for the downward leaching of CO₃- or P-rich solutions.

Just an aside -- that is such interesting chemistry, like the organic materials and ash are melting down into the underlying deposits.

The association between microfacies 2 and 4 suggests that the sedges, grass, or reeds that were brought into the cave for bedding were usually burned and probably by humans when they no longer used the bedding. This observation explains the sequence seen in samples SS-6 and SS-5 of laminated nonburnt fibrous organic material grading into laminated burnt fibrous organic material with phytoliths (microfacies laminated type 2B); the sequence is finally capped by a layer of laminated phytoliths.

Why did they burn the stuff? The authors guess that they were trying to cut down on parasites:

Together, this evidence shows that not only were the occupants of Sibudu bringing grass or reeds into the cave—likely for the construction of bedding—but they were periodically burning them, possibly as a means to remove pests or insects that had colonized the beds. (Smoldering goat dung and organic matter can be observed in many parts of the Middle East, including Hayonim, where tick removal is one of the important objectives; P. Goldberg 1992, personal observation.)

The MSA at Sibudu dates to between 45,000 and 65,000 years ago, with the best evidence for bedding in the units that OSL puts around 50,000 years ago. The implications of the "site maintenance" and spatial characteristics of the site are mentioned in the paper's conclusion:

Organization of living space, and particularly a deliberate use of space, has been suggested by Wadley (2001) and also Binford (1996) as an important trait of culturally modern behavior, reflecting a more complex social organization. While the evidence from the laminated units at Sibudu may reflect such organization, the lack of evidence for such spatial organization, such as is the case for the lower homogeneous layers at Sibudu, should not automatically suggest that occupation in these units was any less complex.

If spatial organization of living space is a "modern" behavioral feature, it is one shared by Neandertals (I noted that briefly in a 2006 post). But then, it's shared by any number of invertebrates, also. I think the interpretation of this kind of behavior will have to wait until we have more sites investigated with comparable methods. As the introduction to the current paper points out, a lot of spatial information could be brought out of these micro-scale studies, if they were conducted routinely.

References:

Goldberg P, Miller CE, Schiegl S, Ligouis B, Berna F, Conard NJ, Wadley L. 2009. Bedding, hearths, and site maintenance in the Middle Stone Age of Sibudu Cave, KwaZulu-Natal, South Africa. Archaeol Anthropol Sci 1:95-122. doi:10.1007/s12520-009-0008-1