Sahelanthropus

The rate of neutral mutations varies across the genome. When studying a single gene, this variation in rates is not especially important -- it is generally possible to obtain an estimate of the neutral rate for a single locus by comparing just that locus among closely related species.

But some comparisons involve looking at the pattern of variation among different loci. For instance, testing hypotheses about the ancestral populations leading to living species (like the common ancestor of humans and chimpanzees) involves comparing the amount of divergence among many independent loci. The variance in divergence times among loci gives an estimate of inbreeding in the ancestral population.

I discussed this particular example two years ago this week, after the paper that proposed extended hybridization between ancestral hominids and chimpanzees. The conclusion of the paper was that the X chromosome displays much less divergence between humans and chimpanzees than the autosomes, and this might reflect a late introgression of the X chromosome into hominids from another population that (mostly) was ancestral to chimpanzees. The autosomes, by contrast, averaged very old genetic divergences, although there was substantial variance. As I concluded then, the data look consistent with a large population size in the human-chimpanzee ancestor species, coupled with greater selection on the X chromosome. The interpretation of large population size (or alternatively, the interpretation of long-term population structure) comes from the low inferred inbreeding in that ancestral population -- which caused the variance in divergence dates among loci.

But there is another reason for a large variance in divergence dates: variance in mutation rates. Whenever mutation rates vary among loci, this variance adds to the variance among loci in their between-species genetic differences -- that is, the substitution rate. And as long as we are excluding selected sites (as we always try to do for these kinds of comparisons) we will overestimate the genetic diversity in ancestral species whenever the mutation rate varies among loci.

A new paper by Svitlana Tyakucheva and colleagues looks at human and macaque genomes to find patterns underlying the variance in mutation rates among regions of the genome. They find that a number of factors may cause such variations, including chemical factors like the CG content of the genome, functional causes such as male versus female rates of recombination, and large-scale structural causes such as telomeric proximity:

While a complete understanding of all biological mechanisms leading to variation in neutral substitution rates across the genome remains elusive, it is plausible that at least some of these mechanisms are conserved over relatively long evolutionary distances. For instance, both mouse-specific and rat-specific substitution rates are positively correlated with rodent-primate substitution rates [14], suggesting shared mechanisms persisting over ca. 90 million years [15]. Additionally, a positive correlation exists in substitution rates of homologous X- and Y-chromosomal introns that diverged from each other ca. 100 million years ago [16] (Tykucheva et al. 2008: R76).

Their finding that male recombination is an important contributor to mutation rate heterogeneity puts the focus on the X chromosome -- which has little recombination in males -- as unusual. X versus autosomal position did not explain a large fraction of the variance in this study (only around 2 percent, controlling for other factors) but the deviation was in the right direction to help account for the low X chromosome divergence between humans and chimpanzees.

Altogether in this study, a large fraction of variation in the human-macaque substitution variability could be explained by phenomena that affect the rate of mutations, including the structural and functional factors listed above as well as the corresponding homologous variability between mice and rats, and dogs and cattle. If these variations were explained by inbreeding in the human-macaque ancestral species, they would be random with respect to the dog-cow or mouse-rat divergences, and with respect to structural causes. So current estimates of the effective sizes of human-chimpanzee and other ancestral populations are almost certainly inflated. The amount of inflation is not clear, but a good estimate will require correcting for a large number of factors -- a complicated analysis.

Since the date of the human-chimpanzee divergence depends on our assessment of the diversity within the human-chimpanzee ancestral population, it may be a while before we can settle the issue of human-chimpanzee divergence time. That may or may not provide hope for Sahelanthropus, Orrorin, and Ardipithecus kadabba -- all supposed hominids that would predate 5 million years ago, the current best genetic estimate of the human-chimpanzee divergence time. To be sure, if the date is simply in error, that error might encompass older dates consistent with a 7-million-year divergence. But I'm not sure we should believe that the error is biased toward an older divergence -- "error" might lean in either direction, and a younger species divergence remains possible.

References:

Tyakucheva S, Makova KD, Karro JE, Hardison RC, Miller W, Chiaromonte F. 2008. Human-macaque comparisons illuminate variation in neutral substitution rates. Genome Biol 9:R76. doi:10.1186/gb-2008-9-4-r76

There's nothing especially surprising about the functional interpretations in Richmond and Jungers' paper about the Orrorin BAR 1002'00 femur. They conclude it was an australopithecine-like biped, because it shared several features with australopithecine femora: in particular, it has a long, narrow, anteroposteriorly flattened neck and a broad thick proximal shaft.

In this, they mirror the conclusions of the original description of the Lukeino fossils by Senut et al. (2001). Richmond and Jungers also reiterate the evidence for arboreality in the Lukeino fossils, including the well-developed musculature of the distal humerus and the chimpanzee-like curved finger bone. I wonder why their analysis could not have made something more out of the other two femoral fragments, one of which is fairly large (but lacking the head). Still, the paper reiterates the quite good evidence for bipedality in the most complete femoral specimen.

I wonder sometimes how closely people actually read the papers they comment on. The associated coverage, including Ann Gibbons' article, has made a lot out of a small point in the paper, but I think that the commenters have it wrong.

Here's the story: When the Orrorin materials were first published, Brigitte Senut and Martin Pickford put forward the argument that these may be more closely related to Homo than to known australopithecines. They based their argument mainly on Orrorin's relatively thick-enameled molars, which they viewed as different from the thin-enameled molars of Ardipithecus, but lacking the enlarged dentition of Australopithecus. So, they suggested that Orrorin might be a plesiomorphic ancestor of Homo, and that Ardipithecus and Australopithecus represent divergent lineages derived in their dental anatomy.

I don't find that suggestion very compelling, because it seems to put too much faith in the absence of evolutionary reversals. There's no reason why a large-molared australopithecine should not have given rise to small-molared Homo, particularly since smaller-toothed Homo habilis is apparently derived from earlier, larger-toothed "Homo" specimens like A. L. 666-1 and Omo 75-14. And Haile-Selassie, Suwa and White (2004) claimed that the Orrorin, Sahelanthropus, and Ardipithecus dentitions were so similar that they might represent one taxon. So the dental contrasts among these early hominids are probably not great enough to justify the idea that Orrorin is an exclusive Homo ancestor.

The femur also formed a part of this phylogenetic story, with Senut and Pickford having noted the lack of extreme australopithecine-like features in the femur. The Orrorin femur has a less exaggerated neck length than many australopithecine specimens, it is larger than many, and appears to have a higher neck-shaft angle. To the extent those features differ from later Australopithecus, they resemble the human anatomy.

Richmond and Jungers address this argument very briefly in their last paragraph, by noting that the functional elements of the Orrorin femoral anatomy are entirely consistent with the australopithecine pattern of bipedality:

The similarity between O. tugenensis and australopith femora weakens support for scenarios in which O. tugenesis is ancestral to Homo to the exclusion of A. afarensis (4). Instead, the overall primitive hominin morphology of the O. tugenensis femur, along with primitive dental anatomy, is consistent with the more parsimonious hypothesis that it is a basal member of the hominin clade.

I think that's fair, as far as it goes. The overall morphological pattern of this femur, with its long neck and broad shaft, is much like known australopithecine femora. But to go a bit further, their metric comparisons show BAR 1002'00 to be the most Homo-like of the early hominid femora they examined, and their phenetic cluster puts it basal to the other australopithecines. That's pretty much exactly what Senut et al. have consistently said. So I have a hard time understanding how those observations refute the idea that Orrorin has a more Homo-like femur than later australopithecines!

Again, I don't put much stock in the phylogenetic argument for an Orrorin-Homo link. I don't see any difficulty deriving Homo from Australopithecus, especially given the likely effects of body size evolution on the locomotor pattern. And at least one or two early Homo femoral specimens, like KNM-ER 1481, share most of the Australopithecus-like pattern of proximal femur anatomy. But this paper surely doesn't add anything new to the critique of Senut and Pickford's preferred phylogenetic hypothesis. The details simply don't detract from their story.

References:

Richmond BG, Jungers WL. 2008. Orrorin tugenensis femoral morphology and the evolution of hominin bipedalism. Science 319:1662-1665. doi:10.1126/science.1154197

Gibbons A. 2008. Millennium ancestor gets its walking papers. Science 319:1599-1601. doi:10.1126/science.319.5870.1599

Haile-Selassie Y, Suwa G, White T. 2004. Late Miocene teeth from Middle Awash, Ethiopia, and early hominid dental evolution. Science 303:1503-1505.

Senut B, Pickford M, Gommery D, Mein P, Cheboi K, Coppens Y. 2001. First hominid from the Miocene (Lukeino Formation, Kenya). C R Acad Sci Paris, Sciences de la Terre et des planètes 332:137-144.

It's that time of year again -- the time when those boring ``Year in Review'' magazines are on newsstands, and when pundits make fools of themselves predicting what will happen in the next year.

Well, I'm not too proud to join the fools, as I've shown the last two years. In 2006, I got five predictions right out of ten. Not bad for my first outing, but you'll see that last year's predictions fared even better:

• 10. Sahelanthropus postcrania will be published. I'm frankly shocked that this didn't happen. I don't doubt the rumors, but I'm starting to wonder whether this story is more interesting than it looks....
• 9. Two words: Holocene evolution. OK, this was a little unfair, considering that my work was an important part of making this prediction come true. Still, Discover made ``recent human evolution'' one of its top 100 science stories of the year, even before our December paper came out -- mainly on the strength of the paper by Scott Williamson and colleagues from earlier this year. And "Human genetic variation" was Science's "Breakthrough of the Year" -- most of that variation representing recent evolution.
• 8. Despite (or because of) the success of the Neandertal genome project, there will be no genetics of any kind published on early modern skeletal material. Puzzling, isn't it? But then, considering the trouble with Neandertal contamination reported in August, maybe we're better off leaving the early Upper Paleolithic alone for a while.
• 7. The mitochondrial history of human dispersals will become more and more detailed, but no paper will test against other loci. D'oh! Reading this one a year later, it's pretty obvious that I should have included Y chromosome in this one, since those two get compared all the time! Proofread, Hawks!
• 6. Another (yes, another) paper about the chimpanzee-human divergence will peg it between 5 and 7 million years ago. Will they never tire of these? Hobolth et al. (2007, PLoS Genet 3:e7) pegged the divergence at 4.1 million years. That's too recent to fit my prediction. Instead, I have to turn to Ebersberger et al. (2007, Mol Biol Evol 24:2276), who placed the divergence at 5.7 million years ago. Both estimates are too recent for Sahelanthropus, which the geneticists have started to figure out....
• 5. Three papers with new Ethiopian fossils. The last few years, one annual Ethiopian find seemed to be predictable enough. So I figured, why not three? We got a not-nearly-noted-enough paper this summer by Gen Suwa and colleagues descringing the Konso Homo erectus remains. Then, Suwa brought us Chororapithecus -- hey, I didn't say "hominid!" That's two. But despite the long-ago announcement of the Woranso-Mille skeleton, its appearance in a meetings abstract and a mid-summer press release about further Mille fossils, all we got from the peer review system is a lousy faunal list. Well, the faunal list does include the hominids. Should it count as a "paper with new Ethiopian fossils?" I'll say yes -- hey, unlike Aramis, at least the Mille fossils are new!
• 4. Another early Upper Paleolithic specimen will emerge from a museum collection. The only bizarre thing about this one was the location: South Africa. Hoffmeyr may not be that convincing as a European early Upper Paleolithic skull, but it was sure sold that way. Weird.
• 3. A big year for Miocene apes, which will look increasingly important in the story of human brain evolution. No brains, but it sure was a big year for Miocene apes, with two significant East African discoveries claiming to push back the timeline of African ape divergence.
• 2. Maturation rate in early Homo becomes a dead issue, because of the variation in dental and skeletal maturation in living people. Wishful thinking. Still, did Tanya Smith (2007) breathe new life into perikymata? Let's just say that unresolved questions remain.
• 1. The year will end without a single new hominid species having been named. This one was like dodging a bullet, since new species riffle out of paleoanthropologists' minds all the time. From 2001 to 2006, there were six (six!). In 2007, none.
• BONUS: A dramatic development in the problem of pre-2.0-million-year-old Homo. Rats.

OK, that's seven out of ten. It's beyond belief that I did better in the top five than the bottom five -- I picked those because they were far out there. I mean, really -- a new Upper Paleolithic specimen from a museum collection? After Muierii, that's like calling lightning to strike twice. But there it is, and in January, no less.

I'm clearly going to have to pick stranger predictions this year. And I'll have to be careful about that "dramatic development" line -- I mean, it's appropriately Delphic, but what is it supposed to mean, really? I wonder whether "operatic development" might be better.

And do I dare call down my non-lightning strike for a third year? It's ruining my percentage! It's starting to reek of desperation -- I mean, it starts to look like the stopped watch effect even if it happens.

Oh, well. I mean, those are just the risks of predictions, right? Suppose in the preseason I had picked Kansas to win the Orange Bowl!

• 10. A dramatic development in the Sahelanthropus story.
• 9. Both major-party candidates for the 2008 U.S. Presidential election will accept evolution.
• 8. This year's featured piece of anatomy: the femur.
• 7. No new hobbits, at least, not from Flores.
• 6. An incisive example of introgression in East Asia.
• 5. A viral insertion in the human genome will tell us about a disease of the australopithecines.
• 4. Another language gene joins FoxP2. No word on whether Neandertals have the human version.
• 3. Homo habilis: an endangered species?
• 2. This year, something new from three A's: A. afarensis. A. africanus. Atapuerca.
• 1. Oh, and one more A. Ardipithecus.
• BONUS: A big, big year for Neandertals. I mean, besides the election.

There you have it. I'm not sure which of these is the riskiest, but I'm sure they're more out on a limb than last year!

That depends on whether these teeth are really from a gorilla, I suppose.

Chororapithecus abyssinicus teeth compared to gorilla mandible. Photo credit: Gen Suwa/University of Tokyo.

Oh yeah, sure, "saved paleoanthropology" is overdramatic. But what am I supposed to write? Over four years, we have had a series of genomic comparisons narrowing down the age of the human-chimp common ancestor to something like 2/3 the age of Sahelanthropus. I said it was a crisis, and it is: these data sources must agree. Either we have to cast out a bunch of hominids, or we have to wrench the genes by around a factor of two.

Now, Suwa and colleagues show up with a 10-million-year-old gorilla. A 10-million-year-old gorilla works just fine with 7-million-year-old hominids. It doesn't work at all with a 7-million-year-old human-gorilla common ancestor. So there's no doubt about the centrality of this particular ancient gorilla -- if it is one.

So far, all the articles I've seen have someone on the record expressing some reluctance to accept the teeth belonged to the gorilla lineage. Reuters has Peter Andrews; Nature has Jay Kelley; National Geographic has Richard Potts.

Should we be skeptical? Well, there are lots of convergences among Miocene apes. Many of the dental convergences are detailed in our paper about Sahelanthropus, available open-access from PaleoAnthropology. These convergences make it difficult to identify hominids based on the teeth alone. They also make it hard to say that any particular big-toothed, leaf-eating ape is definitely a gorilla. After all, if it eats like a gorilla, why shouldn't it have teeth like a gorilla?

Suwa and colleagues go to some pains to demonstrate that the dental similarities with gorillas are more than enamel-deep. Their strongest argument is that the tooth morphology exhibits a derived gorilla-like condition well below the surface, at the enamel-dentine junction. That is, while the tooth was forming, the initial growth surface took on a distinctive shape which was then reflected by the form that the growing enamel took.

The most distinctive features of the Chororapithecus dentition are the derived shearing structures seen in portions of its molars (Fig. 2), despite a generally low cuspal topography (the latter is apparently a primitive retention).

Examination of internal morphology by micro-computed tomography (micro-CT) demonstrates that these occlusal features were underlain by distinct enamel-dentine junction (EDJ) structure (Fig. 2). In particular, the straight to weakly concave mesial protocone crest seen in the EDJ of CHO-BT 4, -BT 5 and -BT 6 is gorilla-like, and is formed by a mesiobucally located junction of the mesial protocone crest and mesial marginal ridge. Such spatial placements are best considered to be regulated by enamel-knot-related signalling patterns during early morphogenesis [23, 24], and may be one of the underlying causes of the mesiodistally elongate upper molar shape generally characteristic of folivorous primate species. In the lower molars, the most distinctive EDJ topography occurs at the trigonid crest, the structural counterpart that occludes with the upper molar mesial protocone crest. The high trigonid EDJ crest is continuous between the metaconid and protoconid cusp tips (Fig. 2). Because recent experimental and quantitative genetic studies suggest significant degrees of morphogenetic independence between corresponding upper and lower molar structures [25, 26], the presence of a functionally integral inter-jaw pattern of morphological expression, as seen in the Chororapithecus molars, suggests adaptation by natural selection, as opposed to chance emergence of neutral morphological minutia.

Still, "minutia" is a loaded term. Why shouldn't an ape that evolves the same shear characteristics as a gorilla molar use the same developmental process to achieve them? The more that development of the teeth are constrained by these genes, the more likely it is that different lineages will evolve in parallel.

Nor is it entirely obvious that Chororapithecus is actually gorilla-like in these characters. The paper compares two ratios involving cusp dimensions measured internally beneath the enamel cap. That's high-tech, but the ratios do not sort out gorillas from chimpanzees, don't sort Chororapithecus from either of those apes or early hominids, and -- even worse -- it's not even clear how these ratios may vary with size. Does Chororapithecus look sort-of like a gorilla on these ratios because it's a sort-of gorilla? Or because it's big? The enamel is relatively thicker than gorillas, like other Miocene apes and orangutans. Clearly the specimen is much less derived than gorillas, but could that be because it isn't a gorilla?

Well, there's the problem: there's not too much to go on with these teeth. I think Suwa et al. laid out as good a case as there is. A 10-million-year-old gorilla can't be expected to look just like gorillas today. It's not like the teeth look more like something else besides a gorilla. Gorillas really are far more derived in these dental characters than the Chororapithecus dentition, which makes the comparison more difficult. And so, the conclusion of the paper is equivocal:

The similarities seen between the two genera raise the possibility that Chororapithecus is a Miocene member of the Gorilla clade. Alternatively, with its combination of thick enamel and distinct molar cresting pattern, Chororapithecus may represent a unique adaptation that is convergent with gorillas in molar structure and function. Although the evidence for phylogenetic affinity between Chororapithecus and Gorilla is inconclusive, it may be that the basal members of the gorilla clade shared large tooth size and incipiently enhanced molar shear as a part of an herbivorous diet that accompanied (presumed) larger body size. Chororapithecus may then represent one example of adaptational (and perhaps phyletic) differentiation within that clade.

I don't know about anybody else, but I don't think this helps us with our little problem very much. Here's what I think: the problem is not so much the 10-million-year-old gorilla, as it is the 17-million-year-old orangutan that it necessitates. Here's the very next paragraph of the paper:

Acceptance of Chororapithecus as a basal member of the gorilla clade would push back the gorilla species split to >10.5 Myr ago. Because this is a minimum date established from a meagre fossil record, the actual divergence would have predated this by an unknown time gap. From the currently available evidence, we consider that a species split of 20 Myr ago for Pongo, 12 Myr ago for Gorilla, and 9 Myr ago for Pan are all probable estimates (see Supplementary Information). We consider that the early divergence hypothesis is congruent with both fossil and molecular data, and should be further evaluated using both sides of the evidence.

I think those dates don't really need to be so old. A 10.5-million-year gorilla divergence could easily correspond to a 17-million-year orangutan divergence. Still, for those of us who have gotten used to the idea that Dryopithecus might have something to do with the origin of African apes, this idea might seem a little troubling. So, let's look at the part of the Supporting Information that, well, supports their assertion that all these dates are "congruent":

The above summarized molecular predictions are in concert with the notion that the Pongo lineage existed in Africa prior hominoid migration to the Eurasian continent, the earliest such opportunity for dispersal (barring significant rafting) being at circa 17 Ma (44). If in fact the Gorilla split was 12 Ma, then the OWM split estimate (33.6-43 Ma) largely predates the earliest known definitive occurrence of catarrhines (Propliopithecus and Aegyptopithecus) (45), and many would consider this to be somewhat outside an acceptable boundary condition. However, it may be indicative of variable molecular rates of evolution across lineages (46, 47), with higher mutation rates in the OWMs (48) (and early hominoids) because of their shorter generation lengths (48, 49) and/or higher metabolic rate in relation to smaller body mass (50).

Well, that's a tricky bit of argument. We might believe that African apes never left Africa and that all the dryopithecines are therefore on the orangutan line. At least, that makes some biogeographic sense. But it's hard to argue that any of these dates are "congruent" with genetic evidence as we currently understand it. Many of the recent methods don't make any prior assumptions about "calibrated" divergence times like the orangutan-human divergence. Worse, Hobolth et al. (2007) found a human-chimp speciation time of 4 million years even considering an orangutan-human divergence of 18 million years.

The "shorter generation lengths" explanation doesn't help much -- after all, if we infer that the current great ape lineages existed as early as 20 million years ago, then almost all of the divergence time is occupied by long-generation-length species. Much faster evolution in Old World monkeys should show a strong signal of acceleration in that lineage (with a higher number of derived substitutions), and we don't see it.

If we believe these interpretations of the genes, a 10-million-year-old gorilla did not exist. Chororapithecus was something else.

If we believe that Chororapithecus was a gorilla, then these genetic interpretations are simply wrong. And Dryopithecus was on the orangutan lineage. And hominoids diverged from Old World monkeys in the Eocene.

And Sahelanthropus could have been a hominid.

References:

Suwa G, Kono RT, Katoh S, Asfaw B, Beyene Y. 2007. A new species of great ape from the late Miocene epoch in Ethiopia. Nature 448:921-924. doi:10.1038/nature06113

Hobolth A, Christensen OF, Mailund T, Schierup MH. 2007. Genomic Relationships and Speciation Times of Human, Chimpanzee, and Gorilla Inferred from a Coalescent Hidden Markov Model. PLoS Genet 3(2): e7. doi:10.1371/journal.pgen.0030007

Out of this week's Science Times special on evolution, I clicked into John Noble Wilford's article first, titled "The Human Family Tree Has Become a Bush With Many Branches".

Now, I don't know about you, but that seems like a boring headline to me. They've been talking about human evolution being a bush for going on 20 years now. It was an old idea when I was in graduate school. So it seems like, if this is all we have going on, the "new frontier" of paleoanthropology must be pretty dull.

The writer doesn't write the headlines, and the headline doesn't describe Wilford's story, which is basically a verbal slide show of fossil discoveries over the last decade or so. Some bone pictures (of the actual species discussed) accompany the article, and it's a good enough sort of account of new finds since 1990, framed around the tension between fossil finders and molecule mavens.

But I'll be a little critical. The thesis is that paleoanthropologists suffered a crisis of confidence after molecular data came online in the 1980's, and "a rapid succession of fossil discoveries since the early 1990's has restored" it.

Well, OK, maybe. But consider the listed discoveries: Kenyanthropus, Ardipithecus ramidus, Ardipithecus ramidus, Orrorin tugenensis, Sahelanthropus tchadensis, Homo floresiensis, and Australopithecus anamensis. Of all of these, only Ar. ramidus and Au. anamensis have gone without significant controversy.

We can set aside H. floresiensis for a moment -- the controversy about it being possibly the loudest, it also stands apart as the only species listed younger than 3.9 million years. All of these early Pliocene and Miocene species have also been challenged -- by the discoverers of the others, by old hands, and by young upstarts like me. At least one research group has claimed that all of the Miocene "genera" may actually belong to one species. Another has claimed that most of these "hominids" may actually be apes.

Whether there was any crisis of confidence among paleoanthropologists, all this disagreement is certainly business as usual.

And, contrary to the article, every one of these species would be thrown from the hominid line, if we believe the molecules. Here's the text from the article:

Genetic clues also set the approximate time of the divergence of the human lineage from a common ancestor with apes: between six million and eight million years ago.

Fossil researchers were skeptical at first, a reaction colored perhaps by their dismay at finding scientific poachers on their turf. These paleoanthropologists contended that the biologists' "molecular clocks" were unreliable, and in some cases they were, though apparently not to a significant degree.

...

The new finds have filled in some of the yawning gaps in the fossil record. They have doubled the record's time span from 3.5 million back almost to 7 million years ago and more than doubled the number of earliest known hominid species. The teeth and bone fragments suggest the form -- the morphology -- of these ancestors that lived presumably just this side of the human-ape split.

It is true that the new fossils date as far back as 7 million years; with Sahelanthropus just under that date, Orrorin at around 6 million, Ar. kadabba at 5.5, Ar. ramidus at 4.4, and Au. anamensis at around 4.1.

But it has been many years since a genetic comparison indicated a human-chimpanzee common ancestor as old as 6-8 million years. This year's study by Holbolth et al. (2007) estimated a human-chimpanzee speciation time of 4.1 +/- 0.4 million years. That makes Au. anamensis possibly too young to be a hominid. The rest of those species would presumably be just so many apes.

Now, I don't believe for a second that Au. anamensis is an ape and not a hominid. It just looks too much like Au. afarensis -- so much so that some would put them in the same species. The evolutionary transition between these two is well documented, and will be more so when some as-yet-unpublished fossils come out. So anything younger than 4.1 million years is almost certainly not right for the human-chimpanzee divergence.

But the 4.1 million year estimate is not unusual compared to other recent studies. My post from last May covers many of these recent studies, including last year's problematic "hominid-chimpanzee hybrid speciation" paper by Nick Patterson and colleagues. The conclusion in that paper about hybridization was certainly wrong, but the date of 5 million years was right in line with other estimates.

These genetic comparisons are not easily dismissed. Possibly there has been a rate deceleration of mutations in the human lineage that means that the estimated dates are too recent. Maybe 4.1 million years can be stretched into 6 million. Maybe it can even be stretched into 7 million. But all this stretching does have other effects -- on the estimated dates of earlier divergences -- and those are compounded by a large multiple of the few million years we may try to push the human-chimpanzee speciation date. That 4.1 million year estimate is calibrated from an African-Asian great ape divergence at 18 million years ago. Push the human-chimpanzee divergence to 7 million, and you push the orangutan-human divergence back into the Oligocene. Are silent sites in humans evolving more slowly than cercopithecines? Probably. Are they evolving that much slower than orangutans? I suppose nothing is impossible, but maybe we should take another look at those fossils.

All this is to point out that there really is a conflict between these Miocene "hominids" and genomic evidence about human-chimpanzee speciation time. I don't see any magic solution to this problem from the molecular side -- those dates keep coming up again and again from different regions, and from comparisons across many regions -- including estimates that are not calibrated by other fossil divergences. This is not an easy "the molecular clock must be wrong" kind of problem.

Nor are the fossils an easy problem. There is pretty good evidence for vertical posture or hindlimb-dominant movement in all of these "hominids." Up to now, we've accepted these kinds of features as de facto evidence of bipedality, and assumed that bipedality is such a unique character of hominids that it is unlikely to be any older. Yet few of these fossils provide really good evidence for obligate bipedality, and some of them provide none at all.

Is it possible that bipedal apes long preceded the divergence of humans and chimpanzees? Was the common ancestor of the two lineages a biped? Or was significant vertical posture a common feature of many Miocene apes -- making Sahelanthropus a possible homologue of Oreopithecus?

Which feature is the important one? The long nuchal plane of Sahelanthropus? The femur neck cortical bone distribution of Orrorin? The toe bone of Ar. kadabba? Heck, I can hardly convince my undergraduates about that toe bone!

I've talked to people about this. Some think that all the molecular stuff is just jibberjabbing, and any day now we will find out that the date estimates were wrong all along.

I think it may be time to start doubting our confidence again.

UPDATE (6/28/2007): I've gotten into rather an interesting e-mail discussion about whether I should have included Homo georgicus on the list of new species. Frankly it didn't occur to me: Wilford didn't mention it.

Actually if you start to think about all the new names that have been proposed in the last 15 years, it is a quite bushy list. It will be no surprise that I think this bushiness has more to do with the listers than the listees.

Anyway, there is something interesting about early Homo right now that goes beyond the simple splitter/lumper questions. I'll have more to say about it in a few days.

References:

Hobolth A, Christensen OF, Mailund T, Schierup MH. 2007. Genomic relationships and speciation times of human, chimpanzee, and gorilla inferred from a coalescent hidden Markov model. PLoS Genet 3:e7. doi:10.1371/journal.pgen.0030007

Patterson N, Richter DJ, Gnerre S, Lander ES, Reich D. 2006. Genetic evidence for complex speciation of humans and chimpanzees. Nature 441:1103-1108doi:10.1038/nature04789

I'm about two-thirds of the way through Mike Morwood's new book, The Discovery of the Hobbit, and I'll be posting a review when I'm through. Generally, I have a positive opinion of the book so far.

Henry Gee has reviewed the book in this week's issue of Nature. I wanted to point out my generally positive attitude about the book, so that you'll know that my miserable opinion of Gee's review has little to do with the book's merits.

Consider how Gee starts his review:

The unicorn, wrote Jorge Luis Borges (in Kafka and His Precursors), is universally regarded as a supernatural being of good omen. But there's a problem: despite its folkloric familiarity, we wouldn't know how to recognize a unicorn if we met one in real life. It "does not figure among the domestic beasts, it is not always easy to find, it does not lend itself to classification," Borges continues. "It is not like the horse or the bull, the wolf or the deer. In such conditions, we could be face to face with a unicorn and not know for certain what it was."

Is Gee smoking crack? What kind of blather is this?

First of all, I know I'm being terribly literal, but a unicorn is a horse with a horn. One horn. Not so hard to recognize! Maybe my 3-year-old daughters could help edit at Nature.

Let's see, where have I seen one of those that Gee might recognize? Oh, yeah:

Photo credit: Simon Stratford (via stock.xchng)

There it is, sound as a pound.

Next, Gee spends several paragraphs expositing on his own role in the publication of the Homo floresiensis announcement. We learn some interesting little facts, like how the authors wanted to name the species "Sundanthropus floresianus" until a reviewer pointed out that future students would confuse the name with a flowery butt.

I kid you not. Nature has a layer of reviewers to take tushie references out of taxonomy. Somehow they can't tell a left femur from a right, but they're on the watch for sphincter-species!

The review is entirely self-serving -- there are only three paragraphs that include any reference to the book! In the midst of this babbling about unicorns and hobbits, Gee tells us that skepticism at new hominid discoveries should be dismissed as the predictable result of "mindsets" of the skeptics:

Such reaction is common in the wake of new hominid discoveries, which are routinely dismissed either as pathological humans (Homo neanderthalensis) or apes (Australopithecus africanus and Sahelanthropus tchadensis). Such reactions say less about the facts than the mindsets of commentators, who might be unwilling to have their comfortable views of the world so forcibly changed. Confronted with what might be a genuine unicorn, many would prefer to see a pantomime horse with a spike glued to its head.

Ooooh! Since I'm one who has been notably skeptical of Sahelanthropus and have approached H. floresiensis skeptically, I'm obviously a prime target for this paragraph. It is so comfortable to stay in my view of the world where hominids interbreed with each other. Clearly, a bestiary that includes small-brained island bipeds must shake me out of my comfort zone.

How could I have been so wrong! When every species ever proposed has faced the same resistance? Sure, Tim White says that Kenyanthropus is a glued-together matrix-filled A. afarensis, but that's just his mindset. Or how about Eoanthropus? Sure, Franz Weidenreich thought that it was just a concoction by "English authors," but couldn't he tell that it was more than just a pantomime skull with an orangutan jaw? Why couldn't I see that these petty minds were just holding back the important work of taxonomy!

No, no, no. You see, if we approach things skeptically, we won't dare to dream about the unicorns:

The unicorn remains as it always did, frustratingly elusive. This year, the researchers will return to Liang Bua to see if they can discover more. But stories such as this demand a mythological beast altogether less serene. It is as if the researchers had set out to discover some new form of fossil mouse, only to find that they had grabbed a dragon by the tail instead. And as any devotee of Harry Potter will remind you: Draco dormiens nunquam titillandus.

The theme of the review is perhaps to be expected from Gee, otherwise known as the author of The Science of Middle-Earth. But I find his mixture of fantasy and science to be especially malaprop in the context of the Flores fossils, since with every fantasy word he detracts from the credibility of the journal's review process!

Some of you will have seen the episode of The Simpsons, titled "Lisa the Skeptic," where Lisa excavates an "angel" from the ground. Here's part of the synopsis from Wikipedia:

As Homer attempts to get a motor boat, a new shopping mall in Springfield is being built on an area where a large number of fossils were found. Lisa condemns and protests the building of the mall. Thanks to her protest, it prompts the school to conduct an archaeological dig. When Lisa is digging, it reveals a human skeleton with wings. Springfield's residents are convinced it is an angel, and Homer cashes in by moving the skeleton into the family's garage; however, Lisa is skeptical, believing it may not actually be an angel, and even has Stephen Jay Gould test a sample of the skeleton. The next day, Dr Gould runs to the Simpson house and said the tests came out inconclusive and after Lisa on television compares belief in angels to belief in unicorns and leprechauns, Springfield's religious zealots riot and destroy all of the scientific institutions.

Later, we find out that the "angel" is a publicity stunt for the new mall; Guest voice Gould confesses that he never really performed any tests on the "angel". This is one of my favorite episodes: it's a rare one where Lisa's preachy skepticism is entirely justified, and the "expert" doesn't care enough to do anything at all.

Now I know, that the episode was missing a scientific editor to encourage Lisa to forget about her doubts, and just to accept the "angel" for what it is. After all, every new discovery has its skeptics.

Well, there is a lesson to take away from all the unicorn talk. If you are in Cardiff and find the skeleton of a giant, be sure to send your report to Nature, where you'll find a receptive editor. Despite what they may say, there's not one of those born every minute.

UPDATE (4/26/2007): A reader e-mails, "Remember that Borges was blind." True. Perhaps we can extend this analogy further?

Another reader: "Well, at least we can expect a fair set of reviews on the Sahelanthropus postcrania...D'oh!"

References:

Gee H. 2007. In a hole in the ground.... Nature 446:979-980. doi:10.1038/446979a

Well, I guess they've got a plot for the pilot of that caveman show:

Humans caught pubic lice, aka "the crabs," from gorillas roughly three million years ago, scientists now report.

Rather than close encounters of the intimate kind, researchers explained humans most likely got the lice, which most commonly live in pubic hair, from sleeping in gorilla nests or eating the apes.

"Sleeping in gorilla nests." Yep, that's the ticket.

The quote is from a LiveScience article by Charles Q. Choi. The article talks a lot about "monkey business" but really spends more time on the hominid-eating-gorilla scenario:

"Unfortunately, even today among modern humans there's a bush meat trade where gorillas are killed for their meat," he said. "If archaic humans were butchering or scavenging those animals 3.3 million years ago, it would be a simple thing to transfer those lice from prey to predator."

UPDATE (3/7/2006): Carl Zimmer's post is great (he's all about the parasites) and mirrors some of what I wrote below. He also includes probably the best snarky quote: "Is this evidence of a Pliocene love that dare not speak its name?"

To telegraph my conclusion a bit, I still think the flawed assumption is that the hominid-gorilla interaction occurred when the hominid and gorilla Pthirus diverged. The interaction works a lot better later, assuming within-gorilla parasite variation. Since there is a lot of within-human variation in the other louse genus, Pediculus, the idea of a couple million years of delay between louse genetic divergence and lateral transfer is not at all unlikely, even without invoking ancient gorilla speciations.

Thoughts

I downloaded the research paper in BMC Biology by Reed and colleagues. Here's the 'Conclusions' section of the abstract:

Reconciliation analysis determines that there are two alternative explanations that account for the current distribution of anthropoid primate lice. The more parsimonious of the two solutions suggests that a Pthirus species switched from gorillas to humans. This analysis assumes that the divergence between Pediculus and Pthirus was contemporaneous with the split (i.e., a node of cospeciation) between gorillas and the lineage leading to chimpanzees and humans. Divergence date estimates, however, show that the nodes in the host and parasite trees are not contemporaneous. Rather, the shared coevolutionary history of the anthropoid primates and their lice contains a mixture of evolutionary events including cospeciation, parasite duplication, parasite extinction, and host switching. Based on these data, the coevolutionary history of primates and their lice has been anything but parsimonious.

There is actually a much more interesting story here than is indicated in either press account or abstract. The genera Pediculus and Pthirus were thought to have diverged at the time that gorillas diverged from the chimpanzee-human clade. This would explain why gorillas have Pthirus and chimpanzees Pediculus. The fact that humans have both ... well, that remained unexplained. The purpose of the study was to test whether humans retained the two genera ancestrally, or if instead they picked one up later.

What they found is that the two genera didn't diverge at the gorilla-chuman split, but instead way earlier. Their estimate for the Pediculus-Pthirus divergence is 13 million years. Thirteen million is as much as twice the age of the human-gorilla common ancestor. This estimate is probably biased toward the recent side, since it is calibrated against a divergence between hominoid and baboon lice assumed at 22.5 million years ago -- probably more recent than the true hominoid-baboon divergence.

The paper considers it likely that the human-gorilla-chimpanzee common ancestor lineage maintained this pair of lice species for the intervening time period, with one genus being lost in each of the two (gorilla and chuman) descendant clades. This ancestral lineage would be similar to humans in that respect -- host to two distinct parasite lineages, both of which stemmed from a single ancestor species.

But much later than the chimpanzee-human divergence, humans apparently picked up the gorilla lice somehow. The paper doesn't belabor this point or attempt to explain it, beyond this:

Evidence suggests that Pthirus pubis has been associated with humans for several million years, and likely arrived on humans via a host switch from gorillas. Despite the fact that human pubic lice are primarily transmitted via sexual contact, such contact is not required to explain the host switch. Parasites often switch from a given species to a predator of that species [17], and are sometimes found to switch to unrelated hosts in communally used areas, such as roosting or nesting sites [18]. The host switch in question could have resulted from any form of contact between archaic humans and gorillas including, but not limited to, feeding on or living among gorillas. Regardless of how the transfer occurred, suitable habitat had to be available on the new human host for the host switch to be successful. For example, it is possible that the switch of Pthirus from gorillas to humans coincides with a change in available niche space in humans, such as the loss of body hair. Further study, however, is required to test such a hypothesis (Reed et al. 2007:7).

Hominids were certainly not hunting gorillas 3.3 million years ago. At least, not the hominids we know about. That date is a bit older than Lucy; it's 700,000 years older than the earliest evidence of flaked stone and 800,000 earlier than the earliest evidence of antelope butchery. Hominids weren't hunting gorillas because they weren't hunting any large mammal species then.

What's worse, gorillas and hominids weren't sympatric 3.3 million years ago. At least, not the gorillas and hominids we know about. Unless gorillas ranged into open woodland, and in particular the East African coastal forest, or hominids ranged into the central or west African rain forest, they never came into contact with each other at all.

If anything, we might expect that gorillas and chimpanzees would have been likely to come into contact and exchange parasites. They are currently sympatric, they eat the same foods, and they even build similar sorts of nests. It's like they share the same locker room. But they didn't have this parasite exchange.

It's all very strange. First we have this long period of divergence of the two great ape louse genera (orangutans don't have their own louse species). Then we have a divergence of the human and chimpanzee Pediculus species just exactly when it should have happened. And then there is this lateral transfer of lice from gorillas to humans 3 million years ago - when hominids and gorillas weren't apparently sympatric and had no credible mechanism for lice exchange.

Here's my hypothesis: cryptic African hominoids. The apparent craziness all comes from the assumption that the only species that existed are the ones we know about. For Africa 3 million years ago, that means two or three hominid species, one gorilla lineage and one chimpanzee lineage. We don't have any fossils that old for the apes; we can only infer their existence from the fact that they exist now.

Let's consider what we know. We know that 3 million years ago there weren't any chimpanzee or gorilla relatives in the Rift Valley, and plausibly (but not definitely) not in South Africa or the Sahel.

We don't know how extensively hominids ranged into the west African or central African forests, particularly from the north and southeast. We don't know how extensively gorillas and/or chimpanzees may have ranged outside the core forested areas where they have historically existed. In the absence of Homo, the competition between these apes and hominids at the forest boundaries may have been a close game.

We don't know how many species of ancient chimpanzees and gorillas there may have been. The present subspecific variation of chimpanzees seems to reflect recent colonization of the eastern range from central Africa, and some substantial population interchange between central and western ranges. Gorilla subspecies now seem to have emerged within the same time frame, with a possible colonization from their western range into their eastern range within the past million years.

Bonobos are only ca. 850,000 years old (Won and Hey 2005). To summarize, the current eastern chimpanzees weren't in East Africa half a million years ago, and the bonobos weren't south of the Congo a million years ago, and eastern gorillas weren't there a million years ago either.

Who was? It seems to me that the best candidates would be ancient species of gorillas and chimpanzees that no longer exist. A second-best (and maybe more interesting) candidate is some variety of hominid. A third-best (and even more interesting) candidate is an ancient ape lineage dating from before the G-C-H divergence.

Three million years ago, any one of those possibilities is credible. Here's my favorite: two gorilla species (or subspecies) became isolated enough for louse divergence 3.3 million years ago, and continued to coexist. Sometime after 2 million years ago, Homo encountered one of these species and picked up its lice. That gorilla lineage later became extinct, perhaps by range expansion from Homo.

Oh, and the long divergence time between the two lice genera? I like a long divergence and later lateral transfer from some pre-H-C-G Miocene ape lineage. There were likely several in Africa to choose from. Maybe it was Sahelanthropus...

References:

Reed DL, Light JE, Allen JM, Kirchman JJ. 2007. Pair of lice lost or parasites regained: the evolutionary history of anthropoid primate lice. BMC Biol 5:7. doi:10.1186/1741-7007-5-7

Won Y-J, Hey J. 2005. Divergence population genetics of chimpanzees. Mol Biol Evol 22:297-307.

It's a hazardous business, making predictions -- all the moreso because New Year's predictions have a deadline. If they don't happen this year, well, that's too bad, because we'll be checking back a year from now to see how well you did.

Last year, I did pretty well. My 2006 predictions are listed below. I ordered them originally "from most certain to most speculative". As you can see, the first five (i.e., the more "certain" ones) all came true; the last five (i.e., the wild-arsed speculations) didn't. So let's check them out:

• 10. We will see a name for the Flores pathology. OK, we got several names, and the issue is far from settled, but this was the year that the Homo floresiensis doubters struck with their papers on the remains.
• 9. There will be two Neandertal genome-related announcements. I undercalled this, since there were three -- the initial announcement in June of the Neandertal Genome project, the announcement and publication in November of the initial sequence results, and the announcement about possible introgression of microcephalin.
• 8. No Ardipithecus. Sometimes, predictions write themselves.
• 7. "Population cluster" will become the new "race". This one is debatable, but enough papers on multi-ethnic SNPs have used the term this year, that I think it is emerging as the replacement for the race concept for a certain class of geneticists. I expect it will continue -- "cluster" has such a neutral computer-program-centric connotation, that people like to use it.
• 6. There will be another paper (yes, besides the one last month) using genetics to estimate the time of the human-chimpanzee divergence. The date will be 5 million to 7 million years ago. Oh, my. There have been bigger messes than the Patterson et al. 2006 paper, but not many. Yes, it was yet another paper with a 5-million to 7-million-year-old divergence, but it had so much more!
• 5. Evidence of recent selection will be found for several Y chromosome genes. Wishful thinking or prediction for the next year? You decide!
• 4. Sahelanthropus postcrania will be published. This one didn't happen this year, but I'm carrying it over onto the 2007 list.
• 3. There will be an ancient DNA announcement from China. Someday it will happen, but not this year or next.
• 2. StW 573 will be proposed as a new species ancestral to all later hominids. Well, we got the opposite -- with a new younger date, StW 573 was proposed as the ancestor of...nobody! Which was by far the smaller of the redating stories this year.
• 1. A Hawks weblog post will be cited in a peer-reviewed research paper. We can only hope this happens in the coming year, but carrying it over just seems desperate...
• BONUS: A new Georgian hominid will be a robust australopithecine. I still think somebody will find an australopithecine outside Africa in the next decade, but it's not to be from Dmanisi -- the hominids are too localized in a single feature.

So that should give some indication of how to read the list for the next year. I'm listing from more certain to more speculative again, and again I'm excluding most of my own work. The main effect of this is just that I'm not including secrets that I know will be coming out this year. Once again, the predictions are Delphic -- if only I were cleverer, I could make them come out right no matter what!

• 10. Sahelanthropus postcrania will be published.
• 9. Two words: Holocene evolution.
• 8. Despite (or because of) the success of the Neandertal genome project, there will be no genetics of any kind published on early modern skeletal material.
• 7. The mitochondrial history of human dispersals will become more and more detailed, but no paper will test against other loci.
• 6. Another (yes, another) paper about the chimpanzee-human divergence will peg it between 5 and 7 million years ago.
• 5. Three papers with new Ethiopian fossils.
• 4. Another early Upper Paleolithic specimen will emerge from a museum collection.
• 3. A big year for Miocene apes, which will look increasingly important in the story of human brain evolution.
• 2. Maturation rate in early Homo becomes a dead issue, because of the variation in dental and skeletal maturation in living people.
• 1. The year will end without a single new hominid species having been named.
• BONUS: A dramatic development in the problem of pre-2.0-million-year-old Homo.

I ended the year with just a shade fewer than 1 million visits since last January 1. The Neandertal women brought me over 10,000 readers in a single day -- the most ever. I know a few of the big stories from the coming year, but there will be many more that nobody can predict. There's no doubt in my mind that 2007 will be a big year!

Despite all the trouble I had traveling (or maybe because of it), I got to have a really enjoyable time finishing Ann Gibbons' new book, The First Human. For a while I was really afraid I'd lost it in the backpack without knowing how it ends! But what a relief, it was in another suitcase so I can report on the whole thing.

I've read most -- not all -- of the recent trade books about paleoanthropology, and this is definitely one of the top few in terms of being fun to read. It follows a familiar form: the quest for the source of the Nile. The book even mentions Burton and Livingston, whose explorations were to some of the earliest anthropologists what the Leakeys discoveries were to the current generation. Like the quest for the solo transatlantic flight, the summit of Everest, or the race to the Moon, the paleoanthropologists here all are trying to capture the same prize: the earliest hominid.

The book appeals in large part because it is well-written. Instead of beginning with the long dry history of finding bones in old dry places, Gibbons' first chapter plunges us right into the middle of three discoveries of the mid-1990's -- all happened within six months of each other, but the events of January 1995 brought them together. The chapter even ends with a cliffhanger!

Then comes the long dry history, with the usual cast of characters: Haeckel, Dubois, Dart, Louis and Mary Leakey. I was apprehensive about this -- no book ever seems to skip this stuff, and it's usually the same boring slog -- but Gibbons adds some details that most people haven't seen before. She's mercifully light on the "Dart courageously fighting the scientific establishment" theme, and brings us a great description of Dart excitedly opening the crate containing the Taung fossils at a friend's wedding. We get rather less of Louis Leakey's long struggle for recognition and more of his behind-the-scenes support from LeGros Clark.

Most notably, Gibbons brings us sketches of many of the paleontologists that the usual accounts miss. We see Bryan Patterson find not one, but two of the earliest hominids, and the episode that caused him to leave Kenyan field work, with his site of Kanapoi lying fallow for 30 years. We are led down the blind alley of Ramapithecus with Elwyn Simons and David Pilbeam. And we follow Yves Coppens to the Omo, Hadar, and Chad. Indeed, one of the real highlights is the account of field research in Chad, which I haven't seen described elsewhere in English so well.

The soap opera really begins with the origins and education of the current fieldworkers, who are as interlinked as characters on Days of our Lives. Pilbeam plays a Kevin Bacon-like role connecting Michel Brunet, Andrew Hill, and Martin Pickford. Pickford and Richard Leakey were old schoolmates, and -- maybe or maybe not, according to the book -- Hill comes between them. The chief fossil hunter from Hill's team goes to work for Pickford. The son of the chief fossil hunter for Richard and Maeve Leakey goes to work for Hill.

We see quite a bit less of the soap opera in Ethiopia, which describes the current Middle Awash work extensively but has little to say about Hadar or other current field sites. Donald Johanson's perspective on events of the last twenty years is very noticeably absent. We see Mary Leakey's anger at White and Johanson for naming her Laetoli discoveries Australopithecus afarensis, but the section does not explain the justification for the anger -- attaching the name to LH 4 as the type specimen removed any chance of naming the Laetoli hominids anything else.

Ian Tattersall raised an important point in his Nature review of the book: Any reporter who depends on access to subjects faces a possible conflict of interest. Report bad things about the subjects, and they may restrict access. Gibbons has obviously received exceptional access to some of the book's subjects -- indeed, the book mentions the famous lack of journalistic access to some of the research teams. Has this exceptional access affected the narrative?

I think that the book has a fair account of many events, but omits other well-known incidents that might have been described. For most of these, there is little that Gibbons could have done -- after all, if some subjects don't talk to you, and others won't give details about certain events, then what are you going to write about? In fact, there must be an intense incentive for many people not to cooperate with a book like this, especially those hoping to continue fieldwork in Ethiopia or begin there in the future. The accounts that are in the book make quite plain that one misplaced word can result in field permits being revoked, or access to collections being revoked, or even worse. As a result, the book puts on the record many arguments that were aired in public -- like the dispute over the Galili field site, for instance -- but doesn't necessarily give the whole story.

There is pretty obviously one overarching prize that shapes the entire narrative. The introductory chapter ends with the world on Alan Walker's "tenterhooks" -- in 1995! -- waiting to see the Ardipithecus skeleton. The book describes on four occasions just how fragile the skeleton was. Twice we hear how the condition of the skeleton "tempered" the Middle Awash team's excitement, twice it is described as "the most fragile skeleton ever found," twice as "roadkill." Early in the book White emerges as a secretive Svengali; at the end -- during an event White himself describes as "theater" -- we see him casting aside the velvet curtains to show his specimen at last to his skeptical colleagues.

Except, well, we don't get to see it. A reader might be forgiven for thinking the obviously crushed skull on the book jacket is the centerpiece of the book -- its "crushed" skull is twice mentioned. Sadly, no, the cover shot is just Sahelanthropus. Ardipithecus is still locked in its fortress of solitude, unseen by the unwashed. This does raise some concern for me -- since Gibbons will undoubtedly be writing the story of this fossil when it at last surfaces.

But some of the best moments are those that shine light on the relationship of the science to journals and the media. Two of the major research teams make a point of rejecting the taint of National Geographic and its film crews. In counterpoint, the book repeatedly notes the long association between National Geographic and the Leakey family, including a direct contrast between the histories of Richard and Maeve Leakey and Tim White. Amid descriptions of media-savvy scientists, we see Henry Gee, editor of Nature, commenting on fossils, prognosticating on future discoveries, "prodding" researchers, and having one incredible meeting that was hard for me to believe even after reading it. If one wonders about possible conflicts of interest for Gibbons, how much more must one wonder about the chance of one of these papers being rejected by Nature's vaunted six "peer reviews"?

At its bottom line, the book really raises two substantive issues. The first is the real danger of today's field work. Paleoanthropology is not merely a game today, it is "the Great Game" replayed. Field teams divide up "Connecticut-sized" research territories, hem opponents into areas with younger sediments, and -- when bullying, scientific name-calling, and bureaucratic manouvers fail -- finally agitate local people, enlist bandits, or pull their guns. To me, the book's most touching moment is its description of Michel Brunet's feelings after losing a colleague on his field team. In another episode, a young graduate student (who deserves recognition for her science and not this) personifies a near-miss with violence in the field. The two cases together bear rereading: if paleoanthropology continues along its current path, then who can doubt that some people will be killed in the field?

The other issue is the relationship between these field teams and the science as a whole. As depicted in the book, they clearly do resemble explorers looking for the source of the Nile. They know what the goal is -- at one point, Pilbeam even sketches what the ancestor will look like, at another Henry Gee opines about it. It is still out there waiting to be found, and these teams will be searching until they find it. It's "the First Human" of the title.

But these fossils aren't human -- and it's darned hard to tell whether they are even the more humanlike kind of apes! In the book, we see that the science turns against the scientists sometimes. Ramapithecus is no longer considered hominid by anybody -- it's not even a valid taxon anymore. Louis Leakey's Kenyapithecus wasn't a hominid either.

Can it be that all of these new fossils are really hominids? Or have some of these scientists in their quest for older and older fossils overshot the mark? The current scientific debate over specimens is only glossed here -- the book sketches what the disagreements are, but gives no details to judge the arguments. (If you want those details, you'll need to read the blog!) Instead, the science appears as another forum for the scientists to misbehave -- accusing each other of holding "creationist positions" and the like.

Many readers will surely be puzzled to read how these men and women, who brave disease, bullets, broken families and years of denial, can be so poorly composed in the face of scientific examination. Again and again we see them squirrel the fossils away, withdraw them from the world, or give up on paleoanthropology altogether. How can it be that this story is repeated so many times? But the reader should consider: No one can take away Hillary and Norgay's summit photos. But even after all the years of work, the lowliest graduate student might turn one of these "hominids" into an ape.

Even I make a brief appearance in this book -- blink and you'll miss me dancing through to aggravate Brunet's heart condition.

And it comes from me! My paper with Milford Wolpoff, Brigitte Senut, Martin Pickford, and Jim Ahern is now available online from PaleoAnthropology! The PDF is freely accessible -- a big advantage with this journal. So go download!

Here's the abstract:

The Toumaï cranium TM 266 is the first known from any Late Miocene African hominoid clade, and is one of the best preserved crania of any Miocene hominoid. Since its publication there has been debate in the scientific literature and discussion in the popular press over the assertion that this cranium is significant because it is the earliest known hominid. The basis of the hominid assessment rests on two interpretations of the anatomy: a hominid-like, small, flat-wearing canine; and, cranial features reflecting an upright stance and bipedal locomotion. In fact, it is widely reported that the specimen is an upright hominid biped (Haile-Selassie et al., 2004; Kimbel, 2004; Lieberman, 2002), although this is yet to be verified by independent observations and study. The history of paleoanthropology may be relevant to this assessment, because there have been similar claims for other extinct primate species. Here, we evaluate the hypothesis that Sahelanthropus (the genus TM 266 is attributed to) is a hominid by examining features of the canine and of the cranial base that are said to reflect canine reduction and change of function, and upright posture and bipedal locomotion. These are hominid autapomorphies and their presence or absence in late Miocene hominoids has fundamental importance for identifying the hominid clade (Wolpoff et al. 2006:36).

There are two important differences between our analysis and earlier analyses of the skull. First, we provide more comparative data from Miocene primates. Some of the apparent similarities with hominids -- especially considering the morphology, size, and wear on the canines -- are clearly present in other Miocene ape lineages. This is of course the primary difficulty in defining hominids on dental remains alone, since several lineages of Miocene apes appear to have been convergent on some hominid dental features. These similarities don't preclude Sahelanthropus as a hominid, but they remove a major support for that hypothesis.

And second, we provide a biomechanical assessment of the reconstructed skull and its relevance for bipedality and posture. Rather than simply looking for similarities with hominids or chimpanzees, we actually developed a model for how the skull and neck musculature must have functioned. One angle in reconstruction makes the skull look like modern humans -- the foramen magnum - orbital plane angle. But regardless of this angle, the skull actually cannot have functioned in a vertical posture because of the length of the nuchal plane and vertical height of inion. Also, this angle in Toumaï doesn't look anything like early hominids -- australopithecine skulls have FMOP angles similar to chimpanzees!

I think this line captures the point:

The point is not that the TM 266 cranial rear and posterior portion of the cranial base was unlike hominids because the region looks like apes, but that TM 266 had a posture that is not upright because the region reflects nuchal functions similar to those of apes (Wolpoff et al. 2006:46).

There's lots of other interesting stuff in the paper also -- including a section about the hominid-chimpanzee divergence date that I never thought we would say, but is looking very prescient considering recent data. It was a real pleasure to explore several different topics and bring them together, and we'll have much more to do!

References:

Wolpoff MH, Hawks J, Senut B, Pickford M, Ahern J. 2006. An ape or the ape: is the Toumaï TM 266 cranium a hominid? PaleoAnthropology 2006:36-50.

Rex Dalton has a great two-page article in Nature about the bush vs. ladder dispute. It keys off of the Middle Awash Australopithecus anamensis article by White and colleagues from a couple of weeks ago.

If you recall that one, White et al. posited that Ardipithecus was likely ancestral to Au. anamensis, and that the two did not overlap in time. Here's the key exchange in the Dalton piece:

This month's Nature paper makes a bold argument, and shows the Awash team seeking to put its mark on the record. Others in the
field are impressed. "When you find 30 new hominid fossils, you are allowed a certain amount of conjecture," says Bernard Wood, a palaeoanthropologist at George Washington University in Washington DC. "As always, they have done a fantastic job."

But he and others are unconvinced by the Awash team's conclusion: "This is only the first half of the rugby match," says Wood. Meave Leakey, lead author on the Au. anamensis discoveries in Kenya, is more blunt. "I don't believe this," she says. "We do not have the specimens to fill the gaps."

Leakey and Wood are among those who believe that other, as yet undiscovered hominid species may have lived at this time, from 4.4 million to 2.9 million years ago. The existence of other species would cloud or eliminate the argument for a direct lineage. "My prejudice is there are more lineages rather than fewer -- more diversity," says Wood. "I have to concede these new data are dramatic. But we should beware coming out with a complete explanation when we don't have all the
evidence."

This argument frustrates White. "There were Martians there back then too," he says. "And spacecraft all over the Pliocene -- we just haven't found them yet."

Waiting for Monte Cassino

In a series of articles since 2000, White and colleagues have laid out a systematic attack on the "bushy" phylogeny model. Their arguments have extended across four million years and seven species, with a breadth that rivals the Allies breaking the Winter Line.

Consider the angles of attack:

1. Au. anamensis -- Au. afarensis. Everyone basically accepts that Au. anamensis is a direct ancestor of Au. afarensis. And the two species are really not very different from each other -- for instance, they are more alike than either is to Ardipithecus. The transition between these species would look to be a simple case of anagenesis, except...

...for Kenyanthropus (Leakey et al. 2001). This small-toothed, flat faced hominid needs an ancestor, too. Au. anamensis might have been the common ancestor of Kenyanthropus and Au. afarensis. If so, then both these later species originated by cladogenesis from Au. anamensis. A similar argument might be made for other species, like Australopithecus bahrelghazali (Brunet et al. 1996) or the Sterkfontein Member 2 hominids. But Au. bahrelghazali is only known from a partial mandible and only differs from Au. afarensis by a three-rooted premolar, which is considered by many to be weak evidence, and the Sterkfontein Member 2 sample has not yet been taxonomically assigned -- they might turn out to be Au. afarensis, for example. Kenyanthropus remains the strongest case for cladogenesis (i.e., a "bush"). Yet...

...White (2003) denied that the Lomekwi skull KNM-WT 40000 was a distinct species. In particular, he argued that the extensive postmortem deformation of the skull made it impossible to substantiate an anatomical difference from Au. afarensis, and even if it was different, the anatomical diversity of living hominoid species is so great that it would probably encompass the difference between KNM-WT 40000 and known Au. afarensis crania.

2. Earliest hominids. At the moment, the earliest putative hominids include three genera: Orrorin (Senut et al. 2000), Sahelanthropus (Brunet et al. 2002), and Ardipithecus, represented in the Late Miocene by Ar. kadabba (Haile-Selassie 2001, Haile-Selassie et al. 2004). Evidence for obligate bipedality has been challenged (by different researchers) for each of these three (I'm one of those who has questioned bipedality for Sahelanthropus).

So far the only comparable anatomical parts from all three samples are teeth...

...which were examined by Haile-Selassie, Suwa and White (2004). They concluded that the variation among these three genera

is no greater in degree than that seen within extant ape genera. Despite claims of molar enamel thickness differences among these late Miocene fossils, we question the interpretation that these taxa represent three separate genera or even lineages. Given the limited data currently available, it is possible that all of these remains represent specific or subspecific variation within a single genus (Haile-Selassie et al. 2004:1505).

Additionally, Ohman, Lovejoy and White (2005) challenged the interpretation of the internal anatomy of the Orrorin femur, which had been suggested to be more derived than that of Au. afarensis. They wrote:

We agree that the Lukeino femur's external morphology suggests some form of bipedality. Yet the more detailed original scans appear to show a distinct superior cortex different from Australopithecus and humans, with the cortex distribution being more primitive than that seen in any other hominid, including Australopithecus.

The relevance of this argument to the phylogenetic diversity of early hominids depends on the anatomy of the Ardipithecus femur, which none of the rest of us are in a position to know. But one may speculate that if all these early "hominids" had femora with similar morphology, it would further reinforce the interpretation that they belong to a single lineage.

3. Ardipithecus -- Au. anamensis. This is the current example. Here's how Dalton discusses it:

The latest Afar discovery is exciting experts because it shows that the three hominids existing in the same area, but in successive time periods. Tim White of the University of California, Berkeley, co-leader of the Awash team, believes this points to a direct lineage between the three -- a process called phyletic evolution. The new Au. anamensis fossils are only 300,000 years younger than Ar. ramidus, meaning that if one became the other, the changes would have had to happen that fast. But the key point, says White, is that fossils of Au. anamensis and Au. afarensis have never been found in sediments the same age as those containing Ar. ramidus. If fossils of the different species were found together, that could show that they belonged to multiple lineages existing simultaneously.

Finding remains of all three species in the same area but not from the same time period suggests they did not coexist, says White.

...

The specimens also provide anatomical clues to evolutionary history. "The new Au. anamensis fossils are anatomically intermediate between the earlier Ar. ramidus and the later Au. afarensis," says White. For example, the teeth of the newly discovered Au. anamensis fossils seem adapted to chew tougher and more abrasive foods than Ar. ramidus. The researchers believe this shows that Au. anamensis had a broader diet. "All this strengthens the view that there is phyletic evolution from Ar. ramidus through Au. anamensis," says White. He believes he has nailed down the relationship between the two later species, although he says that further specimens are needed to prove the earlier link (Dalton 2006:1100).

Of course, it would help matters if we knew in more detail what Ardipithecus looked like. But one must imagine that the stage is being set for its revelation. The unilineal interpretation places Ardipithecus at the critical point as an ancestor to the major mid-Pliocene australopithecine lineage. Extending the unilineal interpretation earlier into the Late Miocene would make Ardipithecus the earliest hominid as well.

It is not necessary to think that taxonomic uniformity means anatomical uniformity, though. Ardipithecus already encompasses a trend of decreasing canine size and less sectorial P₃ for example. A trend toward fuller skeletal adaptation to bipedality may also be imagined. But in that context, it is important to note that the time interval between the Orrorin femur and the unpublished Aramis skeleton is longer than the time between Aramis and Hadar. Those relative times may become quite important in thinking about the evolution of those postcrania.

The Winter Line was broken at Monte Cassino, after many failed attempts from different approaches. The Aramis fossils are either the heavy shoe waiting to drop, or they are the uncomfortable foot that all this talk about phyletic evolution is meant to shoehorn into place.

Commentary

If all these cases are added together, they imply a single evolving lineage encompassing at least four anagenetic taxa, Ar. kadabba -- Ar. ramidus -- Au. anamensis -- Au. afarensis. This last would presumably be followed by a cladogenesis into a robust australopithecine species (Australopithecus aethiopicus) and Australopithecus africanus.

One could add Homo erectus to this list, since White and colleagues argued in their description of the Daka skull (Asfaw et al. 2002) that the Asian and African samples represent one cosmopolitan species.

But then one species sticks out as a surprising exception to the pattern: Australopithecus garhi (Asfaw et al. 1999). It will be interesting to see a close argument showing why this species is really different from South African Au. africanus. Say, more different than KNM-WT 40000 is from the Hadar crania. It's quite glaring, really, that this species should be there mucking up such a simple phylogeny.

I have to say, after reviewing all these papers in one sitting -- this entire bush vs. ladder thing is getting very tiresome! I mean, isn't there something else that we could organize early hominid discoveries by? These are all papers in the top journals, and this is the (fairly specialized) discussion that has been promoted as the central issue in the field!

The subtitle of the Dalton piece suggests that it is merely a philosophical difference:

Deciding whether our ancestors evolved as a single lineage may depend more on philosophy than fossils.

But that's not really true. There is a clear null hypothesis here, quite directly drawn from William of Ockham:

entia non sunt multiplicanda praeter necessitatem

Which of course means:

Sometimes fossil samples really do form ancestor-descendant relationships.*

(*) It doesn't really. It means "Entities should not be multiplied beyond necessity."

References:

Asfaw B, Gilbert WH, Beyene Y, Hart WK, Renne PR, WoldeGabriel G, Vrba ES, White TD. 2002. Remains of Homo erectus from Bouri, Middle Awash, Ethiopia. Nature 416:317-320. DOI link

Asfaw B, White T, Lovejoy O, Latimer B, Simpson S, Suwa G. 1999. Australopithecus garhi: A new species of early hominid from Ethiopia. Science 284:629-635. DOI link

Begun DR. 2004. The earliest hominins -- is less more? Science 202:1478-1480. DOI link

Brunet M. and 37 others. 2002. A new hominid from the Upper Miocene of Chad, Central Africa. Nature 418:145-151. DOI link

Brunet M, Beauvillain A, Coppens Y, Heintz E, Moutaye AHE, Pilbeam D. 1995. The first australopithecine 2,500 kilometres west of the Rift Valley (Chad). Nature 378:273-275. DOI link

Dalton R. 2006. Feel it in your bones. Nature 440:1100-1101. DOI link

Haile-Selassie Y. 2001. Late Miocene hominids from the Middle Awash, Ethiopia. Nature 412:178-181. DOI link

Haile-Selassie Y, Suwa G, White TD. 2004. Late Miocene teeth from Middle Awash, Ethiopia, and early hominid dental evolution. Science 303:1503-1505. DOI link

Leakey MG, Spoor F, Brown FH, Gathogo PN, Kiarie C, Leakey LN, McDougall I. 2001. New hominin genus from eastern Africa shows diverse middle Pliocene lineages. Nature 410:433-440. DOI link

Ohman JC, Lovejoy CO, White TD. 2005. Questions about the Orrorin femur. Science 307:845. DOI link

Senut B, Pickford M, Gommery D, Mein P, Cheboi K, Coppens Y. 2001. First hominid from the Miocene (Lukeino formation, Kenya). Comptes Rendus 332:137-144.

White T. 2003. Early hominids -- diversity or distortion? Science 299:1994-1996. DOI link

A concise 4-paragraph article by Mathieu Schuster and colleagues reports on dune deposits that show the Sahara formed during the Late Miocene.

After the mid-Holocene humid period (6000 years ago), arid conditions developed throughout North Africa, culminating in the formation of the Sahara, which is the largest warm-climate desert on Earth (9,000,000 km2). However, earlier desert recurrences in the region are also documented. Direct evidence for eolian deposition is given by thermoluminescence dating for the Late Pleistocene; e.g., in Mauritania [25 to 15 thousand years ago (ka)] (1) or in Tunisia (86 ka) (2). The latter is currently considered as the oldest terrestrial record for desert conditions in the Sahara (2), even if firm evidence exists for a pre-Quaternary Great Western Sand Sea in Algeria (3). Some earlier arid episodes (Miocene-Pliocene) were also suggested by marine records off West Africa (4); but until now, no contemporary in situ eolian deposits were known in the Sahara region. In the northern Chad Basin, we recently identified and dated widespread outcrops of eolian dune deposits that are distributed over an area more than 2000 km². Our results testify that the onset of recurrent desert conditions in the Sahara started at least 7 million years ago (5-7) (Schuster et al. 2006:821).

The desert comes and goes, expanding and contracting -- and those vacillations are recorded by this earliest evidence, also:

In the Toros Menalla region, these eolian sandstones are conformably overlain by a horizon bearing abundant vertebrates fossils, including Sahelanthropus tchadensis, the earliest known Hominid [sic] (5, 7). In this horizon, named the Anthracotheriid Unit, biostratigraphic correlation of the mammalian fauna indicates an age of 7 Ma (5–7).

Now, this isn't news (which I'm sure Science didn't bother to check) since Vignaud and colleagues (2002) published the same evidence, complete with the wind direction chart:

The lower part of the section (at least 4 m thick) is composed of fine to very fine white sands, poorly cemented, and is mainly constituted by numerous quartz grains, without matrix. The grains are well sorted, well rounded, matt and frosted, and are strong evidence for aeolian modelling. The foreset laminations (avalanche laminations in front of the aeolian dune) represent a typically aeolian deposit. These sands show cross-beddings that progressively decrease in size from the bottom (1 - 2 m) to the top (20 cm). This facies exhibits typical alternations of grain-fall and grain-flow laminations, characteristic of aeolian dune deposits. Our interpretation is confirmed by frequent wind ripples at the foot of the fossil dunes, whose crests are perpendicular to the direction of dune progradation. These fossil dunes are, to our knowledge, the oldest evidence for desert conditions in the southern Sahara area (Vignaud et al. 2002:152).

I guess this is the science journal equivalent of getting "punk'd" -- "Ha ha! You published what we printed four years ago!"

I opened up the Vignaud paper to double-check the paleoenvironment in the fossil-bearing layer. From the faunal list, they conclude this:

The oldest known East African hominids (Ororrin [sic], Ardipithecus) are contemporary with faunas associated with wooded environments. Younger australopithecines lived in a wider range of habitats. In contrast, the TM 266 vertebrate fauna contemporary of the Toros-Menalla hominid suggests a mosaic of environments from gallery forest at the edge of a lake area to a dominance of large savannah and grassland. Determining the precise habitat of the TM 266 hominid locality among the mosaic of environments available to it constitutes a research challenge to be met by further laboratory and field studies currently in progress (Vignaud et al. 2002:155).

They (Vignaud et al. 2002) interpreted the succession of dune and lacustrine deposits to mean that the hominids lived in a mosaic environment near sandy desert, but locally including marshy/swampy, lake, and gallery forest. An alternative interpretation might be that the desert really receded (or disappeared) during the later time period when the hominids were there. In either case, the paleoenvironment is interesting, because it means that the Sahelanthropus-like primates colonized (and possibly repeatedly recolonized) areas that were periodically dune desert (and therefore probably not habitable by large primates). This may not mean much in terms of locomotion -- the hominid-bearing unit is clearly water-rich, and we can't refute the idea that the surroundings were as woodland-like as those preserved in the Late Miocene Middle Awash localities.

But I think it is a good hypothesis that all of these apes (or hominids) were very cosmopolitan compared to extant chimpanzees and gorillas. The question is whether their actual dispersal abilities were different from chimpanzees. Prehistorically, genetics would seem to indicate that chimpanzees had long-distance dispersal; the only fossil evidence of chimpanzees has been found in a region that historically did not support chimpanzees; and they today successfully utilize relatively open savanna at the eastern end of their range.

So it is by no means obvious that the cosmopolitan nature of these Late Miocene lineages would have required a specialized terrestrial adaptation -- at least not beyond the specialization of knuckle-walking. So why become bipeds?

References:

Schuster M et al. 2006. The age of the Sahara Desert. Science 311:821. Full text (subscription)

Vignaud P et al. 2002. Geology and paleontology of the Upper Miocene Toros-Menalla hominid locality, Chad. Full text (subscription)

The weblog didn't start from zero a year ago; the sections related to my courses and the Flores files long predate that. But it has been a year since I started daily updates and regular reviews. It has been a great year here, with an immense growth in readership -- December ended with over 1500 daily readers. I want to give my thanks to everyone who has helped, by reading, contributing ideas, or sending papers. Please keep it up!

In the coming year, you'll be seeing more of my writing elsewhere, in addition to some very interesting (and long-awaited) research papers of my own that will be coming out. It should be a year of great announcements, and maybe a few discoveries.

So I think in lieu of a look back over last year, it would be appropriate to start 2006 off with some predictions. Here is a list of my top ten predictions for 2006, ordered from most certain to most speculative. As with most predictions, I've tried to keep an appropriately Delphic tone. And I've excluded almost everything related directly to my own work, which makes the predictions more fair, but leaves a couple of fairly obvious gaps.

• 10. We will see a name for the Flores pathology.
• 9. There will be two Neandertal genome-related announcements.
• 8. No Ardipithecus.
• 7. "Population cluster" will become the new "race".
• 6. There will be another paper (yes, besides the one last month) using genetics to estimate the time of the human-chimpanzee divergence. The date will be 5 million to 7 million years ago.
• 5. Evidence of recent selection will be found for several Y chromosome genes.
• 4. Sahelanthropus postcrania will be published.
• 3. There will be an ancient DNA announcement from China.
• 2. StW 573 will be proposed as a new species ancestral to all later hominids.
• 1. A Hawks weblog post will be cited in a peer-reviewed research paper.
• BONUS: A new Georgian hominid will be a robust australopithecine.

This month's Discover came in the mail today. In celebration of their 25th anniversary, their issue is devoted to "Frontiers of Science", with articles covering the (speculated) cutting edge in different fields. One spread is dedicated to "Human Origins", with a short piece by Carl Zimmer, an interview of Tim White, and a graphic.

Zimmer's article, "Digital ancestors walk again" covers the increasing use of CT imaging and reconstruction of hominid fossils. The subtext is that anything high tech must be better -- for example, the article labors under the misimpression that we cannot study endocranial contours without cutting a skull open.

Also, a read of the article gives the impression that every finding from this new advanced technology supports splitting hominids into several species (in particular, it mentions the Liang Bua endocast reconstruction and the virtual Neandertal growth series assembled by Ponce de Leon and colleagues). Probably this trend will continue as long as few people work with scans except the people who do them.

Here's the conclusion:

As the use of CT scans expands, paleoanthropologists are developing new avenues for uncovering clues to our past. They are discovering signs of healed wounds, of toothless old hominids who must have been cared for by others. Some researchers are even producing full-length virtual skeletons to which they can attach virtual muscles and make the ancient hominids walk again. Most significantly, CT scans can liberate hominid fossils from museum drawers. Once a research team makes a scan, they can post the data on a Web site for other researchers to analyze, bringing a precious hominid fossil to new sets of eyes and new sets of questions.

So utopian....

But Tim White has a different view of the kind of technology changing the field:

What technology advances are changing the way you study evolution?

W: The global positioning satellite system. WIth GPS, we no longer have to worry about the position of a fossil. Some of the biggest blunders in the history of paleoanthropology were made by people who lost the place where a fossil came from. There's no excuse for that anymore. The other big advance is in geochemical dating....

No CT scans there.

Personally, I think CT will have a limited set of impacts. The best thing is that it will allow any lab in the world to have as full a set of comparative data as have been released. Currently, it's useless for that purpose; there's just not enough access. But that is changing, and CT scans are as useful to a practiced eye as casts -- which are much less available today even as CT increases. In fact, high-resolution CT may essentially end casting of new fossils, since that is one of the major sources of damage. We'll be doing a lot of comparative work with imaging in the future.

On the other hand, I think CT will have a really limited impact on the study of new fossils. For one thing, those of us who are used to studying fossils are trained to deal with fragments. People do reconstruct fossils, but reconstruction is not essential to studying most morphology. Another thing we are trained to deal with is distortion. Especially plastic deformation can affect the very shape of the fossils we work with. We correct for it by examining which morphology is affected or unaffected, and by making conservative estimates. CT imaging is too tempting in this scenario -- it encourages people to think they have corrected problems, when instead it is merely adding geometric assumptions.

CT imaging and reconstruction is often proposed as a way to deal with distortion and fragmentation: Zimmer mentions the "sophisticated mathematical software to find the best way to assemble" reconstructions. But these can end up just as biased as any handmade reconstruction, even when the distortions are fairly apparent. There has been substantial disagreement about the CT reconstruction of Stw 505; we may find that the reconstruction of the Sahelanthropus skull faces similar problems.

There are three main benefits of CT reconstruction: it allows repeated trial and error assembly without actually grinding bone contacts against each other, it allows mirror-image substitution for missing parts, and it allows a skull to be geometrically fit to a model. The repeated trials are very valuable: they give an experienced anatomist a chance to try slightly different configurations to sense the range of variation resulting from the state of preservation. The mirror-image reconstruction is valuable for visualization, but potentially misleading for scientific comparison -- try taking a flat mirror and reflecting half your face: notice how hard it is to align the mirror properly and how odd it makes you look and you'll have an impression of the problem. The widespread use of geometric fitting is a potential disaster: by encouraging the use of a model, it reduces the range of biological variability expressed in fossils. When these mathematically-fitted reconstructions are then fed into mathematical comparisons, the structure of the data will be biased by the reconstruction technique in ways that may not be visually apparent.

Anyway, despite all the math, the computer is only as good as the scientist running it -- the principle of "garbage in, garbage out" is everlasting.