Ardipithecus

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!

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

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!

Did I miss a meeting?

Thanks to efforts in Ethiopia and elsewhere, we already know a good deal about A. afarensis. It has been called an 'archaic' hominin for at least two reasons. First, it is old: its fossils date from between 4 million and 3 million years ago. Second, its morphology is archaic, in the sense that its brain case, jaws and limb bones are much more ape-like than those of later taxa that are rightly included in our own genus, Homo. When adjusted for its body size, the brain of A. afarensis is not much larger than that of a chimpanzee, and although it has lost the large canines that distinguish apes from hominins, other aspects of its dentition, such as its relatively large chewing teeth, are still primitive (Fig. 1) (Wood 2006:278).

Every other reference on the internet to "archaic" hominins, hominids, or homininos refers to Middle Pleistocene Homo. So what's going on with this?

I guess that "australopithecine" no longer appeals to folks who want to simultaneously refer to Australopithecus, Kenyanthropus, Ardipithecus, Orrorin, Paranthropus, and whoknowswhatelseensis. So maybe some people are casting around for another term, besides the boring "early hominid" -- oops, "hominin".

It doesn't make sense to redefine "archaic" to mean non-Homo hominids -- oops, hominins. So I thought I would look in my thesaurus for some alternatives:

age-old, aged, antediluvian, antiquated, antique, archaic, back number*, been around*, bygone, creak, dated, decayed, done, démodé, early, elderly, erstwhile, fossil*, hoary, moth-eaten*, obsolete, old goat*, old-fashioned, older, oldie*, out-of-date, outmoded, primal, primeval, primordial, quondom, relic, remote, rusty, sometime, stale, superannuated, timeworn, unfashionable, venerable, vintage

Now, sure "fossil" is out -- but there are a lot of good options here. I think "hoary hominids" is a bit catchier than "old goat hominids", er, "hominins". But maybe "quondam hominins" is the way to go.

References:

Wood B. 2006. A precious little bundle. Nature 443:278-281. Full text (free)

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.

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

Tim White and colleagues (2006) report on new fossils from Aramis and a new site, Asa Issie, with estimated dates between 4.1 and 4.2 million years ago.

In addition to the paper, there are articles in the New York Times (by John Noble Wilford), the Associated Press (by Seth Borenstein), and BBC (by Paul Rincon).

The story is being played as another "missing link" -- this one between Ardipithecus and Australopithecus. From the Times:

Tim D. White, a paleontologist at the University of California, Berkeley, who was a leader of the team, and his colleagues said the 4.1-million-year-old fossils were anatomically intermediate between the earlier species Ardipithecus ramidus and the later species Australopithecus afarensis, the Lucy family. The newfound bones and teeth are the earliest remains of the most primitive Australopithecus, known as anamensis.

"This new discovery closes the gap between the fully blown australopithecines and earlier forms we call Ardipithecus," Dr. White said in a statement. "We now know where Australopithecus came from before four million years ago."

The fossil specimens are a partial maxilla from Aramis, ARA-VP-14/1; two partial maxillary dentitions from Asa Issie numbered ASI-VP-2/2 and ASI-VP-2/334; and a large femur shaft fragment, ASI-VP-5/154. There are also several postcranial bones -- phalanges, vertebrae, a metatarsal -- that are pictured in some of the press accounts and briefly discussed but not pictured or numbered in the paper. The postcanine teeth in the maxillary specimens are larger than the known sample of Ardipithecus, but the canines are larger and more mesiodistally elongated than in Australopithecus afarensis. The best anatomical match for these features is with the Kanapoi and Allia Bay samples assigned to Australopithecus anamensis, and White and colleagues assign the new fossils to that species.

So why are these fossils important? On the surface, there isn't very much to them. Three piecemeal upper dentitions don't tell much. They have big molars and big canines, both within the range of Au. anamensis. Neither they nor the femur shaft extend the known range of variation in early hominids.

Remembering that every fossil fragment is a precious relic of a bygone age, the main importance of these is that they may address hypotheses about the biogeography of Early Pliocene hominids. The maxillae show that a large-molared hominid existed in the same geographic location at a later time than the small-molared Ardipithecus. That could be interesting, and it is the hook for the news stories and the team's press statement.

The strongest part of this story is the geographic -- finding them in the Middle Awash instead of Kenya -- and the paleoenvironmental. There is some suggestion in the paper that there may be a paleoenvironmental difference at the sites that currently have evidence of Au. anamensis:

Palaeoenvironmental circumstances surrounding Au. anamensis ~1,000 km to the south in Kenya have been described for Allia Bay as a mixed assemblage sampling aquatic, forest, grassland and bushland. Nearby Kanapoi conspecifics were found in another mix of environments described as dry, possibly open, wooded, or bushland conditions with a wide gallery forest in the vicinity. Habitat preferences in such mixed assemblages are difficult to ascertain despite the assertion that the hominids "favored mosaic settings". In contrast, the Ethiopian occurrence of Au. anamensis described here allows its tight spatial and temporal placement in a vertebrate assemblage with taphonomic integrity. Its relative abundance suggests that it was a regular occupant of a wooded biome that appears to have persisted in this part of the Afar during the 200,000-yr interval subsequent to Ar. ramidus at Aramis (White et al. 2006:887-888).

This points to two salient facts about the Australopithecus lineage: they were able to disperse effectively across relatively long distances, and occupy at least those habitats where wooded cover and resources were available.

On the other hand, the fossils don't really "fill a gap" between Ardipithecus and Australopithecus, because they are pretty firmly within the time range of known Au. anamensis, being around the same age as the Au. anamensis sample from the Lake Turkana area -- the oldest Kanapoi hominids may be between 4.1 and 4.2 million years old also. The paper points out the other East African examples of Australopithecus at or above 4 million years ago; but it omits the Sterkfontein Member 2 remains, which are also conceivably in the age range of Au. anamensis. Or, for that matter, the Lothagam mandible, which might be the earliest australopithecine even if its date weren't as high as the >5 Ma estimate.

The paper attempts to close off -- for the moment -- the idea that there were allopatric species of early (ca. 4 Ma) australopithecines with differing dietary adaptations. But the paper cannot reject this hypothesis without caveats:

Two phylogenetic hypotheses concerning the origin of Australopithecus can be offered to account for the available data. The first hypothesis derives Au. anamensis phyletically from Ar. ramidus within a 200,000-yr interval [i.e., between 4.4 and 4.2 Ma]. The second involves cladogenesis of Au. anamensis from an ancestor (presumably Ardipithecus or some close relative) even deeper in the Pliocene or Late Miocene. Under the latter hypothesis, Ar. ramidus would represent a relict species in an ecological refugium (White et al. 2006:888).

This latter alternative is the only "bushy" interpretation -- the idea that known species of Ardipithecus can't really be the direct ancestors of Australopithecus, but that there must be some as-yet-undiscovered hominid (or better yet, hominids) that are the common ancestors, cousins, and other bushy relatives of the known species. White and colleagues cannot reject it, but they clearly do not favor it.

In its place, they suggest Ardipithecus ramidus as a lineal, possibly anagenetic ancestor of Au. anamensis, and Au. anamensis as the anagenetic ancestor of Au. afarensis. It's a ladder from primitive to derived, small-molared to big-molared, big-canined to small-canined.

I tend to think this is the null hypothesis -- we have sampled adaptations that differ because of evolution in what is essentially a single lineage of successive species. I say "essentially" because there was not necessarily a wholesale transformation of one species to another across its entire range. Instead, dispersals of new adaptive packages by population movements were probably important biogeographic aspects of evolution in these early hominids. But I think it important to recognize that one species can indeed be the ancestor of a later species.

People who like their phylogenies bushy and their speciations punctuated can take solace in that 200,000-year gap. The finding of Au. anamensis within the already-known time range of Au. anamensis means that the new fossils haven't really added much to the question of phylogenetic diversity in early hominids.

As a postscript, I have a nomination for "most significant sentence" in the paper:

At Aramis, the lone hominoid and largest primate was Ar. ramidus (109 of 6,156 identified specimens so far) (White et al. 2006:888, emphasis added).

References:

White TD and 21 others. 2006. Asa Issie, Aramis and the origin of Australopithecus. Nature 440:883-889. 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)

There's a new paper by Tim White in the "In Press" portion of Comptes Rendus Palevol, titled "Early hominid femora: The inside story". It has a short introduction to the importance of the Orrorin proximal femur to understanding the evolution of hominid bipedality.

That short introduction is followed by an four-page-long description of White's correspondence attempting to get photographs, scans, and measurements of Orrorin. He quotes his own e-mails. With dates. I've never seen anything quite like it in a journal.

The review ends with this paragraph:

It is unclear why the Orrorin discovery team and its associates will not publish the comparatively very simple conventional radiography and conventional photography of the unglued BAR 1002'00 femoral neck that we have urged on numerous occasions (see above) since 2001. Martin Pickford and Brigitte Senut mysteriously did not join the list of authors who responded to our last, published request for these data in our February 2005 letter to Science. Their American colleagues responded: "it is our understanding that the initial studies were carried out under serious constraints of time and other resources [...] and we have made it clear that we plan to rescan and study the existing fossils if funds are made available" [5 (p. 845)]. We were again disappointed because we had asked for the publication of new data, not the promotion of a funding request for documentation long overdue.

This quote refers to the 2005 exchange between Ohman, Lovejoy and White on the one hand and Eckhardt, Galik and Kuperavage on the other. Read it too.

The cited response ends with this paragraph:

As far as phylogenetic speculations, a fuller understanding of the first several million years of human ancestry awaits the outcome of studies (already under way by other members of our research group) of the equivocal hominoid remains from Chad, as well as some much more comprehensive results from the by now decade-long analysis of the Ardipithecus (née Australopithecus) ramidus fossils, the reported fragility of which nonetheless should not preclude the making of CT scans and publication of what they show.

Well, I know which of these folks have shared data with me...

I have an idea for a contest. Please send your best punchline for the following joke, and I'll post the top ten (let me know if you want credit!):

How is Bigfoot different from a Miocene hominid?

References:

Eckhardt RB, Galik K, Kuperavage AJ. 2005. Questions about the Orrorin femur. Science 307:845. Full text

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

White TD. 2006. Early hominid femora: the inside story. Comptes Rendus Palevol (in press). Full text (subscription)