Sahelanthropus

OK, I'm going to live-blog this show. I've been looking forward to it for a while -- I loved the old NOVA series with Don Johanson and have often showed it in classes but I had to stop several years ago because it's getting out of date. These are great overview-type programs, unlike the more special-purpose one-topic shows.

The producers gave me the opportunity to review the program's script a few months ago (that's explains the acknowledgement at the end), so I'm not expecting any unpleasant surprises.

The pre-credits opening: Naked people smiling. Naked chimps grooming...

7:01: "What set us on the path to humanity? The questions are huge, but at last, there are answers..."

"For millions of years, many human-like species coexisted on our planet, until one day, there was only us."

7:03: "Apes that had walked on four legs stood up and walked on two." We see apish CGI hominins. Then, to the Sahara to see Toumaï. Michel Brunet is describing the skull.

"We, Homo sapiens, are the first ever to be alone."

7:06: To the Afar, explaining the Rift Valley and its erosive contexts. The Insta-Zoom effect across the desert is actually kind of cool. We see Zeresenay Alemseged driving an SUV, then walking in badlands with scattered bones. Nice photographs of the Dikika skull in context.

7:09: Zooming backward into a timeline, as if the years are sucking us back, the program explains the timespan of human evolution as a series of doublings backward in time.

7:10: Alemseged is in the National Museum of Ethiopia, preparing the skull. It's a nice video treatment, shoing the slow preparing with dental drill. The long shots of the postcranial elements are very illustrative -- this is a good demonstration of how the anatomy informs us about the developmental schedule and lifeways.

7:13: Don Johanson is explaining how he found AL 129-1. Then, he explains the difference between the chimpanzee and human pelvis. Too bad they couldn't have included Ardipithecus; it would be interesting.... I'm really liking the fact that you have people interacting with actual casts instead of lots of CGI images. You have a much better impression of the scale

7:15: Now the scene moves to Kenya, this is going to be about paleoenvironments. Yannic Garcin and Daniel Melnick are describing how the now-desert landscape was once much wetter. We go back to the Afar, with Alemseged explaining the fauna that's just eroding up out of the ground (wonder how set up that scene was...).

7:18: Bipedalism. It's like Saturday Night Fevur. Brian Richmond appears to explain theories about why bipedality was adaptive. This is all accompanied by contemporary dancers wiggling around. Chimpanzee-like ancestors are illustrated with video of actual chimpanzees (wonder what Lovejoy is thinking...). Dan Lieberman is talking about energy budgets. People and chimps on treadmills hooked up to oxygen meters.

7:22: Mark Stoneking explains the molecular clock. "The dates that one almost always gets are 5 to 7 million years ago for when humans and chimpanzees shared a common ancestor."

7:24: We go to Chad. Brunet explaining why they needed to recover fossils from somewhere other than East Africa. "Everyone said 'no', there just aren't any [human-like] fossils there."

7:26: "There were no bones apart from the skull..." Er...

7:27: The skull is reconstructed with a CT scanner and then cast. Oops...the rest of the shots of casts are all taken directly from the skull, not the 3-d scan version. Nice artist's rendering of Toumaï here.

7:30: I'd hate to be one of the dancers walking by on the screen with the voiceover, "Walking upright didn't mean that they had big brains."

7:33: Brain growth in Selam. Hints of a longer childhood -- of course, at 330 cc, it's almost the size of a full-grown chimpanzee. Todd Preuss is discussing the evolution of the brain, showing us actual pickled brains of human and chimpanzees. Lunate sulcus -- was Selam like a human or a chimpanzee?

7:35: Ralph Holloway is describing the brain reorganization -- great shot of him with his collection of endocasts. The conclusion is that the lunate sulcus was human-like.

7:37: Now we have stone tools appearing, Brian Richmond explains how we recognize tools. Unlikely they were made by Australopithecus, because they didn't make them earlier. Skip forward to KNM-ER 1470, "the dawn of a new era, beginning around 2 million years ago." Tools were used for meat processing. Homo habilis was small in body size, but had a much bigger brain than Australopithecus.

7:41: Viktor Deak is reconstructing Homo habilis. I like it, more apish than the usual rendering.

7:43: "Africa's gradual drying trend was punctuated by bursts of rapid climate fluctuation." We see Rick Potts explaining the stratigraphy of a lake alternating with desert and volcanic layers over time. The idea of "variability selection" is explained.

7:45: Analyzing diatoms in layers of rock -- the species tell the alternation of shallow and deep lake levels. It's a record of strong fluctuations. We see rapid clips of three different scientists (Potts, John Kingston, and Mark Maslin) talking about water fluxes. It's a good way of explaining the climate instability -- although they could have gone a bit further: when they mention "Lake Victoria-sized lakes appearing and disappearing", for example, they might have pointed out that Lake Victoria itself has appeared recently.

7:48: Dust from ocean cores. Once again, it comes down to tiny sea creatures whose anatomy correlates with date.

7:50: We get a rapid montage reviewing the climate instability idea. Hmmm...I have to say that the very fast cutting of clips and louder music doesn't really add to the credibility of the idea -- it seems like something is being left out.

7:51: Rick Potts restates the variability selection argument. "Simple but revolutionary idea -- human evolution is nature's experiment with versatility...we are creatures of climate change."

That's the end. I think the paleoenvironment story was well done. The shots of how this science is done were very illustrative -- from the field to the lab, the program showed the fine layers of sediment and careful study of microscopic creatures.

On the other hand, the show may have gone a little too far in the "climate made everything happen" direction. I don't think the "variability selection" idea explains the origin of Homo, and while the program did briefly list alternative views about the adaptive value of bipedality, it left no doubt that African desiccation and loss of forest was the ultimate cause.

I think everything with actual fossils, dirt, or rocks was well done. In particular, we got a good view of most of the Selam skeleton, with the notable exception of the hyoid bone. These are the best available images of the specimen to date. Holloway's descriptions of endocast evolution were well done, placed in the middle of a big table of fossil casts. I like the solidity with which the program showed the fossil record. Hopefully the next two segments will also follow this technique -- much preferred over the CGI-reconstruction technique.

I will be out of the country for the next two parts of the trilogy, so I'll have to see if I can get them online. The NOVA Evolution website has the first episode online now, so there's some hope.

Now if you really want to beat the science press, it helps to have readers who take really obscure journals.

I haven't written much here about Sahelanthropus. In 2002, I joined a number of other people who doubted the evidence for bipedalism in what was then (and has since been) touted as the "earliest known hominin." Maybe it is, maybe it isn't. But we weren't convinced by the evidence. Dental similarities with early hominids were also shared with a number of Miocene ape genera, while the evidence for vertical posture, based on the position of the foramen magnum, seemed weak. We made our case in a 2004 paper in PaleoAnthropology, which is open access. Since no new evidence has come to light (or at least, to print), that's where matters stand as far as I'm concerned. I still think the evidence for bipedality in Sahelanthropus is equivocal, and that the specimen's date may be too old to be a member of the hominin lineage.

Meanwhile, also in 2004 began a strange series of events involving Alain Beauvilain, a geographer working with the original Sahelanthropus excavation project. I have no personal knowledge of the details, aside from the published record -- so that's what I'll stick to.

Yes, this story does end with camelherders burying fossils....

Dental associations of Toumaï questioned.

First, Beauvilain teamed up with a dentist named Yves Le Guellec, publishing a short paper in the South African Journal of Science that questioned the assignment of a tooth to the Toumaï mandible. Their suggestion was that the tooth must represent a second individual, and that the original research team had mistakenly glued it into the mandible. They did not question other aspects of the original research, and concluded their paper:

The present analysis, based on some Sahalanthropus paratypes, is only one element of the discussion about this genus. It does not modify the basis of the debate concerning the systematic position and palaeoecology of Sahelanthropus (Beauvilain and Le Guellec 2004:144).

That seemed pretty milquetoast to me, and if I'd been blogging at the time (which I wasn't), I might not even have taken notice.

What was newsworthy about Beauvilain's comment was the reply by Brunet's Sahelanthropus research team. They provided images, including CT scans, that supported their original assignment of the tooth to the mandible. And then, the reply was followed by a statement signed by 28 other paleoanthropologists, led by F. Clark Howell. Here is that statement in its entirety:

Sir, — We, the undersigned, have carefully examined the photographs and digital crown images of a fossilized third molar from the upper Miocene of Chad. This tooth was originally identified by the discoverers (Brunet et al. 2002, Nature 418, 145–151) as a right third mandibular molar. A recent paper by Beauvilain and Le Guellec (2004, S. Afr. J. Sci. 100, 142–144) claimed that this tooth had been misidentified and was in fact a left lower third molar. Based on crown anatomy evident in the images examined by us, we confirm the identity of this tooth as a right molar, as originally published by Brunet et al. (2002).

Rex Dalton covered the reply in Nature, in a piece titled, "Brickbats for fossil hunter who claims skull has false tooth". All I can add is, I hope this crew never comes after me.

This exchange continued for a couple more letters, as Beauvilain and Le Guellec found errors in the analyses published by Brunet and colleagues. They may not have been correct about the tooth, but they had showed a string of apparent mistakes made during the research on Toumaï.

Stratigraphy of Sahelanthropus questioned.

That was all in 2004. In 2008, Beauvilain published an article in the South African Journal of Science in which he asserted that neither Toumaï (the type specimen of Sahelanthropus tchadensis) nor the type specimen of Australopithecus bahrelghazali had been found in situ by Brunet's team. This short article was apparently a reply to a PNAS report on the dating of the two discoveries by Anne-Elizabeth Lebatard and colleagues (2008). That paper placed the two fossils within stratigraphic sequences and provided dates for the strata. Beauvilain (2008) wrote that the placement could not be certain:

From January 1994 to July 2002, the author of the present note was in charge of every palaeontological field expedition that took place in the Chadian desert. In the interests of scientific accuracy, he is compelled to state formally that neither of these fossils was found in situ. The most appropriate word to employ for the two fossils would be that they were 'collected' from their respective localities, which are the sites TM 266 in Toros Menalla and KT 12 in Koro Toro for S. tchadensis and A. bahrelghazali, respectively.

...

The photograph taken by the author at the moment of discovery has been widely published but never with a proper explanation. He thought that the image was sufficiently evocative to indicate the lack of stratigraphical context of the fossil. It is time to correct this omission. In the image, the hollow in the sand to the right of the skull is the exact spot at which Toumaï was found. Almost all the fossils from TM 266 were in similar situations with respect to the substrate. By a curious choice, the photographs of Toumaï published by [Brunet's team] as representing its field context are in reality those of a darkened resin cast, posed in the desert on a ridge of sand created by the northeast winds in February 2004.

Beauvilain did not in this article suggest that the true ages of the fossils might be radically different than those of the strata reported by Lebatard et al. (2008) -- a substantial depth of deposit around the Sahelanthropus site falls within a time range between 6.8 and 7.12 million years ago. He does suggest that the aeolian pattern of erosion in the area and some faulting may have caused undercutting of some fossils and substantial movement against the vertical section.

Cosmos magazine picked up this story last year, although I have not seen it reported elsewhere. I noted it here at the time. I haven't seen any reply to Beauvilain by the rest of the excavation team.

That's the published record. What did Beauvilain accomplish with this paper? He reiterated his position on the field project and gave clear reasons why he had stepped forward to publish his note on the specimens' provenience. This additionally presented the context in which he took his own photos of the Toumaï discovery. The photos are the crucial element.

Buried facing Mecca?

Now the story gets weird. Beauvilain's new paper with Jean-Pierre Watté has been blowing into paleoanthropologists' inboxes around the world. My library doesn't get the Bulletin de la Société Géologique de Normandie et des Amis du Muséum du Havre, but I've gotten copies of it from three different sources. Considering how widespread my sources are, I know that a good fraction of my colleagues have at least had their chance to read it.

I don't tend to pass around mere rumors, even if they're really juicy (you know who you are....). But this is no rumor -- it's a real publication. Nobody quite seems to know what to think about it. So let's get it out in the open.

Here's the English version of the abstract:

Was Toumaï (Sahelanthropus tchadensis) buried?

Photographs taken when the skull of Toumaï was discovered establish that the holotype of one of the earliest known hominid species was probably reburied in the recent past. Taphonomic analysis reveals the likelihood of one, perhaps two, burial(s) which seemingly occurred after the introduction of Islam in the region. Two other hominid fossils (a left femur and a mandible) were in the same "grave" along with various mammal remains.

I'm going to give a fairly long summary of their argument, providing English translations of the key points. They have no direct evidence that the bones were altered in position or deliberately arranged, but they do present an interesting circumstantial argument. There are essentially four points:

1. The skull was associated with an unusual concentration of fossils. As Beauvilain and Watté (p. 20, my translation) introduce their paper:

While fossils of this site are distant from each other, scattered at random and without concentration of large size, Toumai was part of a pile of bones. In general, with the exception of skeletal remains belonging to a single individual, concentrations of fossils are rare in Djourab sites. That is why the Chadian coauthors to the discovery said this cluster was a "dustbin of palaeontologists." This expression does not correspond to reality. The "garbage" of paleontologists, and geologists, experienced in this area of Toros-Menalla, as well as that of KB, are of a different nature: broken bones, brittle, "digested" by the desert crust, not identifiable, with sometimes broken glass of beer or demijohns. A skull could not have been left there, unless it was completely hidden beneath the crust. An examination of Figures 1a and 1b can offer a different explanation.

The picture below shows the association. It is not possible to say how unusual this may be without some statistical study of the area around the site. I'm noncommittal, but am willing to accept Beauvilain and Watté's description at face value.

2. The arrangement of the bones. I can't discuss the paper without showing the photographic evidence that Beauvilain and Watté rely upon. So here's Figure 1b from the paper:

According to the paper, this is the layout of the Toumaï skull and surrounding fossil bones at the time of discovery. Beauvilain and Watté suggest that this is an artificial assemblage of fossil bones. As you can see, they have labeled "two parallel lines" along which the bones are arranged. They do not judge this to be a likely result of aeolian erosion in situ, and consider it a possible sign of a deliberate arrangement of the fossils.

As I've noted, Beauvilain (2008) established the reason why he has photos of the original find. With photographic evidence such as this, the constant worry is that some kind of alteration has been made with Photoshop. Has false color been added? Have objects been composited together? I'm no forensic photo expert, but I see no obvious signs of trickery.

Are they really two parallel lines? Clearly there is at least one straight line here (the top one in the figure), with other fossils arranged in rough accordance with that line. It's not random, but whether collection in an aeolian depression might explain the linear arrangement, I don't know.

3. When it was found, the skull was lying on its left side, with its right side exposed. But both sides of the skull and its adhering matrix appear to have been equally eroded and patinated, including a process of "bluing" the dark matrix, a chemical alteration that occurs on surfaces exposed to air and light. This suggests that the entire skull was exposed for some time. This is not explicable if it was eroding out of a rock layer in situ, but only if it had been rotated, either by aeolian forces undercutting it for some time or because the skull was transported from some other location (p. 23-24).

Also, in favor of the skull having been moved, Beauvilain and Watté suggest that the matrix adhering to the skull appears to be somewhat different from the other fossils:

Par ailleurs, aucune trace n'est visible sur le crâne ou sa gangue de ce ciment blanc, très siliceux et très adhérent aux fossiles, qui caractérise les autres pièces collectées à TM 266, comme celle de la mandibule de Sahelanthropus TM 266-02-154-1, et sur un grand nombre d'ossements de la zone dite de Toros-Menalla. Ces différentes pièces n'ont donc pas strictement le même age (23).

In addition, no trace is visible on the skull or its matrix of this white cement, very silicious and adhering to fossils, which characterizes the other pieces collected at TM 266, including the mandible of Sahelanthropus TM 266-02-154-1, and on a very large number of bones from the zone called Toros-Menalla. These different pieces are therefore not strictly the same age (my translation).

I think these points are fairly compelling. That skull ought to be asymmetrically weathered and patinated unless it has been rotating around a lot. It ought to be more weathered than the surrounding bones, which clearly present a lower profile to the elements.

4. The line formed by the fossil bones is oriented toward Mecca. I'll say right off, this is the least convincing element of their argument. The "parallel lines" in the figure are northeast-southwest, which is only "toward Mecca" in the general sense (as described below). Given a line, it has to point in some direction, and there's a good chance that it will be within 45 degrees of any random line.

However, it does help to tie the story together. Here is the full scenario, as outlined in the paper's conclusion (my translation of pp. 24-25):

The location of the skull in relation to the two rows of long bones evokes the disposal of a body reduced to the status of a skeleton.... This arrangement cannot be natural. In view of its configuration, it is suggested that it arose from the desire to give these remains the honor of a burial. The skeleton was reconstructed from the skull conceived as "human". Indeed, this head, which looks like the head of a man, cannot remain indifferent. In this perspective, the authors of the burial -- if that is indeed the case -- would have done their best, according to their anatomical knowledge and the fossils of animals they could find here and there, and that they may have confused with human bones, in order to bring the skull together with the other elements of his body. They put a close to the humeral head and distal end, well-oriented, a femur at the other end of the "corpse". Toumai therefore is subject to a burial in recent times.

Why a burial? The abundance of fossils in this part of the desert makes it so that today, people remark on it but do not think more than that some stones resemble animal bones, including jaws, without giving them much importance. Children play with them. But to the contrary, this skull that so resembles a human skull could not be taken for a toy and abandoned to children.

Who could have been the authors of this burial? The orientation given to the "body" correponds to the diagonal northeast-southwest. This direction is important: it is the line toward Mecca. This burial, if such is the case, could have been done by nomads who cross the region regularly. Muslims since the 11th century with the conversion to Islam of the sovereigns of the first kingdom of Kanem -- the capital of which lay not far from the place where Toumaï was found -- these populations became religious enough to offer a decent burial to their human brothers. In fact, the Muslim religion makes it an obligation to inter cadavers. The precise tradition in which the body must be lain out with the head oriented in the direction of Mecca. Without instruments, believers take the direction of the rising sun, which, near the Tropic of Cancer, is highly variable during the year when it may be in either the northeast or the southeast.

Is it a convincing scenario? It seems a little far-fetched if you imagine that the desert was an untouched wilderness where humans would never have laid eyes on these objects. But to give them as much credit as possible, Beauvilain and Watté note that today's "desert" was before 1900 much more frequently trafficked than today. Human activities are still widespread in the area and were formerly more so. This is not untrammeled wilderness, and according to the paper it might be very likely for a humanlike skull, laying on the desert floor for decades, to have been encountered by local people and reburied.

At this point, it is an ethnographic question: Other burials must periodically emerge from the ground. What happens to them? Is there other archaeological evidence of reburial of skeletal remains, either in this immediate area or elsewhere in North Africa? I haven't found any other citations of such a practice, but it must be rare and episodic in any event.

What does this mean?

At the end I have to say I'm ambivalent. I have no real reason to accept or deny this new report or the original studies. Beauvilain has a record of pointing out genuine inaccuracies in the publications of Brunet's group. Beauvilain and Watté base their specific claim of a recent reburial is only on circumstantial evidence, which is tenuous.

Yet the evidence does seem sufficient to support Beauvilain's 2008 position as regards the surface collection of the specimens and the possible transport of the skull. Beauvilain (2008) claimed that Toumaï had not been discovered in situ, and that is relation to the published stratigraphy was therefore unclear. At one level, this current paper says nothing more.

Sahelanthropus may still be precisely what its discoverers have claimed: A Late Miocene representative of the human lineage. The fossil is what it is, and the nearby sediments are Late Miocene in age. If people had transported it some short distance and reburied it, it would still probably be Late Miocene in age -- although I would want to see somebody do a systematic comparison with local sites to be sure.

At worst, the skull represents a somewhat different date, and is not clearly associated with the accompanying fauna. In that case, a fuller understanding of the local paleontology, along with (hopefully) new discoveries of Sahelanthropus, should clarify matters.

Beauvilain himself appears to be a big booster for Sahelanthropus (having published a trade book on the discovery) and has maintained in each of his critical publications that it is likely the first member of the human lineage. He seems to be aiming at precision in the descriptions, not trashing the species and not offering any new interpretation of its anatomy.

But I'm not a big booster of Sahelanthropus. And there's one significant thing in this paper that even the sharp-eyed might miss. Something that you might reasonably wonder about, considering the discoverers and describers of Sahelanthropus have never once mentioned it in print.

Toumaï's skull was found alongside a femur.

More on this later....

References:

Beauvilain A. 2008. The contexts of discovery of Australopithecus bahrelghazali (Abel) and of Sahelanthropus tchadensis (Toumaï): unearthed, embedded in sandstone, or surface collected? S Afr J Sci 104:165-168.

Beauvilain A, Le Guellec Y. 2004. Further details concerning fossils attributed to Sahelanthropus tchadensis (Toumaï). S Afr J Sci 100:142-144.

Beauvilain A, Le Guellec Y. 2004. Reply to Brunet et al. S Afr J Sci 100:445-446.

Beauvilain A, Watté J-P. 2009. Toumaï (Sahelanthropus tchadensis) a-t-il été inhumé? Bulletin de la Société Géologique de Normandie et des Amis du Muséum du Havre 96:19-26.

Brunet M, Guy F, Pilbeam D, Lieberman DE, Likius A, Mackaye HT, Ponce De León MS, C.P.E. Zollikofer CPE, Vignaud P. 2005. New material of the earliest hominid from the Upper Miocene of Chad. Nature 434:752‑755. doi:10.1038/nature03392

Brunet M and 28 others. 2004. Sahelanthropus tchadensis: the facts. S Afr J Sci 100:443-445.

Dalton R. 2004. Brickbats for fossil hunter who claims skull has false tooth. Nature 430:956. doi:10.1038/430956a

Howell FC and 27 others. 2004. Untitled. S Afr J Sci 100:446.

Lebatard A-E and 13 others. 2008. Cosmogenic nuclide dating of Sahelanthropus tchadensis and Australopithecus bahrelghazali: Mio-Pliocene hominids from Chad. Proc Nat Acad Sci USA 105:3226-3231. doi:10.1073/pnas.0708015105

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

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

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!