Embedded reporting from the Middle Awash
Rex Dalton of Nature has a very interesting article recounting his experiences with the Middle Awash project. For those unfamiliar with the research, there is a quick review of the participants:
Much of [the team's] success can be traced to the project's multinational roots. It represents the best of scientific capacity-building: African scientists receive doctorates at top universities overseas, and then return to work and nurture projects at home. Scientists from abroad, such as [Tim] White -- at the University of California, Berkeley -- are in the minority. The team's other leaders are Ethiopians: there is Giday WoldeGabriel, a geologist at the Los Alamos National Laboratory in New Mexico; the palaeoanthropologist Berhane Asfaw, director of Ethiopia's National Museum in Addis Ababa; and Yonas Beyene, a government archaeologist. Yohannes Haile-Selassie, a key team member who received his doctorate at Berkeley, like Asfaw, is curator of physical anthropology at the Cleveland Museum of Natural History in Ohio, a bastion of hominid research.
It's a very nice picture of paleoanthropological fieldwork, with much detail about the Bouri localities, and it is well worth reading. Here's another snippet:
Back at the trucks, an Afar girl waits for us with a brick-shaped elephant tooth. White jokingly suggests our police guards arrest her, and she is asked to return the fossil to its location. He worries that keeping such items encourages locals to remove fossils from their surroundings, destroying vital geological information.
A week later, the team returns to ensure the fossil was replaced. And there, near the elephant tooth site, they find fresh fuel for their fever -- a hominid tooth shard.
Bushels of earth around it will be sieved for any remaining pieces. Tuff dating will follow, then site maintenance. A published article may be years off, but once again Afar shows where it all began.
Sounds like they should hire the girl. I hope they at least make her a coauthor!
References:
Dalton R. 2006. Ethiopia: Awash with fossils. Nature 439:14-16. Full text (subscription)
Afar by NASA
NASA's Earth Observatory has made an overhead shot of the Afar Depression its image of the day (via MetaFilter):
In eastern Africa, in the Afar region of Ethiopia, a nearly barren rockscape marks the location of the meeting place of three separate pieces of the Earth's crust. This meeting place is known to geologists as the Afar Triple Junction; the central meeting place for the three pieces of Earth's crust is around Lake Abbe, just to the south of the area shown in this image from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA's Terra satellite. The three pieces of Earth's crust are each pulling away from that central point, though not all at the same speed.
Understatement-posing-as-segue of the day: "Besides its unusual geology, the Afar region is famous for its fossils."
Book review of "The First Human"
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.
Ardipithecus remains from Gona
Paleoanthropologist Sileshi Semaw has a new paper in Nature describing fossils of Ardipithecus from the Gona research area in Ethiopia. Semaw is a researcher at the CRAFT Stone Age Institute at the University of Indiana. (I would just like to enter a note here that the web pages of the CRAFT Institute and the Department of Anthropology at Indiana are extraordinarily difficult to find and use. Can you imagine a university department with a Flash-only website? Compared to this, the CRAFT page is easy to use, but hard to find. The most important problem is that the word "CRAFT" appears nowhere on the site or its address (www.stoneageinstitute.org), and so searches don't find it! Maybe it will move up the Google rankings soon....)
The fossils come from a locality called As Duma, and likely date to between 4.3 and 4.5 million years ago, based on stratigraphic and paleomagnetic considerations. The remains are mostly dental, with three substantially preserved dentitions and several isolcated teeth. There is also a fragment of mandibular corpus and a mandibular ramus matched with one of the mandibular series. Otherwise, there are three hand bones (phalanges) and one toe bone.
Is there evidence for bipedality?
As is the case for other remains from Ardipithecus, here it is argued that the toe bone represents a biped, because the proximal articular facet is oriented upward (dorsally). This means that the toes were characteristically dorsiflexed rather than plantarflexed, and therefore were used for walking but not substantially for gripping objects.
Why are the fossils assigned to Ardipithecus?
The teeth resemble known dental remains of Ardipithecus. General similarities include the size of the postcanine teeth, the height of the upper and lower canines, and the diamond-like shape of the upper canine in labial view (the broadest mesiodistal point is about halfway between the apex and base of the crown). These features are not entirely compelling because of the notable variability among early australopithecines, including A. anamensis and A. afarensis. According to Semaw and colleagues,
The P4 crown is mesiodistally compressed with an oval plan, a simple cusp pattern and a single root matching the Aramis (Ethiopia) specimens of Ardipithecus ramidus and distinct from the triangular/quadrangular cervical cross-section of Australopithecus multi-rooted P4.
In other words, the fossils "fit" well with Ardipithecus by virtue of their anatomy and date, and have at least one tooth crown that is fairly distinctive.
Is there any other Ardipithecus insight?
The evolving story of Ardipithecus is that it is a likely ancestor of later hominids. Contrast this with the situation in 1995, shortly after the initial announcement of both Ardipithecus ramidus and Australopithecus anamensis. These two hominid species overlapped in time, but had clear anatomical differences. Although Ardipithecus had an earlier range of dates (4.1 million to 4.4 million years ago according to White et al. 1994), it was viewed to be morphologically distinct from all later hominids, in particular having molars that were described as smaller, with thinner enamel and more chimpanzee-like morphology than any known australopithecine.
But being described as "chimpanzee-like" has obfuscated much about the anatomy of Ardipithecus. Consider the dental features of A. afarensis compared to living apes and humans: it has thick molar enamel, large molars, molars that increase in size posteriorly, canines that are variable in height, ranging on the small end to incisor-sized, and on the large end still much shorter than females of living ape species. Australopithecines evolved from apes with large projecting canines. It is unclear whether the Miocene apes that were ancestral to later hominids had thick or thin enamel, or large or small molars, because these traits are variable among Miocene hominoids. But they certainly had smaller molars and premolars than the australopithecines, and could not have matched their enamel thickness. Therefore, it is exceedingly likely that the early ancestors of the australopithecines had smaller molar sizes, thinner molar enamel, larger canines, and cutting P3's. Compared to A. afarensis these ancestors would be chimpanzee-like in these details.
Is this what the initial description of the Aramis hominids described? Let's consider what White and colleagues (1994) wrote:
Morphology of the known Aramis canines, however, diverges from that of known apes. The upper canines are slightly less incisiform than homologues of A. afarensis but more incisiform than any ape counter part, with occlusally placed terminations of the mesial and distal apical crests. The visual result of apically placed crown shoulders is a low, blunt canine tooth relative to more projecting ape canines, a morphological condition which may have important evolutionary implications. The Aramis upper canine is large buccolingually, formaing a further contrast with mesiodistally elongate African ape canines. Wear pattern also differs significantly from the ape condition. Mandibular canine wear does not show the pattern typical of great apes. (p. 308)
The broken canines and lower P3 in ARA-VP-1/128 and -6/1 exhibit thin enamel distinct from previously known hominid conditions. Canine enamel thickness approximates the chimpanzee condition, with a lack of apical thickening we observe in other hominids. ... The relatively thin enamel and large size of the Aramis canine, together with its primitive P3 morphology, suggest a C/P3 complex morphologically and functionally only slightly removed from the presumed ancestral ape condition. (p. 308)
The ARA-VP-6/1 P3 is markedly more apelike than any A. afarensis homologue in its high protoconid with extensive buccal face and steep, distolingually directed transverse crest. In these features it is indistinguishable from ape homologues. (p. 308)
Molar morphology resembles the A. afarensis condition, but lacks the extreme buccolingual breadth relative to mesiodistal length common in that species. The 'serrate' root pattern and deep dentine wear on the buccal cusps described in A. afarensis, Tabarin, and Lothagam also occur in Aramis specimens. All molars lack the extensive crenulation and broad occlusal foveae characteristic of modern chimpanzees, or the high cusp topography of gorillas. (p. 309)
On these dental characters, the Aramis hominids do correspond to what we might expect of an ancestor of later hominids. The "chimpanzee-like" character is canine enamel thickness. Other characters are either near or within the range of later hominids, or more apelike. In this context, apelike means that these characters are not specifically similar to any living ape, and indeed the living apes are substantially different from the Aramis remains in several characters. In particular, the chimpanzee-like dm1 is not specifically like chimpanzees, but instead is in the range of all living and fossil apes. This is a plesiomorphy, not a derived similarity with chimpanzees.
Aramis enamel thickness is similar to chimpanzees not only in the canines but also in the molars. But enamel thickness itself is a problematic character. White and colleagues (1994) say:
A distinct difference from known hominids occurs in molar enamel thickness. Maximum radial enamel thickness of crown faces can be measured in three fractured Aramis specimens and it ranges from 1.1 to 1.2 mm buccally, at or near the unworn cusp apex, perpendicular to the enamel-dentine junction. These values are comparable to the uppermost range of our homologous enamel thickness values measured on broken P. troglodytes molars (n=22; M1 through to M3). Equivalent measures in A. afarensis range from 1.4 to 2.0 mm (n=5). (p. 309)
Of course, this places the Aramis specimens intermediate between chimpanzees and other early hominids. White and colleagues (1994) did not report the range of the chimpanzee comparative sample, so it is hard to say what the import of the enamel thickness of Aramis really is. Haile-Selassie (2001, 180) writes:
Another candidate for hominid ancestry is the recently described Orrorin tugenensis. The authors report thick molar enamel and suggest that Ardipithecus and African apes are commonly derived in having 'thin' enamel. However, enamel thickness is a complex character and intraspecifically variable, and its within-tooth three-dimensional patterning is characteristically expressed both serially and taxonomically. Therefore the simplistic dichotomous characterization of enamel as either 'thick' or 'thin' on the basis of unspecified measurements of naturally broken sections (as was done in the Orrorin report) is problematic.
Of course we can compare this quote with the report of Senut and colleagues (2001, p. 4), "Enamel thickness at the apex on the paraconid [of the BAR 1000'00 left M2] is 3.1 mm. This is comparable to other hominids, Ardipithecus excluded." There certainly does seem to be a difference between the Middle Awash sample and Lukeino in enamel thickness, based on the limited number of examples. It is a shame that the range of variation in living hominoids has not been better reported in the context of these early hominids. But even if everything rises or falls based on enamel thickness alone, we have one sample (Middle Awash) with an intermediate morphology between australopithecines and chimpanzees, and another sample (Lukeino) with an australopithecine-like morphology, which is probably also more similar to earlier Miocene apes like Ouranopithecus.
White and colleagues have clearly moved toward an interpretation of the early hominids in which the taxonomic diversity was minimal. For example, Haile-Selassie, Suwa and White (2004, p. 1505) wrote:
Metric and morphological variation within available small samples of late Miocene teeth attributed to A. kadabba, O. tugenensis, and S. tchadensis 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.
One may point out that if the Asa Koma and Lukeino hominids actually do represent a single species, they are properly referred to Orrorin (or Ardipithecus) tugenensis, rather than A. kadabba. One may speculate that it is for this reason that Haile-Selassie, Suwa and White (2004) focus on the genus level in their argument, since it is at this level that Ardipithecus has priority. It is worth mentioning that the enamel thickness in the A. anamensis molars is more similar to later australopithecines than to Ardipithecus, with measurements around 1.9 mm on the protocone of KNM-ER 30748 (Ward et al. 2001). Again, the measurements here emphasize the problems with comparison of such values, since measurements on fossils are taken where breaks naturally occur, and with the possibility of attrition on the teeth.
At any rate, this argument continues with the discovery of the Gona Ardipithecus remains. According to the BBC, Tim White
agreed it was becoming apparent A. ramidus was an important species that was a very plausible ancestor to later hominids. "It's already clear that we're seeing the basic grade from which Australopithecus evolved,"he told the BBC News website.
One feature of these hominids points toward that conclusion--even at the expense of A. anamensis. The reconstruction of the GWM-3/P1 mandible has a shape similar to AL 288-1 (Lucy) with diverging (V-shaped) tooth rows. Semaw and colleagues contrast this morphology with A. anamensis, where the tooth rows are oriented parallel to each other.
Is it possible that the Kanapoi and Allia Bay hominids were a side-branch of an Ardipithecus--A. afarensis lineage? Sure, there is actually little to preclude this scenario beyond the relatively thicker enamel and larger molars of the sample assigned to A. anamensis. It would be helpful to have a full characterization of the variability of these samples as they existed over time, to judge the magnitude of change necessary under different evolutionary scenarios.
But there is one more point on which White and colleagues (1994) described Aramis as chimpanzee-like: the basicranium.
The ARA-VP-1/125 and -1/500 specimens represent adult temporal and occipital regions. Both are smaller than their A. afarensis counterparts, but no female temporal is known for that species. The Aramis cranial fossils evince a strikingly chimpanzee-like morphology that includes marked pneumatization of the temporal squama which even invades the root of the zygoma. (p. 310)
And the postcranial remains are described as displaying "a mosaic of characters usually attributed to hominids and/or great apes" (p. 311). In fact, the known postcrania are an area of resemblance between Ardipithecus and A. anamensis, since both these samples appear to have relatively long forelimb bones compared to later hominids (this comparison is imperfect since body size is not known from other evidence). And the basicranium of A. anamensis was probably similar to Aramis, considering that the KNM-KP 29281 temporal appears to have been pneumatized in the same areas as its Aramis counterpart where it is preserved (Ward et al. 2001). Much appears to have been similar between these two samples, just as much is similar among the earlier hominid samples.
Is this a single lineage changing over time? That would be an appropriate null hypothesis to pursue, and there is little evidence tending to refute it now.
References
Semaw S, Simpson SW, Quade J, Renne PR, Butler RF, McIntosh WC, Levin N, Dominguez-Rodrigo M, Rogers MJ. 2005. Early Pliocene hominids from Gona, Ethiopia. Nature 433:301-305. Full text at Nature
Did australopithecines croak?
With apologies to the late Frank Livingstone, I couldn't help but wonder this as I read this passage from Bernard Wood's comment on the Dikika skeleton:
I am especially intrigued by the detailed morphology of the hyoid bone in the throat of the fossil. Does the open space in the body of the hyoid mean that A. afarensis had air sacs in its neck? In the absence of large canines, these air sacs might have been a way in which males established a dominance hierarchy, and females judged the quality of a potential mate.
Most apes have air sacs associated with their larynx, and these were lost sometime during human evolution, because we lack them. So it wouldn't be especially surprising if australopithecines retained this ape-like character.
Hewitt, MacLarnon and Jones (2002) suggested that the loss of air sacs in human evolution was pertinent to fine control of breathing related to language:
A possible function of laryngeal air sacs in apes and gibbons was investigated by examining the relationships between air sac distribution, call rate, call duration and body weight in a phylogenetic context. The results suggest that lack of sacs in the smaller gibbons and in humans is a derived feature. Call parameters in primates, such as rate and duration, scaled to resting breathing rate (and therefore to body weight) only in species without air sacs, which appear to modify these relationships. Apes and larger gibbons may be able to produce fast extended call sequences without the risk of hyperventilating because they can re-breathe exhaled air from their air sacs. Humans may have lost air sacs during their evolutionary history because they are able to modify their speech breathing patterns and so reduce any tendency to hyperventilate.
If this were true, there would really be no reason to expect australopithecines to differ from chimpanzees and gorillas in this regard. One might even imagine call sequences to be more important to them considering the possibility of higher predation (and the adaptive value of alarm calls) in more open habitats.
I don't think there is any need to infer a different function in australopithecines than in apes, based on the anatomy. Here is what Alemseged et al. wrote about it:
The hyoid of DIK-1-1 is only the second example in the hominin fossil record, and this element was previously unknown for any species earlier than Neanderthals. Its similarities with Pan and Gorilla hyoids suggest that the bulla-shaped body is the primitive condition for African apes and humans, rather than the more shallow, bar-like body shown by modern humans and Pongo. The bulla-shaped body almost certainly reflects the presence of laryngeal air sacs characteristic of African apes. However, the function of these structures is not well understood.
Fitch and Hauser (2003) wrote about ape laryngeal air sacs thusly:
A related possibility, the "accessory lung" hypothesis, is proposed here for the laryngeal air sacs of the great apes. Chimpanzees, orangutans and gorillas all have voluminous air sacs (6 liters in orangs, Schön-Ybarra 1995), that can be inflated with air from the lungs. The air sacs connect to the larynx via a long thin-walled channel that opens directly above the vocal membranes and vocal folds. The air sacs extend into the subdermal space in the pectoral region, and are overlain by the sheetlike platysma muscle. Thus, an ape could inflate the air sacs via lung pressure, and then forcibly deflate them by tensing the platysma and other pectoral muscles (or by pounding the chest, as in Gorilla). This anatomy suggests that great ape air sacs may act as "accessory lungs", providing an additional source of expiratory air flow and thus of energy into the source. This hypothesis seems more plausible than that offered by Negus (1949), who suggested that ape air sacs act as storage sites for oxygen during vigorous activity. Because the sacs are inflated with exhaled air that has already been in the lungs, and thus will be low in oxygen and high in CO2, such an air reserve would be of dubious respiratory value (Fitch and Hauser 1995) (Fitch and Hauser 2003:95).
Well, we're not looking at a 6-liter air sac for australopithecines -- but then it is odd that the orang and human hyoids should be similar considering the huge air sac in orangs and the lack of any in humans.
Aside from the problem of figuring out how apes actually use these laryngeal air sacs, I guess there's not much of a story here. But I'm beginning to appreciate the lack of surprises!
References:
Alemseged Z, Spoor F, Kimbel WH, Bobe R, Geraads D, Reed D, Wynn JG. 2006. A juvenile early hominid skeleton from Dikika, Ethiopia. Nature 443:296-301. Abstract
Fitch WT, Hauser MD. 2003. Unpacking "honesty": vertebrate vocal production and the evolution of acoustic signals. In Acoustic Communication, edited by Simmons AM, Popper AN, Fay RR, Springer-Verlag, New York. pp. 65-137.
Hewitt G, MacLarnon A, Jones KE. 2002. The functions of laryngeal air sacs in primates: a new hypothesis. Folia Primatologica 73:70-94. DOI link
Wood B. 2006. A precious little bundle. Nature 443:278-281. Full text (free)
Commentaries on Dikika
The online companion site to Scientific American is running commentaries by a few paleoanthropologists on the importance of the new DIK-1-1 skeleton. It's an experiment in online publishing, and it has turned out some very good material, including commentaries by Owen Lovejoy, Ralph Holloway, Diana Roman, and, well, me.
These other commentators and the press of the last week are hard to follow, but actually my second thought about the skeleton has turned out to be fairly original:
The new Dikika skeleton has me wondering one thing: did Selam sink Kenyanthropus?
If you can guess the connection, you've been reading entirely too much about early hominids!
Congratulations to Kate Wong on a really innovative idea -- they have an opportunity for other folks to contribute as well, so dive in!
Eyes white open
There is a story from LiveScience's Ker Than on the function of white sclerae in the eyes, as an adaptation for communicating gaze direction in hominids:
According to one idea, called the cooperative eye hypothesis, the distinctive features that help highlight our eyes evolved partly to help us follow each others' gazes when communicating or when cooperating with one another on tasks requiring close contact.
...
Results showed that the great apes -- which included 11 chimpanzees, four gorillas and four bonobos -- were more likely to follow the experimenter's gaze when he moved only his head. In contrast, the 40 human infants looked up more often when the experimenter moved only his eyes.
This is Michael Tomasello's work, and it is pretty interesting if the differences are as great as implied here. We'll have to wait to read the article in JHE.
The basic idea is that directed gaze and shared (joint) attention on objects are integral to human learning, so that this would be a strategy that emerged in early hominids to facilitate teaching some adaptive behaviors to young.
We can wonder at what point in human evolution this became important. I would offer a candidate: that toolmaking is a behavior that requires this joint attention on small objects and parts of objects. Of course, another option is that the objects in question are large and distant, such as animals being hunted.
There may be no contradiction here, because these behaviors may have emerged at around the same time -- which may be the real point; the underlying learning adaptation may have been necessary for both foraging and technological strategies.
I suppose that early hominids had apelike eyes, then. Unless there is some other reason (scanning predators?) why they might have needed to communicate gaze.
Lumbar vertebrae in hominids: six or five?
So have we decided that early hominids had five lumbar vertebrae?
The knuckle-walking anteater
Caley Orr (Personal page, Arizona State University) has an advance paper in AJPA examining convergent features in the wrists of knuckle-walking hominoids and the terrestrial giant anteater (Myrmecophaga tridactyla). Did you know that some anteaters were knuckle walkers? I certainly didn't, until I read this!
The background for this paper is the recent finding of certain features in the wrists of early hominids, specifically the distal radius of Australopithecus afarensis and Australopithecus anamensis, that appears similar to those found in chimpanzees and gorillas (Richmond et al. 2001). It has been argued that these features are primitive retentions from a knuckle-walking ancestor, a hypothesis viewed as consistent with the idea that all of the African apes descend from a single agent knuckle-walking species. The current paper gives a good synopsis of this evolutionary problem, including the alternative hypothesis that knuckle-walking evolved in parallel in the chimpanzee and gorilla lineages, so that the hominids did not descend from an age in knuckle-walker.
So why study anteaters? The idea is that different phylogenetics lineage that share a behavior ought to share convergent features to support that behavior. African apes have long fingers for suspensory climbing, which are talked out of the way for quadrupedal movement, hence walking on the knuckles. Giant anteaters have long claws for digging into insect colonies, and these long claws are talked out of the way for a quadrupedal walking. Thus the purpose of this study was:
1. To determine if the locomotion and hand postures of the giant anteater are appropriately analogous to the knuckle-walking of African apes;
2. To identify features of the Myrmecophaga hand and wrist that converge functionally with Pan and Gorilla, and that distinguish these taxa from their non knuckle-walking out groups and terrestrially digitigrade primates; and
3. Through the above analyses, to help determine the traits most likely to be adaptive to knuckle-walking, thereby suggesting which features of the early hominin and modern human wrists might be reliably indicative of a knuckle-walking ancestry (Orr 2005:3).
Did it work? The study did find some features of giant anteaters that made their wrists similar to those of chimpanzees and gorillas. But does this provide support for the idea that early hominids were descended from knuckle-walkers? Orr saves a critical piece of logic for the discussion:
A convergence study can only provide positive or equivocal evidence in testing hypotheses of adaptation for purported knuckle-walking features of the African Hominidae (or any other such study). That is, because all taxonomic groups have unique evolutionary histories, a convergence test cannot truly falsify a hypothesis of an adaptation for a particular lineage. However, convergence study can provide positive support for the hypothesis that structure X is an adaptation to function F, if X distinguishes F- performing taxa from their respective non-F-performing outgroups (Orr 2005:18, emphasis in original).
OK, so we should be wary. Comparing australopithecines to anteaters cannot falsify the hypothesis that hominids had knuckle-walking ancestors. In other words, it is not testing any hypothesis of human origins, although it may provide evidence consistent with one or more of them. Here's the summary of anteater-hominid resemblances:
Morphological features that appear in the hominin lineage shared by Myrmecophaga, Pan, and Gorilla, to the exclusion of their respective outgroups and digitigrade primates, are supported as adaptations to knuckle-walking and provide strong inference of a knuckle-walking last common ancestor (LCA) of Gorilla, Pan, and hominins. Only one such feature (proximal expansion of the non articular surface of the dorsal capitate) appears in the hominin lineage. Human capitates show the African apes state of proximal expansion, and the A. afarensis capitate (AL 333-40) shares the morphology of Gorilla, Pan, and Homo (Orr 2005:19).
Orr discusses the distal radius articular ridge that features in the arguments of Richmond and colleagues (2001), but does not designate this trait as one that necessarily reflects knuckle-walking as opposed to other kinds of vertical hand posture during locomotion, as found in cercopithecid monkeys.
If you're interested in the origins of knuckle-walking, and the question of whether early hominids were knuckle-walkers, this article is for you. As for myself, I think the issue is more likely to be settled with more fossil evidence of Miocene apes and their locomotor styles, rather than the examination of other mammalian lineages. It remains a mystery to me why early hominids should retain features useful only for a knuckle-walking, when their knuckles clearly could not have reached the ground. It seems more likely to me that there is some other function for which these characters might be adaptive related to early hominid locomotion, such as climbing, or other activities. Phylogenetic inertia is never very convincing, especially for early hominids, says they altered almost every other interface between their body and the environment in the pursuit of more perfect bipedal locomotion.
References:
Orr CM. 2005. Knuckle-walking anteater: a convergence test of adaptation for purported knuckle-walking features of African Hominidae. Am J Phys Anthropol (advance before print).
Richmond BG, Begun DR, Strait DS. 2001. Origin of human bipedalism: The knuckle-walking hypothesis revisited. Yrbk Phys Anthropol 44:70-105.
Them's fightin' legs
Elizabeth Pennisi has a short piece in Science describing David Carrier's ideas about leg length and fighting in early hominids.
David Carrier, a comparative physiologist at the University of Utah in Salt Lake City, has jumped into the fray with a provocative idea about Lucy's legs. In earlier studies with dogs, he had found that short legs provide mechanical benefits during fights. Pit bulls' short limbs, for example, aid stability and are tough enough to sustain attack without breaking.
Carrier contends that Lucy and other australopithecines also had bodies built for defense against each other: Their short legs may have provided a competitive edge when males battled rival suitors.
This is basically the wrestler vs. distance runner physique story. And it could be true -- but is it a primary cause or a secondary one? Depends how they were fighting, I should think.
The interesting part is that it is not an argument based on optimal energy expenditure. But is it safe to use sexual dimorphism as a proxy for male competition? And were australopithecines really very sexually dimorphic?
Boy, there sure seem to be a lot of unknowns lately.
References:
Pennisi E. 2006. Was Lucy's a fighting family? Look at her legs. Science 311:330. Full text (subscription)
Lowly origin of bipedalism :: the squatting model
In his 2003 book, Lowly Origin, Jonathan Kingdon presents a model for the origins of hominid bipedality, along with many other possible insights concerning the evolution of both earlier apes and later hominids. The book is notable because of Kingdon's speciality: as a very talented zoologist and perhaps the foremost biogeographer of African mammals, he brings an eye toward the temporal and spatial context of the transition to bipedalism that is generally lacking in other models. The book is also notable because it is recent, and provides a present-day look at many venerable models of hominid origins that well characterizes their strengths and weaknesses with respect to the present pattern of evidence.
An example of his biogeographical knowledge coming into play is his hypothesis for the place that bipedalism may have originated. Many models talk about a hypothetical division between Central African and East African forests or a hypothetical mosaic forest-savanna woodland mix. Kingdon can talk about actual forests where this might have happened. He focuses on the coastal African forest, which stretches from Somalia to South Africa (116-119). His examinations of biogeography of microfauna have shown that this forest has been biologically separate from those of Central Africa for a very long time. Today, the coastal forest is depauperate of large and medium-sized endemic mammals, which Kingdon attributes to human activity during the past 40,000 years. In the past, this forest would have served as a core area for animals spreading periodically into river valleys and forest fragments further inland. It would also have presented a rather different climate regime from the West and Central African forests, with its highly seasonal monsoonal rainfall.
Filling the bill
Any model that attempts to explain hominid origins must provide an account of several distinct things:
- How did early hominid populations become separated from early chimpanzee populations? That is to say, what accounts for the human-chimp divergence?
- What decisive advantage was there in increasing the frequency or importance of bipedal locomotion?
- What exactly was the ancestral pattern of locomotion?
- Why did this ancestral pattern, whatever it was, lose its advantages when compared to bipedality?
To question number 1, Kingdon gives basically the same biogeographical answer as Coppens' East Side Story and many others: namely, that progressive aridification of East Africa led to a separation of East and West African ancestral hominoids. His details about the nature of the East African forest are very welcome and interesting, but do not change the basic picture. Kingdon places the timing of this event in several cycles of aridity beginning at 10.5 million years ago, through recurrent drying at around 7.8 million years ago and 6.2 million years ago (119). These dates do approximately correspond with the time interval preceding the fossil remains of the earliest hominids, which are now some 6 million years old.
To the second question, about the advantage of bipedalism, Kingdon provides an answer based on Clifford Jolly's (1970) seed eaters hypothesis. In this model, and upright posture for the upper body is advantageous for use in foraging for small items, in particular seeds from grasses. Like Jolly, Kingdon envisions a squatting, ground-based ape, which he calls the ground ape. He describes the effects of a small item feeding strategy as follows:
One way of asking how apes might have responded to these limitations is to look at the feeding strategies of living species. For example, when contemporary chimps are under duress from a poor fruit season, they break up into smaller foraging units that scour the environment more thoroughly while trying to maintain their frugivorous dietary preferences for as long as possible. By contrast, the more terrestrial gorillas respond to the same pressure by maintaining their groupings but diversifying and enlarging the range of their foods to include previously ignored and less digestible plants. Another variant, better suited to eastern forests, would have been to diversify (by including more animal and underground foods) but also to spend more time and effort foraging for smaller (but still nutritionally rewarding) items. As observed in contemporary situations, these are stopgap routines for gorillas and chimpanzees. However, I am proposing that similar strategies could develop or be transposed into a sustained and systematic way of using a spatially restricted environment. (122-123)
When Jolly originated the small object feeding model, he focused on the analogy between geladas and savanna baboons as a way of understanding the effects of this dietary change. Kingdon focuses more closely on the range of plant species that may be exploited by such a dietary shift, the ability of ancient groups to exploit the same geographic range more intensively, and the probable ecological diversity of plant species in the East African forest. He notes that chimpanzee groups across Africa appear to use a similar number of fruiting plant species, adjusting their home range in response to habitat richness. This results in a great disparity in chimpanzee foraging ranges (from as little as five square kilometers to as much as 400 square kilometers). Kingdon suggests that a more intensive foraging strategy based on the wider ecological diversity of East African forests may have increased the carrying capacity of these forests for the ground apes, with consequent alterations in their social behavior and ecology. He supports this ecological model with an analysis of the species richness of human-edible plants in this eastern forest (123). His major case is based on the increased availability of ground or near-ground foods in the eastern forest, including both animal and plant resources, compared with the small ratio of time that forced chimpanzees appeared to spend foraging near the ground as opposed to foraging canopy fruits
Perhaps the most important change, in answer to question 4 above, is a change in daily foraging range. As Kingdon notes, "Quadrupedalism would never have been abandoned if substantial distances had to be covered, especially if such journeys involved exposure to predators" (125). Easy terrestrial movement and escape from predation in apes requires the rapid movement of quadrupedal locomotion. It biped faces substantial disadvantages in these respects. This means that a greater reliance on bipedal locomotion would has required both a small home range and easy access to trees. This idea is a 180 degree shift from the Darwinian model of bipedal origins, in which upright posture was a reflection of the challenges of a poor habitat and the need to forage over long distances. Here, it is safe "secure and a rich environment" that is essential to the origin of bipedalism. In Kingdon's view, living apes naturally pursue a number of hand manipulation skills, social interactions, gestural communication, and carrying objects that require them to "squat, lie down, stand on two legs, or become three-legged" (125). For all of these behaviors, bipedal locomotion might well be naturally advantageous. But chimpanzees and gorillas cannot abandon quadrupedal locomotion and its speed advantages because of their large foraging ranges and susceptibility to predation. The commitment to quadrupedalism thereby impedes the further development of manual abilities that apes already have.
This idea provides a slightly different answer from Jolly might have given concerning why geladas are not more hominid-like than they are. Although the foraging style of manipulating small hard seeds and other objects might have been similar between early hominids and geladas, the habitat is very different. Geladas must retains an effective adaptation to quadrupedalism because they do not limit their foraging to areas where trees are readily accessible. Nor do they already show the range of manipulative behaviors shared by apes, which provided further incentives to bipedalism in early hominids.
The thrust from squatting
The squat-feeding model encompasses several untested predictions, which might well provide fertile ground for research. First, this pattern of adaptation should direct attention to the anatomy of the back. In particular, to conserve energy and maximize the use of a single foraging location, the spine should be well adapted to a bright posture, flexible in side to side movements, and capable of providing a stable platform for a wide range of movement for the arms. This may help to answer the question of why early hominids had relatively long spines, and especially in contrast with very short lumbar spines in other living hominoids. It also allows the side-to-side twisting motion of the pelvis during bipedal gait to be examined as an exaptation based on an earlier ability to rotate the upper trunk against a stationary pelvis. Normal arm-swinging upright walking depends on this flexibility of the lower spine, which would appear to be absent from living chimpanzees and gorillas, in which the flat iliac blades and the lower rib cage are strongly connected and relatively inflexible. Kingdon describes the compact, inflexible trunks of living apes (127) and their disadvantages for upright walking, but he does not explore why this configuration in apes would be advantageous for the locomotor behaviors of these apes, such as climbing or knuckle-walking. This difference from hominids is worth exploring, particularly in considering the effectiveness of early hominids as climbers.
The model also places a different spin on the usual anatomical description of the changes involved in bipedalism. Generally, the shortening and broadening of the iliac blades are seen as enabling a shift in muscular action during hip extension, recruiting the gluteus maximus as an extensor of the hip instead of an abductor. Kingdon explains the shortening of the iliac blades as a way of disentangling their action from the motion of the lower trunk, creating two separate functional units. In this way, he also explains the lengthening of the lumbar spine as part of the same anatomical change. This is potentially important because the length of the lumbar spine in the common ancestor of hominids and chimpanzees is not known. If hominids descended from an ancestor with the chimpanzee-like spine, a mechanism for the expansion in length of the lumbar spine is both necessary and welcome.
One of the advantages of bipedal locomotion often cited in explanations of hominid origins is the ability to see distances over tall grass while scanning for predators. Kingdon places a different twist on this also, by suggesting that this scanning behavior was present prior to the evolution of obligate bipedalism, as the ground apes would scan for predators from a squatting position. In this way, the apes habitually made their spines as vertically erect as possible at frequent intervals, and simultaneously required effective side to side head movement. This kind of behavior may have underlain the anterior placement of the foramen magnum and the reconfiguration of the head-spine articulation. This hypothesis would especially be interesting if it were shown that the anterior placement of the foramen magnum significantly predated the origin of the pelvic specializations for bipedalism. This kind of evidence might already be present in Sahelanthropus, Orrorin, or Ardipithecus. Especially in Sahelanthropus, where Brunet and colleagues (2002) have argued for an anterior foramen magnum, and in the Aramis occiput, where the foramen magnum also appears to be relatively anterior. Pelvic evidence is not yet available from any early hominid, and although the Orrorin femora are consistent with the weight-transmission characteristics of later hominids, it is not clear that this anatomical element is necessarily reflective of an entire pelvic anatomical complex.
One might argue that every hypothesis to explain the origin of bipedalism is in some sense an umbrella hypothesis (Langdon 1997), and this is no exception. While the fundamental change hypothesized by the model is a change in foraging strategies, this change is proposed have several effects on other elements of early hominid behavior.
The first of these involves the dynamics of hominid groups. Kingdon speculates that terrestrial life would have involved new adaptations to resist a greater diversity of predators and competitors. This adaptation would likely have involved group coordination with intimidation displays. In particular, a restriction to relatively small home ranges would of reduced the possibility of simply moving on as a response to competition or predation. Climbing would have remained very important in predator avoidance, but it arguably would not be enough to cope with the eastern African ecology.
Speciation among the early hominids
Another consequence adduced by Kingdon is on the pattern of speciation of subsequent hominid lineages after the hominid-chimpanzee divergence. Kingdon describes many of the land areas bordering on the East African coastal forest, along with the prospects for ancestral hominids occupying and spreading among these different areas. He raises an interesting point about the Zambezi basin, which is largely open grassland with extensive floodplains and gallery forests and would therefore have been ideal hominid habitat despite the present lack of hominid fossils from the area.
As Kingdon describes each African region, he makes four basic points. First, the linear movement of ground apes along the coast and into the upland regions would have placed hominid populations at such distance from each other to radically restrict gene flow between them. Second, each of the areas, ranging from the Ethiopian highlands to the Zambesi basin would have presented unique ecological circumstances that would have demanded local adaptations on the part of the early ground apes. And third, the likely habitat of the ground apes extended along river courses. This means that the apes were likely not separated by the river drainages themselves, especially since many of them are highly seasonal, but instead they were separated by the interim habitats that were highly risky and resource-poor for a woodland-dependant ape. Last, the home ranges of the ground apes were probably small, again reducing the possibility of long-range dispersal and contact among populations.
I repeated the term "ground ape" repeatedly in the previous paragraph in reflection of Kingdon's other major assumption. He promotes the ground ape as a genuine stage in the evolution of the hominids. In other words, these apes once differentiated from chimpanzees were themselves highly successful occupants of the East African forest, and could themselves spread into adjacent habitats. All this occurred before the advent of of bipedalism as reflected in later hominids. This would imply that a substantial diversity of ground apes may have once existed, on the hominid lineage, but not themselves obligate bipeds. Kingdon suggests that known fossil samples like Orrorin or Ardipithecus might in fact represent a ground ape in this sense rather than bipedal hominids.
I am unconvinced by the idea that the squatting ground ape lived for a long period of time before evolving the adaptations to effective bipedality. Indeed, Kingdon's argument about the advantages of bipedalism would seem to suggest that it would emerge quickly if the opposing need for quadrupedal locomotion decreased. The idea that the ground ape stage lasted for a long time ignores the likelihood of competition from more effectively arboreal forms.
The biogeographic separation of hominid ancestors from chimpanzee ancestors (and gorilla ancestors) creates a set of interesting problems that Kingdon doesn't address. For example, if the apes on both sides of the East African arid strip were initially the same, did this original ape form survive for some time alongside the new ground apes? Or was that form itself a ground ape (as speculated below). Did this ancestral species survive alongside its bipedal descendants for some period of time? If Kingdon's idea about widespread diversification and long survival of the ground apes were true, then these apes must have coexisted for some long time with their bipedal descendants, especially if the ground apes had significant local adaptations to different African regions.
While Kingdon does support his argument that the early ground apes would have differentiated into different species with several assumptions, I found this unconvincing. Consider that chimpanzees are spread across over three thousand miles of West and Central Africa with clear evidence of recurrent gene flow among different subspecies over the past million years or more. Lowland gorillas also have an impressive geographic range, and orangutans today comprise two long-lasting geographic subspecies, which in the past must have extended to a greater diversity on the Asian mainland as well as across the Sunda shelf. The phylogenetic pattern represented by today's great apes indicates widespread species with highly conservative ecological adaptations. This allows subspecies to remain ecologically similar for long periods of time, and enables the exchange of genes long after the initial establishment of geographically distant (or periodically isolated) populations.
Kingdon does not consider this pattern, but his argument would indicate that the ground apes (or early hominids) diverged from the phylogenetic tendencies of other ape species because of their restricted home ranges and more intensive ecological exploitation of local environments. This hinges on the idea that bipedality really doesn't increase mobility, but instead radically decreases it.
But this argument fails to recognize the energetic consequences of bipedalism after it originates. It may be true that the initial transition to bipedalism would not be possible without the means of abandoning the dependence on quadrupedal movement in foraging and flight. It may also be true that obligate bipeds continued to be at a disadvantage compared to quadrupeds in predator avoidance and daily range. But the movement of bipeds over long distances would if anything have been less costly than that of a quadruped of the same size. And the social correlates of bipedality that Kingdon notes would seem likely to increase dispersal rather than decrease it. That is to say, despite a smaller home range, more cohesive groups with potentially larger group sizes present a higher chance of significant disparities in resource access among groups, a greater variance in group sizes, increased challenges for individuals integrating into new groups, and greater incentives to colonize and disperse over long distances. Bipeds are well equipped to move along linear habitats like gallery forests, and might have done so with maximum energetic efficiency in response to resource challenges or seasonal scarcity. An increased tolerance for higher population densities would have enabled an effective migration strategy in regions where seasonal resource shortfalls in one area may have been supplemented by movement to other areas with enough to go around.
This cuts to the nature of what it is to be a biped. Once the bipedal strategy arose, did it enable greater mobility or not? Were hominid groups highly territorial, and highly sedentary, or were they instead highly mobile? Did hominids tolerate local aggregations of multiple groups, or were they committed instead to intergroup conflict? This is where a chimpanzee model potentially misleads, since chimpanzees are both mobile and territorial, intolerant of contact with neighbors and capable of long-distance dispersal for maturing females. How would bipedalism change a chimpanzee's behaviors? An unanswered question.
Unanswered questions
An unanswered question is to what extent the focus on ground-accessible foods would have precluded the use of canopy foods. As Kingdon notes, canopy fruits are the major food source for chimpanzees today. Presumably, a greater adaptation to terrestrial life including bipedal locomotion would have greatly restricted the ability of early hominids to climb into the forest canopy and exploit fruiting trees. It seems possible that competition from other primates, such as cercopithecoid monkeys, might have precluded the effective dependence on a canopy resources anyway. But this line of inquiry needs to be developed further.
Another unanswered question involves the body proportions of early hominids. Australopithecines were exceptionally short compared to living humans. And there legs were hardly longer than similar-sized apes. These legs were very inefficient for bipedal movement compared to the long legs of subsequent hominids. But one possibility is that australopithecine legs may have been effectively adapted to a squatting posture. As far as I know, this hypothesis remains to be tested. Certainly if Kingdon is right about the small foraging ranges
of early hominids, the energetic disadvantages of short legs may have been relatively minor, because hominids would never have walked very far anyway. In this respect, even the home ranges of chimpanzees would be a poor model for the relatively small home ranges of early hominids. While anthropologists have tended to contrast australopithecines with early humans, who were believed to have had larger home ranges on the scale of those occupied by living hunter gatherers, it remains possible that australopithecine home ranges were smaller even than has usually been assumed.
And of course the biggest unanswered question appeared in the form of a key fossil shortly after the book must have been finished. What about Sahelanthropus? If Sahelanthropus was in fact on the hominid lineage, then it would seem to reject the model of differentiation proposed by Kingdon--Chad is a long way from the East African coastal forest. Conversely, if it is not on the hominid lineage, its importance to the model depends on what it is. If it is ancestral to hominids or to chimpanzees or gorillas, then it potentially informs us as to the anatomy of the common ancestor to these species. If so, that ancestor may have been substantially more ground ape-like than even Kingdon might have expected, at least if Brunet and colleagues (2002) are right about the foramen magnum placement and its implications for vertical posture. One might even envisage the hypothesis that chimpanzees and gorillas themselves are substantially derived from the common ancestor because they colonized the West and Central African equatorial forests long after the common ancestor lived (although presumably before the separation of chimpanzees and bonobos). This is a lot of mileage out of one fossil sample, but the absence of a fossil record for either chimpanzees or gorillas invites speculation.
References:
Brunet M, Guy F, Pilbeam D, Mackaye HT, Likius A, Ahounta D, Beauvillain A, Blondel C, Bocherens H, Boisserie JR, De Bonis L, Coppens Y, Dejax J, Denys C, Duringer P, Eisenmann V, Fanone G, Fronty P, Geraads D, Lehmann T, Lihoreau F, Louchart A, Mahamat A, Merceron G, Mouchelin G, Otero O, Campomanes PP, Ponce de Leon M, Rage JC, Sapanet M, Schuster M, Sudre J, Tassy P, Valentin X, Vignaud P, Viriot L, Zazzo A, Zollikofer C. 2002. A new hominid from the Upper Miocene of Chad, Central Africa. Nature 418:145Ð151.
Kingdon J. 2003. Lowly origin: Where, when, and why our ancestors first stood up. Princeton, NJ: Princeton University Press.
Langdon JH. 1997. Umbrella hypotheses and parsimony in human evolution: A critique of the Aquatic Ape Hypothesis. J Hum Evol 33:479Ð494.
Orrorin opera
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)
Tuber or not tuber? Rats are the question
From a new paper by Greg Laden and Richard Wrangham:
We propose that a key change in the evolution of hominids from the last common ancestor shared with chimpanzees was the substitution of plant underground storage organs (USOs) for herbaceous vegetation as fallback foods. Four kinds of evidence support this hypothesis: (1) dental and masticatory adaptations of hominids in comparison with the African apes; (2) changes in australopith dentition in the fossil record; (3) paleoecological evidence for the expansion of USO-rich habitats in the late Miocene; and (4) the co-occurrence of hominid fossils with root-eating rodents. We suggest that some of the patterning in the early hominid fossil record, such as the existence of gracile and robust australopiths, may be understood in reference to this adaptive shift in the use of fallback foods. Our hypothesis implicates fallback foods as a critical limiting factor with far-reaching evolutionary effects. This complements the more common focus on adaptations to preferred foods, such as fruit and meat, in hominid evolution.
Tubers are not the only kinds of USOs; there are also corms, bulbs, and rhizomes. I tend to use "tuber" as an easier-to-type version of USO, though. I was practically dared to review the paper here (nota bene: I do respond to dares, albeit more carefully and slowly than for most things), and Carl Zimmer has also written a short item on the idea. The mole rats are the lede, but there is much more to it than them, and in many respects they are the least problematic part.
So here is my semi-rambling take.
Take one
In 1999, Wrangham and Laden, along with David Pilbeam, James Holland Jones, and NancyLou Conklin Brittain, suggested that tuber cooking was central to the adaptation of early Homo. The evidence for that suggestion was and remains essentially absent. As Henry Bunn put it in his comment to the paper:
Why is there abundant evidence of hunting and some form of scavenging, carcass transport, butchery, and sharing and consumption of meat and fat in the behavioral and dietary adaptations of early Pleistocene Homo (e.g., Oliver, Sikes, and Stewart 1994 and references therin)? Why are the earliest stone tool kits of the Oldowan dominated by sharp-edged cutting tools? Why is there intensive meat polish on the edges of stone flake knives studied for microwear (Keeley and Toth 1981)? Why is there not microwear evidence of grit or sediment damaged on the teeth of supposedly tuber-feeding hominids themselves, including the robust australopithecines (Kay and Grine 1988)? (Bunn 1999:580)
Additionally there is the problem of the complete lack of evidence for cooking and the weakness of evidence for early control of fire, compared to the strong and substantial evidence for both much later in the Pleistocene.
So early Homo just doesn't show any signs of having been a serious tuber-eater. Not to say it is impossible; just that there isn't any particular evidence for the idea.
Take two
Now, Australopithecus, that's another story. Robust australopithecine teeth in particular have a lot of pits and scratches on them, as if they were eating some hard, gritty foods. Underground storage organs fit that bill. Eating a lot of dirt along with them might well explain the high rate of dental wear that robust australopithecines clearly had -- many had their first molars worn almost completely flat before the third molars came into occlusion.
In this context the fallback food idea seems like an especially good one. The tooth anatomy and microwear evidence indicate that robust and nonrobust australopithecines probably did not differ in most of their dietary spectra, but instead in the accentuation of different food sources that were shared by both. If food shortages were important in the evolution of these hominids, one way that the difference between them might have been sustained was an ecological difference in fallback food utilization. Hominids like A. afarensis and A. africanus undeniably had teeth adapted to heavy grinding, fracturing off brittle foods, and intensive attrition compared to any other living or fossil primate. So it makes no sense to propose that the difference between these "gracile" australopithecines and later robust australopithecines was that the "gracile" ones lacked the high-chewing element. Rather, it makes considerably more sense to suppose that both kinds of hominids were eating the high-chewing foods, with the robust ones making a more intensive use of them, and possibly lacking some of the tough pliable foods eaten by earlier nonrobust species. A difference in fallback strategies might comprise exactly this kind of dietary prediction.
To me, the coolest thing about the hypothesis is that it explains the postcanine adaptations of australopithecines without reference to the now-well-known carbon isotope data. Indeed, the question of C4 versus C3 foods is entirely irrelevant. I discussed the carbon and other stable isotope data in an earlier post; the short story is that all kinds of australopithecines appear to have included around a 25 to 30 percent component of C4 foods, which include grasses, some sedges, and the animals who ate them.
Peters and Vogel (2005) proposed that the C4 component of the early hominid diet could be explained as a sum of several different plant and animal sources, including around 5 percent each of seeds, roots and pith, insects, small mammals and vertebrates, and large mammal meat. That does a good job of describing a diversified hominid diet without reference to tubers.
But the thing about USOs is that relatively few of them are C4 plants. If hominids did eat tubers, in other words, they still wouldn't account for the C4 fraction of the overall diet.
However, they might account for the postcanine dental adaptations of later hominids, under the assumption that they represent a substantial part of the C3 fraction. And the replacement of C3 fruits by C3 tubers would explain why robust and nonrobust hominids both have approximately the same C4 fraction, while differing so greatly in their dental adaptations and dental microwear.
As far as I can tell, nobody has mentioned this implication, but it should be the next thing to test.
The evidence
But although I think Laden and Wrangham's study has some interesting possibilities, I think the data is a bit short of where it needs to be. What about the four lines of evidence used by Laden and Wrangham? Are they to be believed?
The first thing to point out is that a reading of the paper finds little detail to go along with two of the lines of evidence. It is true that australopithecine teeth are not like ape teeth, and that robust australopithecines were different from nonrobust ones. The innovative suggestion here, although brief, is that an enlarged oral cavity in australopithecines, particularly robust ones, may be an adaptation to increase the exposure of masticated tuber to salivary digestion.
But the dental discussion appears less as two independent lines of evidence converging to one conclusion, and more as throwing up whatever seems relevant to see what will stick. A review of early hominid dental evidence also reveals plenty that is less consistent with the hypothesis that USOs were an important food for most early hominids.
For one, the comparative dental evidence is questionable. As Laden and Wrangham review the issue, Hatley and Kappelman originated the argument that the early hominid dentition was adapted to tuber-eating:
In 1980, Hatley and Kappelman pointed out parallels in dental morphology that suggested that bears, pigs, and hominids are all adapted to eating significant amounts of plant underground storage organs (USOs). They summarized their argument as follows: "We believe that postcanine similarities evident among ursids, suids, and hominids are in part an adaptation for processing this tough, fibrous, and gritty plant part. Bears, pigs, and humans are adapted to exploiting plant roots and tubers, although their methods of food gathering are functionally rather than morphologically analogous. Convergence upon the resource of belowground plant storage parts appears to make the responses of nonretractable claws, cartilaginous snout, and digging stick equivalent" (Hatley and Kappelman 1980:380, quoted in Laden and Wrangham 2005:1).
This isn't obviously true. For one thing, Pliocene pigs appear to have been mainly grazers (Harris and Cerling 2002 -- not cited by Laden and Wrangham 2005). They increased in molar size and complexity in several different lineages, as a reflection of their increased reliance on C4 vegetation. The diet of current-day suids in particular seems to share little in common with early hominids, at least as far as their stable isotope ratios are concerned. Nor are large and flat early hominid molars particularly analogous to those of most bears -- perhaps the closest are pandas, which are far from dedicated tuber-eaters.
Then there is the problem with the earliest hominids. These, like the later ones, are found alongside mole rats, at sites like Aramis and Lukeino. But they don't have the postcanine adaptations of later hominids. The essential problem with the earliest hominids is not postcanine specialization, but instead the changing role of the canine-premolar complex, and the reduction of the canines. There is no reason (at least that I can think of) to suppose that small canines are adaptive to tuber-eating (and a search of the paper finds no occurrences of the word "canine").
One way to avoid this problem is to suppose that the USO-eating adaptation was simply a feature of later hominids --- say, A. anamensis and later. Perhaps it's true, but if so, the hypothesis loses some of its punch, and possibly one of the converging lines of evidence, since the expansion of USO-rich savanna central to Laden and Wrangham's paper starts in the Miocene.
And the paper would prefer to displace the importance of tubers earlier rather than later in time:
There is growing evidence that middle to late Miocene hominoids, mainly in Europe, exploited relatively open habitats, and may have exhibited dietary adaptations (Teaford and Ungar, 2000, Smith et al., 2003 and Smith et al., 2004) that we claim here to be related to USO consumption. This lends support to our assertions that a USO niche may have emerged during the Miocene, that this niche may have been important for non-fossorial mammals, and that certain features, such as thick enamel and large teeth, can arise in response to this niche. However, we do not wish to make claims beyond the hominid taxon at this time, other than to note that this may be a fertile area of future research (Laden and Wrangham 2005:13).
If you are a student looking for a thesis topic, don't pick this one.
The most original suggestion is that hominid and mole rat remains are significantly coassociated. On the surface, this looks like fairly convincing evidence that the hominids lived in USO-rich environments, which is precisely what Laden and Wrangham conclude. And indeed, the number of sites either possessing both kinds of animals or lacking both (27) is higher than expected considering the small number that have one kind but lack the other (11).
But wait a minute. Neither "mole rats" nor "hominids" are species, they are groups composed of several species. Let's consider the same kind of comparison for other kinds of animals. How many hominid sites lack bovids? Or suids? Or crocodilians? Keep in mind that some groups are rare at early hominid sites because they hadn't diversified yet, like papionins, or hadn't yet appeared in Africa, like equids. But these groups are found at many later hominid sites. And of course, for many sites the total species list may reflect less intensity of sampling rather than the paleohabitat.
In other words, the mole rats may show that hominids had the opportunity to eat USOs -- at least, if they could compete effectively with the mole rats for them. But they don't show that the hominids actually ate USOs. At least not if we aren't equally willing to believe that the presence of crocodiles at hominid sites meant that hominids swam in rivers and ate migrating wildebeest.
The weaknesses NOT mentioned
I see two significant weaknesses in the hypothesis. The first is the simpler of the two: digging up tubers is a lot of work.
For groups like the Hadza who eat a lot of them, this work takes many hours (at least by some group members). That kind of work seems unlikely for australopithecines, even hungry ones. Especially considering the full scenario: australopithecines digging intensively for savanna-living tubers for hours at a stretch would have been highly exposed to predation and heat stress for hours at a stretch.
Might they have done it if they had nothing else to eat? Sure. But could they have done so efficiently enough to get a net return on their effort? There's a question worth answering.
Might they have banded together into large defensive groups? Maybe, but that would seem likely to decrease foraging efficiency -- how many tubers are there in any small patch of ground? However, there is slight evidence for large multimale groups (chiefly AL 333), as well as pretty good evidence that predation was high and survivorship into adulthood low. Another question worth answering.
There may be a solution for this problem: perhaps the plants themselves have evolved under intensive hominid predation. Maybe today they put their roots further underground, or maybe the plants with tougher and more fibrous roots have predominated since the Pliocene. If so, australopithecines might have had an easier time of digging them up.
The other problem is more vexing. How can we demonstrate that an extinct species was adapted to eat a food that it did not eat very often? Bone chemistry must predominantly reflect the foods that make up the majority of the diet, not those that are consumed only intermittently. Microwear also ought to reflect the majority foodstuffs, although perhaps more weakly -- especially if mortality occurs mostly during periods of dietary stress, when animals are eating more of their fallback foods than usual. This is perhaps worth looking into.
Maybe the most promising test would be variability in tooth wear. Presumably the need to rely on fallback foods would vary in accordance with climatic conditions, on a multigenerational timescale. If so, then some individuals might exhibit relatively great amounts of attrition due to their reliance on fallback foods during long periods of resource stress, while other individuals might have lived in times of relative abundance, and therefore not have experienced significant amounts of wear. This kind of heterogeneity would itself have created differences in selection on tooth size, enamel thickness, and occlusal anatomy over time: perhaps in ways that could be differentiated from alternative strategies. But even so, that kind of comparison is relatively far from the direct evidence, and may be impossible with the fossil record we have available.
Summary
Looking back at the post, I've written a balance of critical comments and supportive ones. I guess my opinion overall is that the USO hypothesis is certainly worth presenting, but it has a ways to go before it is really testable. I think there is a balance of good ideas here and evidentiary weaknesses, and it is certainly worth talking about them, perhaps with a bit more skepticism and documentation than has yet been done.
And if you are serious about tubers, as Wrangham clearly has shown himself to be, then you are going to have to choose a time when they were important. With this paper, I have now read that tubers were the key adaptation for Miocene apes, the earliest hominids, australopithecines, robust australopithecines, early Homo, and recent humans.
It can't be all of these. If it were, they would all look the same. And there wouldn't have been any reason for one to change into anything else! So you have to pick.
And making a choice means more than saying, "well, Miocene apes tasted tubers, early hominids needed them when the fruit ran out, for australopithecines they were a fallback food, robust australopithecines ate them all the time, early Homo cooked them, and recent humans pickled them with vinegar and caraway seeds. As yet, the many tuber hypotheses have been just-so-storytelling at its most self-contradictory.
If I were picking, I would put the best odds on Laden and Wrangham's current argument: USOs were important fallback foods for nonrobust australopithecines like A. afarensis and A. africanus, and equally or more important for robust australopithecines. In contrast, early Homo was adapted to meat eating, and the earliest hominids -- who lack the postcanine specializations of later hominids -- remain as yet a mystery, although a fundamentally apelike diet is a good first guess.
This post doesn't account for all the details of early hominid diets, but some previous posts review other sources of evidence, including:
Stable isotope analyses
Dental microwear
Occlusal anatomy
References:
Hatley T, Kappelman J. 1980. Bears, pigs, and Plio-Pleistocene hominids: a case for the exploitation of belowground food resources. Hum Ecol 8:371Ð387.
Laden G and Wrangham R. 2005. The rise of the hominids as an adaptive shift in fallback foods: plant underground storage organs (USOs) and australopith origins. J Hum Evol in press (online)
Wrangham RW, Jones JH, Laden G, Pilbeam D, Conklin-Brittain N. 1999. The raw and the stolen: cooking and the ecology of human origins. Curr Anthropol 40:567-594.
More microwear from South African australopithecines
Scott and colleagues (2005) examined dental microwear in some Swartkrans (A. robustus) and Sterkfontein (A. africanus) specimens. The interesting part of the study is the use of fractal analysis to quantify the complexity of scanned surfaces. They scanned a very tiny area of each tooth, around 200 micrometers on a side. Then they fed the scans through an algorithm to calculate texture.
The basic link to diet is the same as before: hard, brittle foods leave scars and pits, tough pliable foods leave directional marks like scratches.
Some results:
Fossil hominin results indicate that P. robustus (Asfc 4.29 2.150) has microwear textures more complex (chi-squared = 8.17, P < 0.005; Kruskal-Wallis test) and more variable in complexity (F = 16.82, P < 0.0005) than A. africanus (Asfc 1.686 +/- 0.52) (Fig. 2c, d). These results are consistent with the hypothesis that P. robustus incorporated more hard and brittle foods in its diet. However, some overlap in Asfc for the hominins (Fig. 3b) suggests that P. robustus was unlikely to have been a specialized hard-object feeder. It is more likely that hard, brittle foods were an occasional but important part of the diet. Previous studies have emphasized average differences between species rather than overlap, because low repeatability associated with observer error made assessments of within-species variability difficult.
In contrast, the microwear textures of Australopithecus africanus (epLsar1.8 0.0045 +/- 0.00163) show greater anisotropy (chi-squared = 3.84, P = 0.05; Kruskal-Wallis test) and epLsar variability (F = 7.38, P < 0.01) than P. robustus (epLsar1.8 0.0028 0.00060) (Fig. 2c, d). These data suggest a tougher diet on average for A. africanus compared with P. robustus, but one that is also more variable in its toughness (Scott et al. 2005:694).
The interesting thing is the overlap between the two samples. The authors also compared cebus and howler monkeys, finding extensive variation in both taxa, with minimal overlap in distributions (howlers are leaf-eaters, cebus eat a wider range of foods including some hard items). The two hominids overlap almost completely in "surface complexity" (i.e. whether they are pitted and scarred), with the main difference between the samples being an average greater complexity in A. robustus and an average greater anisotropy (i.e. grooving and scratching) in A. africanus. A third or so of each sample lie in the region of overlap in both variables.
From these measures, the diet variation within each species appears to be more extensive than the differences between them. The authors suggest this pattern of differences may represent a basically uniform diet with different fallback foods:
The greater variation in complexity for P. robustus and in anisotropy for A. africanus suggests that these species altered different components of their diet, but that there was probably substantial overlap in the fracture properties of their preferred foods. Thus, the clear differences between A. africanus and P. robustus microwear may relate, in part, to differences in critical dietary resources consumed only periodically during the year (Scott et al. 2005:695).
That would certainly be concordant with the stable isotope data. I guess it's a good thing for them that these two species weren't contemporaries.
References:
Scott RS, Ungar PS, Bergstrom TS, Brown CA, Grine FE, Teaford MF, Walker A. 2005. Dental microwear texture analysis shows within-species diet variability in fossil hominins. Nature 436:693-695. Full text (subscription required)
Robust australopithecine diet ablated
Sponheimer and colleagues (2006, link) zapped some Swartkrans teeth with lasers to measure their 13C content. I wrote quite a bit here last year about australopithecine diets, including a long review of isotopic evidence for australopithecine diets.
With respect to dietary differences between A. africanus and A. robustus (the two species with any substantial isotopic sampling), there are four essential observations:
- The apparent C4 dietary content of the two species is basically the same, and fairly high.
- High C4 foods are not so easy to come by, they include some grasses and sedges and the animals who eat them.
- The Sr/Ca ratios of the two species are fairly different.
- The postcanine teeth of A. robustus seem to be adapted to crushing and grinding, moreso than A. africanus.
One hypothesis for the difference in Sr/Ca ratios is exploitation of underground tubers (warthogs and mole rats have elevated Sr/Ca similar to A. africanus). A mix of C4 foods has been proposed to solve the grass-eating problem, including seeds, rhizomes, insects, lizards, and herbivore meat. But these don't really solve the postcanine tooth conundrum, and while they may both be true; neither is really testable.
OK, so does the new laser ablation study solve any problems? First, let's read a bit about what exactly it is, and why it might be useful. Ann Gibbons has written a ScienceNOW article:
[A] team of American and British researchers studied the teeth of four individuals of Paranthropus robustus (also known as Australopithecus robustus) from the Swartkrans Cave in South Africa. The team scanned the teeth with a sensitive laser, which did not destroy the teeth but etched them lightly enough to free carbon gases long trapped in the enamel. Because different plants absorb atmospheric carbon dioxide differently, the researchers were able to see what types of vegetation the hominids ate based on the ratio of carbon isotopes in their teeth.
An accompanying perspective by Stanley Ambrose explains:
In contrast to conventional methods, the laser ablation technique used by Sponheimer et al. barely penetrates the enamel surface of an area of less than 0.5 mm2 and is thus nearly nondestructive (2). Laser ablation also avoids the problem of time averaging in large drilled grooves. Moreover, perikymata can be counted, providing a good estimate of the minimum time interval sampled and of the duration of tooth formation.
The Paranthropus teeth studied by Sponheimer et al. show interesting patterns of seasonal variation in diet and climate. All have the isotopic composition of mixed feeders, and two show at least ca. 40% variation in the proportions of C3- and C4-based resources over 1 year. One individual had a predominantly C3-based diet and foraged in a cooler, more humid environment; it may have formed its tooth in a very wet year. The others ate more C4-based foods in a warmer, drier environment. Their average carbon-isotope ratios are similar to those of adaptively versatile savanna baboons (2). Analyses of seasonal variation in teeth of modern and fossil baboons and of other hominin species are necessary to evaluate dietary specialization in Paranthropus and niche overlap with other hominin species.
Back to me. There are two possibilities. First, the differences between 13C values for different samples might be sampling the actual dietary variability of single A. robustus individuals over the course of their tooth development (in this paper, sampled over a course of a couple hundred days).
Or second, they may just be sampling noise.
The paper presents comparative data to suggest that this is actual variability in diet and not isotopic noise. They sampled some steenbok teeth from Swartkrans with the same technique. Steenbok are consistent C3 browsers; their diet doesn't vary much in its 13C proportion over time. And the samples from the steenbok teeth didn't show very much variation across different sampling zones from the same tooth. Hence, it looks like the samples from different perikymata actually may give a consistent picture of dietary 13C composition over time.
Compared to the steenbok, the A. robustus samples show great heterogeneity in 13C content. This heterogeneity is manifested when looking at multiple samples from the same tooth, and it is also manifested when looking at different individuals. So far, that would seem to indicate dietary heterogeneity -- the A. robustus individuals ate a different mix of foods over time, and different individuals ate different foods.
On the basis of the magnitude of difference (particularly within the single specimen SKX 5939), Sponheimer et al. propose that some individuals must have gone from a diet predominantly composed of C3 foods to one predominantly C4 within the span of two years (estimated 644 days).
Here's how their paper concludes:
A dental microwear study of the earlier (3.0 to 3.7 Ma) hominin Australopithecus afarensis found no evidence that its diet changed over time or in different habitats (20). In contrast, stable carbon isotope (3, 4) and dental microwear texture analyses (1) of the slightly younger (3.0 to 2.4 Ma) hominin A. africanus demonstrated that its diet was far more variable. This suggests the possibility that a major increase in hominin dietary breadth was broadly coincident with the onset of increasing African continental aridity and seasonality after 3 Ma (21, 22) and only shortly antedated the first probable members of the genera Homo and Paranthropus (23-25) and the earliest stone tools (26). The undoubted toolmaker Homo is thought to have been a dietary generalist that consumed novel foods such as large ungulate meat and tubers that are abundant in savanna environments (27-30). Paranthropus, in contrast, with its extremely large and flat cheek teeth, thick enamel, robust mandible, and heavily buttressed facial architecture, is often portrayed as a dietary specialist (27-29). Further, it has been argued that this specialization contributed to its extinction when confronted with increasingly dry and seasonal environments later in the Pleistocene, whereas Homo's generalist adaptation was crucial for its success (28, 29). Our results suggest that Paranthropus had an extremely flexible diet, which may indicate that its derived masticatory morphology signals an increase, rather than a decrease, in its potential foods. Thus, other biological, social, or cultural differences may be needed to explain the different fates of Homo and Paranthropus (31).
We have lots of other reasons to believe that robust australopithecines were not dietary specialists, as pointed out by Wood and Strait (2004). Robust australopithecines had broad geographic ranges, were able to disperse over long distances, and persisted despite substantial climatic and environmental changes. The evidence for dietary differences across the lifespan is certainly consistent with this.
It does, however, make for an interesting conundrum: if australopithecines were selected on the basis of their ability to find different foods over the course of years, that suggests a strong role for social learning of more food types and broader geographic ranges. But if this was the path taken by robust australopithecines, what was the path taken by Homo?
References:
Ambrose SH. 2006. A tool for all seasons. Science 314:930-931. DOI link
Gibbons A. 2006. Not just nuts and berries for these hominids. ScienceNOW 9 Nov. Full text
Sponheimer M, Passey BH, de Ruiter DJ, Guatelli-Steinberg D, Cerling TE, Lee-Thorp JA. 2006. Isotopic evidence for dietary variability in the early hominin Paranthropus robustus. Science 314:980-982. DOI link
Wood B, Strait D. 2004. Patterns of resource use in early Homo and Paranthropus. J Hum Evol 46:119-162. DOI link
Chemistry and early hominid diets
The chemical analysis of bones to interpret diet rests on the observation that different foods vary in the composition of different chemical elements or isotopes. Isotopes are differ