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paleoanthropology, genetics and evolution

Kenyapithecus

  • Book review of "The First Human"

    Thu, 2006-06-29 21:27 -- John Hawks

    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.

    Tags: 
    Ardipithecus
    Hadar
    Sahelanthropus
    Kenyapithecus
    A. afarensis

  • Dentition and diet in early hominids

    Sun, 2005-02-06 20:32 -- John Hawks

    Early hominid teeth changed substantially over time. A number of fossil apes of the Middle and Late Miocene had a dental pattern featuring low-cusped, grinding molars with relatively thick enamel. In females of some species such as Ouranopithecus, Kenyapithecus wickeri, and Gigantopithecus, the canine teeth were small in size compared to living apes like chimpanzees. Living chimpanzees, bonobos, and gorillas differ from the pattern of these fossil apes, as they all share molar teeth with relatively thin enamel and high crowns, and large canines that project well beyond the incisors and premolars even in females. These substantial differences between living African apes and fossil Miocene apes make it unclear which pattern may be the ancestral condition for early hominids. But this pattern of diversity does suggest that the dental characteristics of hominoids tend to evolve readily in response to dietary changes.

    By the time of their earliest known fossil representatives, hominids had established their own, unique dental adaptation. This pattern is present at the earliest clear hominid site, Lukeino (Senut et al. 2001), as well as at a number of Middle Awash localities including Asa Koma, dating to between 5.2 and 5.8 million years (Haile-Selassie 2001, Haile-Selassie et al. 2004) and in isolated mandibles from Lothagam and Tabarin, both dating to between 5 and 6 million years ago. The pattern includes molars that are similar in size and morphology to the teeth of late Miocene apes like Ouranopithecus. There has been some suggestion that these teeth may have varied in enamel thickness (Senut et al. 2001), but systematic comparisons have yet to be performed. The main distinguishing feature of early hominids is a reduction in the size and projection of the canine teeth, in both sexes. Although these canine teeth were reduced in size compared to apes, they still projected beyond the crowns of the neighboring teeth and interlocked with each other (Haile-Selassie et al. 2004). Ape upper canines, like those in living chimpanzees and fossil Ouranopithecus, have a sharpened edge resulting from wear against the lower P3. This pattern of wear is called honing. The earliest hominid canines are not only smaller in size, but tend to lack this kind of honing wear. Some of the canines were worn not on their back edge but instead on their tips, showing that they functioned more like incisors than like ape canines. This pattern of canine size and wear is also found at Toros-Menalla, and is the major piece of evidence that Sahelanthropus may be a hominid. The last fossils with dental characteristics similar to the earliest hominids come from Aramis, also from the Middle Awash region dating to between 4.5 and 4.3 million years ago (White et al. 1994, WoldeGabriel et al. 1994).

    After the Aramis hominids there appears to have been a fairly strong change in the hominid dentition. The fossil samples from Kanapoi and Allia Bay, at the southern end of Lake Turkana, are slightly more recent than the Aramis hominids at between 4.1 and 3.8 million years ago. The important changes are in the molar teeth and the size and robusticity of the mandible. Compared to earlier hominids, the molar teeth are larger and have thicker enamel (Ward et al. 2000). The mandible, represented by KNM-KP 29281, is tall--well over twice the height of the molar roots inside the mandible--and like later hominids has a strong reinforcing bar behind its symphysis. However, unlike later hominids, the molar tooth rows are long and parallel, giving the mandibular and maxillary dentitions a very U-shaped occlusal configuration. The canine teeth are similar to those of earlier hominids in size and projection. Like earlier hominids, these canines did not have strong honing wear, but the adaptation to cutting against the lower third premolar was not entirely gone, as evidenced by the single-cusped P3 in the KNM-KP 29281 mandible (Ward et al. 2000).

    The teeth from Laetoli, Maka, and Hadar appear to form a single series of continuous morphology spanning from 3.7 million to slightly less than 3 million years ago. The basic elements of the dental morphology of these hominids make up the core adaptation of one of the most successful and long-lasting hominid lineages. Over a dozen well-preserved mandibular pieces are preserved, including complete or near-complete mandibles from each of these three sites (White 1977, White et al. 2000, Kimbel et al. 1982). These mandibles are large and thick. They have a distinct buttress along the posterior side of the mandibular symphysis--at the center of the mandible--which is clearly visible in several of the mandibles that are broken at the midline.



    The canine teeth are reduced in this sample compared to earlier hominids. There are still large single canines--especially at the earlier sites of Laetoli and Maka--but these increasingly exhibit wear on the tip and project less beyond the other teeth than in earlier remains. In this sample there is rarely a gap, or diastema, between the canine and the incisors (White et al. 2000), and the canine often takes on an incisor-like function. Most other anthropoids have large canine teeth, and these teeth are often strongly sexually dimorphic. They are apparently sexually dimorphic in these early hominids as well, with strong differences in canine size between the larger and smaller mandibles. The large canines of most primates are not principally a dietary adaptation, but reflect the social aspects of directly fighting or communicating threats. The reduction of the canine teeth in early hominids likely indicates that these social interactions had changed.

    One possibility is that social competition, particularly among males, may have reduced in intensity. Such a reduction in male competition is consistent with models of the evolution of bipedalism that involve greater parental investment and provisioning of offspring. On the other hand, competition may have remained strong but may have taken a form for which large canines were useless. For example, the development of weapons such as clubs or accurately thrown rocks would reduce the advantages of large canines. Likewise, the development of more effective vocal communication might reduce the impact of visual signals like the canine teeth. Amid these possibilities, the reasons for smaller canines in hominids remain uncertain, but are clearly linked to the evolution of other features such as bipedality and social complexity.

    The most distinctive dental feature of these early hominids is the large size of their molar teeth. The earliest hominids had larger molars than chimpanzees or most Miocene apes. The molars of the Hadar hominids average nearly twice the occlusal area of the earliest hominid teeth. Unlike living humans or chimpanzees, these molars increase in size from the front of the mouth to the back, so that the entire tooth row is elongated. And their large size combined with the smaller size of the canines lead the tooth rows to have a more parabolic shape, diverging from each other further back in the mouth.

    The premolars are large as well. The third mandibular premolars are sexually dimorphic. Males lack any trace of honing morphology in the P3, with the tooth more similar in orientation to the P4 and having two distinct cusps. Female specimens tend to have a single-cusp P3 that has a higher angle of rotation from the tooth row. Especially the fourth premolars are larger and more molar-like in function than in earlier hominids. In this way, the area of the postcanine teeth has been increased both by increasing the size of each tooth and by changing the function and form of the premolars.

    With low cusps and thick enamel, the large postcanine teeth clearly are used for grinding. These teeth and the powerful jaws that contain them reflect a dietary concentration on lower-energy plant materials, at least during part of the year. The postcanine teeth of the Hadar hominids are perhaps three times as large relative to their body size than most humans, and over twice as large as in chimpanzees. Chimpanzees and humans both eat rather high-energy foods, such as fruits and meat. The large molars of early hominids indicate that such foods were probably eaten more rarely or were unavailable for large parts of the year.

    Finally, the incisors are relatively large, possibly with a role in stripping plant material as in living apes.

    Two samples from between 3.4 and 3.5 million years ago deviate from the pattern established by the Laetoli--Maka--Hadar sequence. One, from Bahr el Ghazal in central Chad, is not well dated but is likely around 3.5 million years old. The fossil is a partial mandible, preserving the front of the mandible anterior to the first molars, and including canines and premolars on both sides. Unlike other early hominid premolars, which typically have one or two roots, both the P3 and P4 of this specimen have three roots. This unusual feature, as well as the relatively vertical symphysis and relatively thin premolar enamel make this central African specimen stand out somewhat compared to contemporary fossils (Brunet et al. 1995). The other sample is the dental sample from Lomekwi. The teeth from this site, including those in the KNM-WT 40000 skull, have similar morphology and enamel thickness to teeth from other sites, but the sizes of the teeth are at or below the minimum size observed at Hadar or Laetoli (Leakey et al. 2001). Both of these sites have been suggested to represent separate species from the Laetoli--Maka--Hadar sequence as discussed below.

    References:

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

    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 418:145-151.

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

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

    Kimbel WH, Johanson DC, Coppens Y. 1982. Pliocene cranial remains from the Hadar formation, Ethiopia. Am J Phys Anthropol 57:453-500.

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

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

    Ward CV, Leakey MG, Walker A. 2001. Morphology of Australopithecus anamensis from Kanapoi and Allia Bay, Kenya. J Hum Evol 41:255-368.

    White T, Suwa G, Asfaw B. 1994. Australopithecus ramidus, a new species of early hominid from Aramis, Ethiopia. Nature 371:306-312.

    White TD. 1977. New fossil hominids from Laetolil, Tanzania. Am J Phys Anthropol 46:197-230.

    White TD, Suwa G, Simpson S, Asfaw B. 2000. Jaws and teeth of Australopithecus afarensis from Maka, Middle Awash, Ethiopia. Am J Phys Anthropol 111:45-68.

    WoldeGabriel G, White TD, Suwa G, Renne P, deHeinzelin J, Hart WK, Helken G. 1994. Ecological and temporal placement of early Pliocene hominids at Aramis, Ethiopia. Nature 371:330-333.

    Tags: 
    Sahelanthropus
    Hadar
    A. anamensis
    Pliocene
    Kenyapithecus
    A. afarensis
    Miocene
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