john hawks weblog

paleoanthropology, genetics and evolution

Hadar

  • 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

  • The Hominid Pelvis

    Wed, 2005-02-02 14:36 -- John Hawks

    The most dramatic illustration of bipedalism is the pelvis, and the most dramatic specimen demonstrating pelvic morphology is the relatively complete skeleton from Hadar, Lucy, AL 288-1. This important fossil preserves a nearly complete innominate (hip) bone and sacrum, which have been used to reconstruct the pelvis of this individual. The upper portion of the innominate bone, called the ilium, is short and curving compared to the long, flattened ilium of chimpanzees and other apes. This curvature provides an anterior attachment for the muscles that pull the femur forward during walking or running. Although the ilia are short in length compared to a chimpanzee, they extend more broadly to the side, resulting in a pelvis that is very broad overall. Indeed, this pelvis is nearly as wide as that of a human female, despite the small body size of the australopithecine. This pelvic width contributes to a very different body shape for early hominids than for humans.

    The width of the pelvis affects the muscular requirements of walking. Whenever one leg supports the body, the trunk of hominids tends to fall away from the supporting leg. The muscles that prevent the body from falling over attach to the lateral part of the ilium and to the femur, pulling the trunk upward around the hip joint. A wide ilium tends to increase the efficiency of these muscles. Their effectiveness is also aided by a long femur neck, just as long handles on a pair of scissors greatly increase the force with which they can cut. This configuration of muscles is similar to the condition found in living people, but it is much more extreme. In living people, the pelvis is much narrower in proportion to overall body height, and femur necks are much shorter in proportion to femur length.

    A number of hypotheses compete to explain why the bipedal adaptation of early hominids should be different from modern human bipedalism in this way. One explanation is that the pelvic width contributes to the length of the stride, by rotation of the pelvis during walking. This rotation would increase stride length without increasing the length of the swing of the legs, allowing an increased stride without lowering the mass of the body--which if lowered would subsequently have to be raised again through greater muscular exertion (Rak 1991). Alternatively, the widely spaced femora may allow a greater mechanical advantage for the muscles that draw the legs medially, toward the midline. Such a configuration would be advantageous for certain leg motions, especially the style of climbing that requires the legs to clamp around a branch or trunk. This kind of climbing would be more necessary to bipeds who lacked the prehensile feet of living apes.

    The wide pelvis of early hominids had one consequence beyond those related to bipedalism, by setting the stage for a major evolution of the birth process. During birth, the infant must pass through the birth canal, entering it through the pelvic inlet. Although relatively small compared to the pelvic inlets of the apes, the oval shape of the australopithecine pelvic inlet probably did not create any problems during birth because australopithecine infants likely had head sizes about the same as those of chimpanzees. Even so, Robert Tague and Owen Lovejoy (1986) have speculated that birth in australopithecines may have involved a more complex series of events than chimpanzee births, with the possibility that the infant head may have had to rotate a quarter-turn to be born in line with the sideways oval in a manner similar to the half-rotation of a human infant during birth. Karen Rosenberg and Wenda Trevathan (2002) suggest that at this time, hominid females began to require assistance with the birthing process. The pelvic constraints associated with bipedal locomotion would come to determine much more about the birth process in later hominids.

    More on bipedalism in hominids

    References:

    Rak Y. 1991. Lucy's pelvic anatomy: its role in bipedal gait. J Hum Evol 20:283-290.

    Rosenberg KR, Trevathan W. 1996. Bipedalism and human birth: the obstetrical dilemma revisited. Evol Anthropol 4:161-168.

    Tague R, Lovejoy C. 1986. The obstetric pelvis of A.L. 288-1 (Lucy). J Hum Evol 15:237-255.

    Tags: 
    bipedalism
    Hadar

  • Forelimbs and climbing in early hominids

    Mon, 2005-01-31 21:45 -- John Hawks

    Compared to their small body mass, the forelimbs of early hominids are both longer and more muscular than those of recent humans. The arms are shorter than in chimpanzees, but the areas of muscle attachment have greater strength. Strength is especially evident in a large humerus from the Ethiopian site of Maka, dating to 3.4 million years ago (White et al. 1993). The prominent muscle attachments on this large specimen indicate that the individual was very strong, but also that most muscle exertion was in a single preferred pattern. The bone is thicker than chimpanzee humeri of equal length, again reflecting its mechanical strength.

    Several other forelimb fossils show a similar pattern. These include a relatively large distal humerus fragment from Kanapoi, a large radius from the contemporary site of Sibilot Hill, and a distal section of humerus exhibiting large muscular crests from Lukeino (Senut et al. 2001). Additionally, the hamate bone preserved at the Kenyan site of Turkwel preserves part of a very large carpal tunnel region, indicating strong tendon attachments into the hand (Ward 1999). Finally, the finger bones of early hominids are curved. This feature occurs wherever Early Pliocene hominid finger bones are preserved (Stern and Susman 1983).

    The most probable interpretation for the strength of the forelimbs among these sites is that early hominids were effective climbers. Hominids do not have grasping feet. Apes use all four limbs in a variety of climbing positions, but hominids use their arms mainly to pull the body upward with the legs providing upward propulsion but no gripping support. Such use would lead to large muscle development in the arms, both because they bore more of the force of climbing and because they functioned in a more specialized way.

    But despite the strength of the large fossil arms, smaller individuals show a somewhat different pattern. For example, the AL 288-1 skeleton preserves much of both humeri and ulnae, and these small bones bear minimal muscle markings. The difference in forelimb anatomy between large and small individuals may mean either that males climbed more frequently than females or that the biomechanics of climbing among malesÑwith a mass much larger than femalesÑplaced much greater muscle requirements on the male forearm.

    Males may have performed other tasks with their arms, including wielding weapons or other competitive behaviors such as threat displays. The hands of early hominids, as represented at Hadar, have fingers that are similar to living humans in their relative sizes. These proportions are very different than in chimpanzees, which have a much shorter thumb that departs the hand much closer to the wrist, as well as much longer fingers. The human-like proportions of the hands underline the fact that climbing in these obligate bipeds was done in a human-like manner, and their hands did not function in a chimpanzee-like suspensory role. Also, early hominids may have gripped clubs or other items that do not require forceful fingertip control. Chimpanzees wield large branches in the context of threat displays, and it is possible that early hominids also had such behaviors or even more menacing ones, enabled by the power of arms like those from Maka.

    However, early hominid hands were clearly not used for making stone tools. Two sources of evidence argue against the ability of these early hominids to modify stone. First, no stone tools have yet been found earlier than about 2.6 million years ago, long after the early Hadar sample. Second, the Hadar hands and other early hominid hand bones lack important features that reflect a powerful grip useful for tool production (Marzke 1983). Most noticeably, the distal finger bones, or phalanges, lack the large fingertip surfaces, called apical tufts, which are found in living and fossil humans (Bush et al. 1982). These large fingertips increase the surface area used for gripping, and allow the forceful grip necessary for tool production. The apical tufts at Hadar are relatively much smaller than those in human fingers (Stern and Susman 1983).

    Tags: 
    A. afarensis
    Hadar
    Orrorin
    Pliocene

  • The Flores find :: more thoughts on Liang Bua

    Wed, 2005-01-05 15:12 -- John Hawks

    I (and others) have had about a week to reflect on this new skeleton and its significance. There is little question that this is one of the most vexing discoveries ever.

    To understand why, compare the situation with that surrounding a major find like Lucy (AL 288-1). Lucy (and the Hadar sample as a whole) presented little new or interesting information about the morphology of australopithecines. The Sterkfontein sample, which had emerged over the thirty years prior to the Hadar discoveries, demonstrated nearly everything important about the australopithecine adaptive pattern--including body size, dental characteristics, evidence for bipedality, and brain size. The story from Hadar was that it was older than everything else then known, and that there was a relatively complete skeleton that associated much of the important morphology into a single specimen. This is why Lucy now has such importance in our textbooks, but the fact is that almost everything interesting was known long before.

    One might say some of the same things about Liang Bua. It looks in many respects like we know some early hominids looked (although admittedly with many differences in detail). But here the differences in time and place are a much greater story, because they upset so much more. A "new earliest hominid" does little but push back the date of hominid origins, especially since none of the "new earliests" have been particularly informative about the ancestral condition (they are all obligate bipeds so far, and none of them is very chimpanzee-like, notwithstanding the dentition of Ardipithecus). But Liang Bua is striking because it has no obvious ancestor. If it is an australopithecine, where are the earlier Asian australopithecines that it descends from? Meganthropus? If it descended from early Javan humans like Sangiran, then what accounts for the necessary selection for smaller brains leading to this specimen?

    Only if LB1 can be attributed to pathology are these questions less pressing. In that case, it represents just one of (potentially) many human populations that lived on Flores or passed through it on the way to Sahul, Melanesia, and points east. The pathology hypothesis has not yet been refuted, and it is important to keep that in mind.

    Do the tools belong to LB1 or its ilk?

    With a 380 mL brain, LB1 is at the bottom of the range of australopithecine brain sizes. After the advent of stone tools 2.6 million years ago, no known hominid has a brain size of less than 400 mL, and most paleoanthropologists have assumed that the manufacture of stone was accomplished by species with brain sizes that averaged 500 mL or more. It is not at all clear to what extent the mass of neural tissue is related to tool manufacture or any other capabilities. Perhaps it is the case that the essential changes affected the structure of the brain rather than its size, and a smaller brain might well have the circuitry to accomplish many advanced cognitive tasks. It is certainly true that chimpanzees are capable of tool use, the mastery of rudimentary symbol manipulation, and cooperative foraging, despite having brains that average 400 mL or less.

    On the other hand, modern humans reached Australia by 50,000 years ago, and were present in island Melanesia by 30,000 years ago. The route to both of these places passes through Flores, so it is highly probable that modern humans were present on Flores 18,000 years ago, when the LB1 lived.

    In this context, I think we can reasonably assume that the stone tools, fire, and evidence of Stegodon hunting can be attributed to these modern people. Colin Groves has sent me a reference to an

    ABC News story     where he makes much the same argument.

    This leaves the two main hypotheses about LB1:

    1. it was a pathological member of this modern human population
    2. it was part of an earlier hominid population alongside these modern humans

    In the second case, we can speculate about the relationship of these populations. Perhaps LB1 was a victim of the modern humans: i.e. it is in the cave for the same reason the Stegodon remains are in the cave. Perhaps the tiny-bodied population avoided the modern humans in the same way as today orangutans avoid modern people on Borneo and Sumatra. This might imply a very different ecological role for these smaller hominids, considering the ecological breadth and travel potential of the modern humans alongside them.

    Tags: 
    Flores
    Hadar
    Sterkfontein
    Ardipithecus

  • <u>Australopithecus anamensis</u> :: overview

    Fri, 2004-09-10 00:08 -- John Hawks

    The sample from Kanapoi, along with other smaller samples predating Laetoli, at 3.7 million years, exhibit several differences from the later A. afarensis sites. Since jaws and teeth are best known for these early hominids, these dental features have been used extensively to define both samples and species. The major differences involve morphology of the mandible, including:

    1. long mandibular symphysis, angled posteriorly
    2. long mandibular surface behind the incisors, called a postincisive plane
    3. long parallel tooth rows

    There are also several dental differences, including:

    1. more projecting canines
    2. more asymmetric P3
    3. small P4

    Postcranially, the Kanapoi sample and other samples from the same time exhibit no specific morphological differences from the much later Hadar hominids. However, the three postcranial fragments from which body mass can be estimated, including a tibia, a distal humerus, and a radius from Sibilot Hill, all result in a body mass estimate larger than the mean size for male remains at Hadar. These findings may imply a larger body size for these earlier hominids, or they may merely represent the chance discovery of three large specimens from these early sites (Ward et al., 2001).

    The dental and mandibular differences in the earlier sample support to some extent the hypothesis that the earlier sites represent a distinct species. This species was named Australopithecus anamensis by Leakey and colleagues, where ``anam'' means ``lake'' in the local Turkana language. Despite the differences, however, these samples represent a hominid that is clearly similar in dental adaptation to the later A. afarensis sample, including:

    1. large molars with thick enamel
    2. tall mandible, over twice the height of the molar roots
    3. relatively small canines compared to Miocene apes, with no diastema

    Most dental differences between the Kanapoi dental sample and later samples are differences of degree, with the Kanapoi hominids exhibiting a form that is clearly like later hominids, but slightly in the direction of Miocene apes, or in some cases in the direction of the Aramis sample. The overall appearance of the Kanapoi dental sample is that it is a primitive form of the later Laetoli, Maka, and Hadar samples. No significant postcranial differences---other than possibly body size---separate the samples. It is therefore reasonable to support the hypothesis that the major sites of Kanapoi, Laetoli, Maka, and Hadar represent a single evolving lineage, developing an adaptation to greater chewing over time. This hypothesis has been supported by analysis of temporal change among the A. afarensis samples (Lockwood et al., 1999), and would imply that the hominids from around 4.0 million years ago in fact represent an early segment of the evolutionary species A. afarensis. Naturally, if it were shown that a significant speciation had taken place between the early and later samples in this lineage, perhaps involving the appearance of hominids in South Africa or Central Africa, then the species name A. anamensis would be justified for Kanapoi and other contemporary samples.

    Tags: 
    Miocene
    Hadar
    A. anamensis
    A. afarensis

  • Australopithecus afarensis : overview

    Thu, 2004-09-09 23:57 -- John Hawks

    The large sample from Hadar overlaps the smaller hominid samples from several sites that are near it in time, including the dental remains from Laetoli and Maka, and isolated finds from many East African localities. The original excavators of the Hadar hominids recognized the similarities with the other important early hominid site known at the time, Laetoli, and promoted the hypothesis that the Hadar and Laetoli hominids belong to a single species (Johanson and White, 1979). They named this putative species Australopithecus afarensis. Australopithecus was the genus name first given to later hominids from South Africa, covered in the next chapter, with which the Hadar and Laetoli samples are broadly similar, but vary in significant aspects. The species name afarensis refers to the geographic location of the Hadar hominids, in the Afar region of Ethiopia, and means simply "from Afar."

    The samples assigned to A. afarensis provide the largest source of evidence for early hominid adaptations, and any general discussion about early hominid morphology is really a discussion about the morphology of this species. A few features distinguish this species from other early samples. These include:

    1. greater development of a lingual cusp on the P3 in many individuals
    2. larger postcanine teeth
    3. smaller canines, with smaller diastemata

    Other possible comparisons with other sites, such as in cranial and postcranial form, are much less clear because of the small samples from these other sites available for comparison. These dental features stand apart from earlier and contemporary samples in the development of an adaptation to molar chewing with less canine cutting. These changes and even greater ones will be evident in later australopithecines also, and may reflect a link between later species and A. afarensis.

    A. afarensis has often been claimed to be a species with a long period of morphological stasis. But recent analyses have shown the existence of temporal trends within the species in both tooth sizes and mandibular dimensions. Charles Lockwood and colleagues (1998) used the ages of different A. afarensis dental fossils to determine whether significant evolution occurred within this species over time. The analysis found that tooth sizes did increase over time, from Laetoli to Maka and Hadar, and significantly within the Hadar sample from the earliest fossils at 3.4 million years to the more recent ones around 3 million years old. Although most aspects of dental form were constant enough to identify the remains as probably a single species, the gradual evolution of size within this lineage appears as a long-term evolutionary trend. Whether the factors that led to this trend may explain earlier or later changes in tooth size is not yet known, but it seems clear that even in a long-lasting, successful hominid species, substantial evolutionary changes may take place.

    Tags: 
    A. afarensis
    Hadar

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