Australopithecus afarensis :: overview
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 Òfrom Afar.Ó
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:
- greater development of a lingual cusp on the P3 in many individuals
- larger postcanine teeth
- smaller canines, with smaller diastemata
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.
Keeping the faith, afarensis-style
From Free Republic: an article from Science News by Bruce Bower covers the recent flap about sexual dimorphism in early hominids. This is a pretty good introduction, if you haven't been following my discussion.
The article goes through the whole story, starting with Owen Lovejoy's bipedal origins model all the way to Plavcan and colleagues' response to the recent work of Philip Reno, Lovejoy and others suggesting a low sexual dimorphism for A. afarensis.
Here's a teaser:
Reports on new fossil finds of A. afarensis and even older hominid species are expected soon. Lovejoy plans to factor skeletal data from these discoveries into a larger examination of ancient sex differences.
Never too soon to blog...
Dentition and diet in early hominids
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.
Forelimbs and climbing in early hominids
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).
Sexual dimorphism in A. afarensis, again
The Journal of Human Evolution early access section has a paper by J. Michael Plavcan and colleagues that critically examines the case for low sexual dimorphism in A. afarensis.
To catch you up briefly with the story, here is a synopsis to date. Before 2004, the consensus about A. afarensis was that the samples from Laetoli, Maka, Hadar, and other smaller samples belonged to a single species with substantial sexual dimorphism (gorilla-like or orangutan-like in extent) and considerable temporal change from the early to late end of the sequence. A minority view was that there were actually multiple species in the sample. Some thought this because they thought that some of the important specimens (especially AL 288-1, Lucy) had been misidentified as to sex. Others were either unconvinced by the morphological similarities among the samples, or were rightly skeptical about the weakness of the test for sexual dimorphism within species. That is to say, the variation would have to exceed that found in gorillas (with males double the mass of females) before the single-species hypothesis would be rejected.
In 2004, Reno and colleagues added another perspective. They applied a resampling technique to estimate the sexual dimorphism in the A. afarensis sample, transforming the sizes of different skeletal elements to a single scale in order to increase the effective sample size. Their conclusion was that the variability in the A. afarensis sample was most consistent with a low level of sexual dimorphism, similar to humans. They used this observation to suggest that the social behavior of early hominids may have included a more humanlike mating system, consistent with Lovejoy's (1981) account of the origin of bipedalism.
Plavcan and colleagues (2005) present several arguments as to why the conclusions of Reno et al. (2004) may be flawed.
- They suggest that the AL 333 sample, upon which Reno and colleagues hinge some of their conclusions, is likely to be biased, In particular, they suggest that there are probably more males than females, and probably many of the elements included as separate individuals by Reno et al. (2004) actually belong to a smaller number of individuals. Plavcan and colleagues suggest that the MNI for the postcranial elements alone at this locality is three adults and one subadult. Their conclusion from this is that estimates based on AL 333 are likely underestimates. However, in my view, this probably does not detract substantially from the results of Reno and colleagues' analysis. Consider that the overall A. afarensis sample was statistically similar to the AL 333 sample alone, and Reno and colleagues attempted to assess the possible effects of sample bias by simulating samples in which one sex was highly overrepresented.
- Plavcan and colleagues argue that the non-AL-333 elements of the A. afarensis sample actually show high variation. This observation is not present in Reno and colleagues (2004), who instead present these remains together with the AL 333 locality as a "Combined Afar" sample. This is more of a problem. Reno and colleagues suggest that the "Combined Afar" sample should be more variable than AL 333 because the combined sample includes specimens across a broad time interval, but as Plavcan et al. note, there is actually little variation over time noted (as yet) for this time span. And Plavcan and colleagues provide an illuminating figure that shows that temporal variation in overall size (without differences in sexual dimorphism) does not result in higher variation in sexual dimorphism.
- The most critical point raised by Plavcan et al. is that skeletal dimorphism is not well related to body mass dimorphism. Presumably it is body mass dimorphism that has implications for social structure. They apply their own range of comparisons to examine the variation of skeletal dimorphism (in particular femoral head diameter variation) with that of body mass dimorphism. In the end, they conclude that the Afar sample is consistent with a body mass dimorphism greater than that in any human population that they examine, and between that of chimpanzees and gorillas.
- Finally, Plavcan and colleagues question the premise that social behavior can be inferred from the level of sexual dimorphism. This problem is well-known, and they give little detail, but the argument is well-taken.
I think the bottom line coming out of this argument is that there really isn't enough to infer much about the level of dimorphism of A. afarensis. For reasons of my own, I think the template method used by Reno and colleagues is problematic. Without this, there are only a handful of specimens for any single skeletal element that can be compared. This is probably a sufficient sample to determine whether humanlike skeletal dimorphism overall can be rejected, but not enough to examine the relationships of dimorphism for different skeletal elements, and probably not enough to infer mass dimorphism.
And the problems with inferring behavior from dimorphism come not from the fossils, but from the comparisons available among living species. Until a model can account for the dimorphism among living hominoids based on their social behavior, there is certainly no point in trying to infer the behavior of ancient hominids in this way. Social groups in all hominoids are flexible to some extent, and in humans they are extensively so. This bodes poorly for a resolution of this problem.
Sexual dimorphism in A. afarensis, again
Earlier this year, Michael Plavcan et al. (2005) had a critique in Journal of Human Evolution of the 2004 paper by Philip Reno et al. in PNAS concerning sexual dimorphism in A. afarensis. Now, in the August issue of JHE, Reno and colleagues have a reply. I have previously written about the Plavcan critique and a news report on the issue.
The reply by Reno et al. (2005) covers the four areas raised by Plavcan et al. (2005) thusly:
- Is the AL 333 sample biased? And how many individuals are there? Reno et al. (2005) present new simulations to show that a very small number of individuals at the site would still result in an interpretation of low dimorphism. So a smaller sample size than they originally assumed does not explain their interpretation of low dimorphism (although it does limit the information provided by the data). They further argue that it is likely the non-AL 333 sample that is biased, because it does not represent the number of intermediate-sized (presumptive female) individuals found at AL 333.
- How does temporal variation affect the estimate of dimorphism? Plavcan et al. (2005) used a clever illustration to show that temporal variation may have no effect on dimorphism estimates. Reno et al. (2005) respond with another clever illustration showing that temporal variation may affect dimorphism estimates substantially, usually by causing overestimation. This point is essential to their argument that the AL 333 site is more representative of dimorphism than the other, temporally dispersed, localities.
- Is skeletal dimorphism well-related to body mass dimorphism? This problem is third on the list generated by Plavcan et al. (2005), but Reno et al. (2005) tackle it first. That may be because they caught the prior authors in an error:
Plavcan et al. make a fundmental error in their Figure 3. This figure shows body mass dimorphism (BMD) plotted against femoral head dimorphism in apes and humans. They also plotted values for A. afarensis calculated from our template estimates of FHD on this figure. They claim that these values imply marked femoral head dimorphism for A. afarensis. They do not. This figure mistakenly commingles FHD estimates for A. afarensis, i.e., template sexual dimorphism (our TSD) and actual values obtained by direct measurement of specimens with known sex, i.e., direct sexual dimorphism (our DSD) (Reno et al. 2005:280).
The two are not comparable, so this is an apples-and-oranges comparison. For the rest, Reno et al. (2005) correctly point out that body size dimorphism is not what they are trying to estimate, since skeletal dimorphism is all that fossils present:
Skeletal dimorphism is only one aspect of size dimorphism. It is not, and can never be, a simple surrogate for dimorphism in body mass. First, these characters have only partial association in hominoids; humans display moderate skeletal dimorphism and low levels of mass dimorphism, but chimpanzees show the opposite relationship (as we discussed, see: Reno et al., 2003). Second, body mass (and therefore body mass dimorphism) is unknowable for fossils. It is therefore impossible to derive a regression with which to estimate it without substantial error, because such regressions must be based on extant species that are likely to be biologically dissimilar to extinct ones. Skeletal dimorphism, however, is dependent only upon skeletal dimensions, which are directly measurable in fossils. Plavcan et al.Õs discussion of scaling and allometry robustly demonstrates the regression problem. That is why we do not use regression. However, we certainly understand why they were able to "corroborate previously reported findings that A. afarensis [body] size dimorphism falls between that of chimpanzees and gorillas." (p. 318). For skeletal dimorphism this range includes virtually all primates (Reno et al. 2005:281).
This is certainly the safe route, although the claim does raise a critical question, discussed below.
- Does body mass dimorphism predict social or behavioral features of primates? Of course, what primatologists usually study is body size dimorphism, and not skeletal dimorphism. So if skeletal dimorphism is what we are limited to studying in fossils, then where is our comparative data? Here, Reno et al. (2005:285) present some theory, focusing on bimaturational patterns:
There has long been a sub rosa assumption that body mass dimorphism is the primary target of sexual selection. However, body mass is a complex character and incorporates several morphological components (Leigh, 1992), particularly (but not exclusively) both skeletal and muscle mass. Muscle mass and skeletal dimorphism can be differentially regulated during mammalian development (McMahon et al., 2003), and our results suggest that this is likely to be the case in chimpanzees and humans. Thus, sexual selection can independently affect skeletal and muscle growth.
That's certainly something to chew on, as are the details of chimpanzee and gorilla developmental hypotheses presented later. They conclude that no extant ape or primate provides a valid model for A. afarensis, and therefore it is necessary to construct one.
At the end, the paper asks an interesting question of A. afarensis:
If its skeleton was largely modern in structure, is it not also likely that much of this derived physiology and anatomy had evolved by the dawn of the Pliocene in ecogeographically unique and cosmopolitan A. afarensis? (Reno et al. 2005: 286-287, emphasis in original).
"This derived physiology and anatomy" includes
...concealed ovulation and permanently enlarged mammary glands implying female reproductive crypsis, elaborated epigamics in both sexes (implying bi-directional mate choice), minimal semen coagulation, moderate muscularity of the vas deferens, relatively small testes and sperm midpiece, retention of scrotal rather than peritoneal testes, a remarkably rapid loss of olfactory receptors, a loss or failure to develop significant vocal sacs, and hormone profiles potentially paralleling those of some extant monogamous mammals (ibid., 286, citations elided).
I have only one thing to say. If the "largely modern" skeleton of A. afarensis is enough to infer all this stuff, then why are we talking about Neandertals? Of course, it's because a "largely modern" skeleton isn't enough to infer anything.
References:
Plavcan JM, Lockwood CA, Kimbel WH, Lague MR, Harmon EH. 2005. Sexual dimorphism in Australopithecus afarensis revisited: How strong is the case for a human-like pattern of dimorphism? J Hum Evol 48:313-320.
Reno PL, Meindl RS, McCollom MA, Lovejoy CO. 2005. The case is unchanged and remains robust: Australopithecus afarensis exhibits only moderate skeletal dimorphism. A reply to Plavcan et al. (2005). J Hum Evol 49:279-288.
AL 438-1
Michelle Drapeau and colleagues (2005) report on the AL 438-1 specimen from Hadar. The specimen consists of "part of the mandible, a frontal bone fragment, a complete left ulna, two second metacarpals, one third metacarpal, plus parts of the clavicle, humerus, radius, and right ulna" (1). At 3 million years, the specimen is one of the youngest of the A. afarensis sample.
The AL 438-1 individual was evidently relatively large compared to the rest of the Hadar sample. The ulna length is 278 mm, which is larger than the mean for any of the human samples examined by Aiello et al. (1999) in their comparative study of the OH 36 ulna. It is about average for a chimpanzee, although chimpanzees have relatively longer forelimbs than would have been true of A. afarensis, so again this is evidence of a relatively large body size.
Drapeau et al. (2005) make a point of the proportion of the ulna and the mandible being similar to that found in AL 288-1 (Lucy), which they take as evidence that large teeth in this late specimen may be attributed to larger body size rather than greater megadonty:
Both Australopithecus afarensis mandibles have a larger corpus (breadth and height at M1 relative to the ulnar size surrogate than those of African apes. Similarly, mandibular corpus shape (breadth/height x 100 at M1) is similar in the two fossils (A.L. 288-1, 57%; A.L. 438-1, 60%). This difference in mandibular size corresponds to what would be expected from two extant ape conspecifics with ulnae of such different sizes. Since there are no differences between the two Hadar skeletons in mandibular to ulnar proportions, there is no evidence for an increase in mandibular size relative to the rest of the skeleton between the points in time represented by these two individuals. We cautiously offer this as support for Lockwood et al.'s (2000) suspicion that the observed temporal trend toward larger mandible size reflects a body size increase late in the Hadar time span of A. afarensis (Drapeau et al. 2005:41-42).
The paper has a substantial discussion of the morphology and comparative anatomy of the ulna. The bottom line of this analysis is that the ulna is similar to that of AL 288-1 in most respects, except for its larger size and somewhat greater curvature. It is, however, smaller and somewhat less curved than the later Omo L40-19, and substantially less curved than the OH 36 ulna. The authors write this about its similarities to other homionids:
While phenetically A.L. 438-1 presents a mix of ape-like and human-like morphology, when considered in a phylogenetic context, the Australopithecus afarensis forelimb shares synapomorphies exclusively with humans among extant hominoids taxa. It resembles non-hominins only in plesiomorphic character states. In this context, it is apparent that A. afarensis forelimb anatomy reveals the results of selection for a more human-like humeroulnar joint, larger thumbs, and altered carpometacarpal joints that reflect an emphasis on manipulative aptitude at the expense of forelimb-dominated climbing ability (Drapeau et al. 2005:43).
That is a relatively powerful statement of the adaptive qualities of the A. afarensis forelimb, which appers more or less necessary to explain the differences between early hominids and apes in this respect. If the early hominids were really climbing a substantial proportion of the time, then we might hypothesize that their forelimbs ought to look more like ape arms. But they don't; there are clear differences that make the early hominid arms look more similar to human arms. Thus, the authors turn to the hypothesis that the A. afarensis forelimb is additionally adapted to "an emphasis on manipulative aptitude."
At the moment, this hypothesis remains to be strongly tested. Most of the human-like features of the A. afarensis arm are arguably the result of not being used in quadrupedal weight support. Thus, the fact that "the Australopithecus afarensis elbow joint appears to reflect habitual loading the elbow at or near 90 degrees, rather than optimization for loading in a more extended posture as in extant apes" (43-44), as well as the anatomy of the joint and the form of the carpometacarpal joints may all be explained by the fact that early hominids were not knuckle (or fist) walkers. The large thumbs are the strongest piece of evidence for any kind of manipulative behavior in A. afarensis.
On the subject of retained similarities with apes, Drapeau et al. (2005:46) have this to say:
The retention of African ape symplesiomorphies in A. afarensis may be attributed to either stabilizing selection fore a partially arboreal locomotor repertoire, or to lack of selection against these traits (see discussion in Stern, 2000; Ward, 2002). It is inherently difficult to test these alternative hypotheses. Thus, the significance of these retained traits for reconstructing the behavior of A. afarensis is difficult to determine with certainty in the context of demonstrable selection for a human-like elbow and hand joints. Australopithecus afarensis shares some apomorphies with humans that suggest emphasis on use of the forelimb in flexed postures, and improved grip capability relative to apes. The presence of these synapomorphies suggests similarities in forelimb function among hominins, likely reflecting selection for expanded manipulative capabilities and flexed forearm postures relative to that found in apes and a diminished capacity for ape-like arboreal behaviors. Only later did humans display evidence of further selection for manipulation coupled with reduced forelimb robusticity. We conclude that in Australopithecus afarensis, selection for natural manipulation outweighed selection for arboreal activities, but that selection for refined manipulative ability had not yet come into play in human evolution.
A fine balance, if it is true, and fitting within the generally understood picture that, with regard to its arm and hand functions, A. afarensis was either Homo habilis nor a chimpanzee.
References:
Aiello LC, Wood B, Key C, and Lewis M. 1999. Morphological and taxonomic affinities of the Olduvai Ulna (OH 36). Am J Phys Anthropol 109:89-110.
Drapeau MSM, Ward CV, Kimbel WH, Johanson DC, and Rak Y. 2005. Associated cranial and forelimb remains attributed to Australopithecus afarensis from Hadar, Ethiopia. J Hum Evol Advance before print.
Lockwood CA, Kimbel WH and Johanson DC. 2000. Temporal trends and metric variation in the mandibles and dentition of Australopithecus afarensis. J Hum Evol 39:23-55.
Paleoecology at Hadar
The coming attractions bin at Journal of Human Evolution includes a paper by Kaye Reed, reviewing the evidence of paleoenvironment in the Hadar formation:
Habitat reconstructions of 12 submembers of the Hadar and Busidima formations (˜3.8-2.35 Ma) are presented here along with faunal differences in these submembers through time. Habitats with medium density tree and bush cover dominated the landscape through much of the earlier time period in the Hadar Formation. The lowermost Sidi Hakoma Member is the most closed habitat. The Denen Dora Member shows the influence of frequent floodplain edaphic grasslands with high abundances of reducin bovids. There is an influx of ungulates in the Kada Hadar Member (˜3.2-˜2.96 Ma) that indicates a more arid habitat populated by mammals that were recovered from earlier deposits further south in Ethiopia and Kenya. In the younger deposits from the Busidima Formation at Hadar, the landscape was open wooded grassland with some floodplain environments. The fossil assemblages from the Busidima Formation show a substantial species turnover. Although high numbers of A. afarensis specimens are associated with the lower Sidi Hakoma Member, they clearly inhabited a variety of habitats throughout the entire Hadar Formation. Australopithecus afarensis from Laetoli through Hadar times appears to have been a eurytopic species.
This is a nicely detailed paper, focusing on the amount of wooded/bush habitat, the relation of the hominids to those habitats, and the relative lack of early faunal exchanges with areas further to the south.
The discussion focuses on the range of paleoecologies in which fossil A. afarensis has been found -- including not only Hadar but also nearby Maka and Dikika, and more distant Koobi Fora and Laetoli. Altogether, these localities cover a long time (from before 3.5 up to around 2.9 million years ago). From the range of paleoecologies reconstructed in this paper at Hadar, Reed concludes that A. afarensis did not have a "narrow" habitat preference. It is found in relatively closed woodland, open woodland/bush, and wet grassland/marshland.
There are some differences between localities. At Koobi Fora, relatively few specimens of A. afarensis have been found in the Tulu Bor Member, despite the fact that it occupies the same time as the Hadar sequence. Based on the paleoecological data, Reed suggests that Hadar was a wetter, more closed woodland habitat than Koobi Fora at that time -- Koobi Fora would have included more scrubland punctuated with wetlands and floodplains (here she cites her own 1997 paper).
The early end of the A. afarensis sample is represented at Laetoli. Reed gives a brief review of the paleoecology of that site, which has been interpreted differently by different authors but broadly appears to have had a fairly high amount of rainfall and some patches of forest amid closed woodland:
Thus, the earliest known A. afarensis material was found in deposits showing habitats in which trees and or bushes were fairly plentiful. It is also interesting to note that while the deposits of A. afarensis at Laetoli and Hadar share some perissodactyls, giraffids, suids, and proboscideans, the bovid taxa and those primates other than A. afarensis are not very similar.
Reed concludes that A. afarensis was a "eurytopic" species -- one that inhabited a wide range of habitats and moved broadly across space. It contrasts with the more habitat-selective ("stenotopic") species, which include most of the bovids.
White et al. (1993) suggested broad habitat tolerance for A. afarensis, and indeed, the species has thus far been recovered from regions in which the reconstructed habitat ranges from closed woodland through more open, but wet woodland and shrubland. There is no direct evidence that A. afarensis only existed in riverine forests or grassland habitats, or that they preferred one habitat over another. It is tempting to equate the aridification in the Kada Hadar Member with the extinction of A. afarensis. However, sediments at Hadar are sparse or missing altogether from ˜2.90-2.35 Ma thus obscuring details of the species' demise. All that can be said is that they are no longer present at 2.35 Ma and most of the fauna, including hominins, has been replaced.
References:
Reed KE. 2008. Paleoecological patterns at the Hadar hominin site, Afar Regional State, Ethiopia. J Hum Evol (in press) doi:10.1016/j.jhevol.2007.08.013
John Hawks Department of Anthropology
University of Wisconsin—Madison
Copyright © 2007 John Hawks