john hawks weblog

paleoanthropology, genetics and evolution

A. afarensis

  • Putting together australopithecine diets

    Sat, 2011-11-26 18:06 -- John Hawks

    Peter Ungar and Matt Sponheimer earlier this fall [1] reviewed the evidence for diet in early hominins, from both microwear studies (Ungar's specialty) and stable isotopes (Sponheimer's forté). I wanted to point to this article because it is a very useful short review that illuminates the cases in which these two sources of evidence lead to a single interpretation.

    [T]he isotope data also suggest enormous and unanticipated differences between contemporaneous taxa with strong morphological similarities, notably the “robust” australopiths P. robustus and P. boisei. Despite their attribution to the same genus, there is no overlap in their carbon isotope compositions (41), which is a rarity for congeners among extant mammals.

    Maybe this should give pause to those who insist that A. robustus and A. boisei are sister species. Ungar and Sponheimer here reiterate the observation that microwear is very similar between A. boisei and A. afarensis:

    The apparent continuity of microwear pattern through the putative lineage Au. anamensis–Au. afarensis–P. boisei could even suggest that morphological changes reflect increasing efficiency for grinding large quantities of tough food. Although living primates that eat tough items typically have sharp shearing crests, eastern African australopiths and especially P. boisei may have evolved a different solution for processing such foods, given the flattened, thickly enameled teeth of their close ancestors (23). Natural selection must work with the raw materials available to it. Thus, the present-day ecomorphological diversity within the primates may not be sufficient for making some paleoecological inferences, which is not surprising given that the vast majority of all primates, especially apes, that have ever lived are now extinct.

    This idea was raised earlier, for example in the context of the stable isotope findings on A. boisei ("'Nutcracker Man' debunked"). Until we have more stable isotope results from the known sample of A. afarensis or A. anamensis, we won't be able to test this "tough C4 food" hypothesis. "Ecomorphological diversity" refers to the match between food types and the topological properties of tooth crowns among living primates. Generally speaking, primates with high crowns and high cusp relief with shearing crests are thereby well-suited for eating tough foods like leaves and stems. That's the common ground between gorillas and colobines, for example. A. afarensis and especially A. boisei have exactly the opposite morphology from what would seem to be the "tough foods" pattern. So why do these species seem to be acting like grazers? Very peculiar.

    My own attitude is that if we can't clearly make sense of the anatomy of A. boisei, then we won't be able to untangle the diets of the other species. Early hominins evolved along a distinctive trajectory toward larger molars, smaller canines, and bigger jaw musculature within a common body plan. A. boisei represents the extreme of this trend. So if A. boisei is the logical morphological extreme, why does it seem to have such a different dietary strategy than every other hominin with stable isotope evidence?

    Meanwhile, if Ungar and Sponheimer are correct in asserting a common dietary strategy in the East African species, then it seems pretty clear that early Homo shares a dietary commonality with the South African species, not the East African ones. One might argue that Homo differentiated from other hominins within East Africa by adopting a fundamentally South African dietary strategy. But I would be more inclined to suppose a South African-derived hominin made incursions into East Africa, possibly repeated ones, as Homo was emerging. Ungar and Sponheimer are correct that natural selection works with the materials available. Population growth and migration are vastly more rapid than in situ evolution. What if the apparent "early Homo" record actually represents a series of successive dead-end migrations from southern Africa?

    References

    1. Ungar PS, and Sponheimer M. 2011. The diets of early hominins. Science (New York, N.Y.) 334:190-3.
    Tags: 
    diet
    A. boisei
    A. afarensis
    A. anamensis
    stable isotopes
    microwear
    Synopsis: 
    A review of microwear and stable isotope evidence of diet prompts questions about early hominin relationships.

  • Aging juvenile fossil hominins

    Tue, 2011-10-25 00:27 -- John Hawks
    Synopsis: 
    Laboratory exercise giving the opportunity to examine the development of juvenile hominin jaws.

    The fossil record is not made up only of adults. We have abundant skeletal evidence from juvenile individuals of a broad range of ages. At this station you will find model mandibles and maxillae from human children of a range of ages. These provide a comparison for the casts at the station, each of which represents a fossil hominin specimen from Africa, between 3.6 million and 1.5 million years ago.

    The mandibles represent several different species. They include:

    1. OH 7, from Olduvai Gorge, Tanzania. This is the type specimen of Homo habilis, around 1.75 million years old.
    2. MLD 2, from Makapansgat, South Africa. This is an early specimen of Australopithecus africanus, around 2.7 million years old.
    3. LH 2, from Laetoli, Tanzania. An early specimen of Australopithecus afarensis, it is around 3.6 million years old.
    4. SK 47, from Swartkrans, South Africa. This is a juvenile specimen of Australopithecus robustus, around 1.5 million years old.

      5. A selection of other mandibles, including some adult mandibles of the same species, is also available. Examine these in comparison with the modern dental models. Which teeth are present in the fossil specimens? What teeth are in the process of eruption? What do they tell you about the ages of the individuals?

    Study terms: 
    eruption
    occlusion
    Laetoli
    Koobi Fora
    Swartkrans
    Homo habilis
    Australopithecus afarensis
    Australopithecus africanus
    Makapansgat
    Tags: 
    A. afarensis
    A. africanus
    Australopithecus
    Homo
    development

  • The feet of Australopithecus afarensis

    Wed, 2011-09-07 11:57 -- John Hawks
    Synopsis: 
    Early hominins had feet that were adapted to a humanlike pattern of bipedality

    The australopithecines were several species of human relatives that lived in Africa between 5 million and 1.5 million years ago. One of the best-represented species of australopithecines is Australopithecus afarensis, which is known from Ethiopia and Kenya between 3.9 million and 3 million years ago. Although it is not the earliest hominin species, A. afarensis provides some of the earliest evidence about the evolution of bipedality in our lineage.

    Human feet are very different from ape feet. The differences are in the toes, the "arches" of the midfoot and the ankle.

    1. An ape foot bears an opposable big toe and they can grasp objects or branches with their feet. Humans have a stout big toe in line with the other toes, and our feet function as a stiff lever that lets us "toe off" powerfully into our next step. The rest of the toes are long and curving in apes, relatively short in humans.

    2. Ape feet are relatively flat and flexible in the middle section of the foot, letting them conform (and sometimes grip around) the surface they are on. Human feet have strong ligaments and bones packed into arch structures, both along the length of the foot (the longitudinal arch) and across the foot from side-to-side (the transverse arch).

    3. Apes have ankle joints that can flex upward (dorsiflex) more than human ankles, allowing them to climb more effectively on vertical tree trunks. Humans are more limited in this movement and have ankles that are directed more toward front-to-back movement with less side-to-side mobility.

    Paleoanthropologists have found evidence about the feet of A. afarensis from fossil footprint tracks, at a place called Laetoli, Tanzania. These footprints are nearly 3.7 million years old, and were uncovered by Mary Leakey during the late 1970's.

    View Larger Map

    The marker shows the location of the Laetoli footprint trail, in Tanzania. Laetoli is just south of Olduvai Gorge and west of the Ngorongoro crater.

    The footprint trackways are preserved in a series of overlapping surfaces that are the result of falls of volcanic ash, very quickly cemented by rainfall. These layers preserve the footprints of many kinds of animals, including ancient horses, birds and hominids. The best preserved hominin footprints are in trail G, representing two hominids walking in a straight line for a distance of about five meters.

    The Laetoli footprints are strikingly human-like, suggesting that these early hominins had feet that were functionally equivalent to ours. The imprints of the toes show that the big toe of A. afarensis was in line with the others. The big toe did not jut out from the foot as these creatures walked, it would have helped "toe off" each successive step. Many of the prints show that the feet were arched, which suggests an anatomy similar to the human anatomy in which the midfoot functions as a lever (White 1980; White and Suwa 1987).

    The paths have short spaces between successive prints, indicating a slow walking speed. One of the tracks is substantially smaller than the other, and since the two individuals matched each other stride-for-stride, it is likely that the smaller individual set the pace, with the larger one following (Leakey and Hay 1979). The larger track appears to preserve the footprints of at least two individuals, one literally following in the footsteps of the other.

    Fossil foot bones from A. afarensis add detail to what we've learned from the Laetoli footprint trails. For example, Carol Ward, Bill Kimbel and Don Johanson [1] described one of the bones of the midfoot from Hadar, Ethiopia, that represents A. afarensis. This bone, the fourth metatarsal, is the one that connects the fourth toe to the bones of the ankle. In humans, these ankle bones are higher, and transfer the body's weight downward into the arching midfoot. So the fourth metatarsal has to be slightly twisted as it arches down toward the lateral (outside) side of the foot. A chimpanzee's foot is much flatter, so the bone doesn't twist. Ward and colleagues found that the A. afarensis bone was twisted in a humanlike way.

    Hadar is the place where one of the most famous skeletons in the world was found: the "Lucy" skeleton, discovered by Don Johanson in 1974. Lucy presents evidence across much of her skeleton for a humanlike manner of walking. Jeremy DeSilva and Zach Throckmorton [2] looked at the tibia (shin bone) of this skeleton, along with other fossil tibiae of A. afarensis, to try to determine whether their ankle bones were structured to create a rearfoot arch like humans. What they found was interesting: Most A. afarensis tibiae met the ankle in a humanlike orientation, but they varied. Lucy's ankle in particular looked like her feet were relatively flatter. Just as human feet vary in their shape, this early species of hominins varied as well.

    Ape feet are made for climbing. Their flexibility allows the sole of the foot to conform to a tree trunk or branch, giving them more friction and a stronger grip. Ape ankle joints allow them to walk up a trunk while holding it with their arms, which means their feet need to carry weight while they are flexed upward and tilted to the side. Humans who are good climbers tend to shimmy up a tree by gripping the trunk tightly between their legs. Our feet just don't work very well when climbing a truly vertical surface. Rock climbers look for footholds where they can place their feet; loggers can use metal spikes clamped to their boots.

    What about A. afarensis? Jeremy DeSilva [3] examined the ankle joints preserved for this species and found that they did not have the orientation that chimpanzee ankles do. Where chimpanzees can bear weight effectively with their feet tilted to the side, for A. afarensis this orientation just wouldn't have worked. If these early hominins needed to climb trees — which might explain the powerful arm bones of some individuals — they must have done it in a different way than chimpanzees and other apes.

    Yet, the feet of A. afarensis were not entirely like ours. One major difference is that their toe bones were curved more than ours (Stern and Susman 1983). This curvature is less than the great curvature of the foot bones in chimpanzees, and may be correlated to the relatively long length of australopithecine toes compared to body size. Or, curved bones may just be a retention from their ape ancestry that hadn't been eliminated because it didn't impede walking. The curvature of the bones and a few other apelike features of the A. afarensis foot show that human feet did not instantly evolve in one moment of our evolution. Instead, human feet represent a long evolutionary history with different steps at different times.

    References

    1. Ward CV, Kimbel WH, and Johanson DC. 2011. Complete Fourth Metatarsal and Arches in the Foot of Australopithecus afarensis. Science [Internet] 331:750–753. Available from: http://dx.doi.org/10.1126/science.1201463
    2. DeSilva JM, and Throckmorton ZJ. 2011. Lucy's Flat Feet: The Relationship between the Ankle and Rearfoot Arching in Early Hominins. PLoS ONE 5:e14432.
    3. DeSilva JM. 2009. Functional morphology of the ankle and the likelihood of climbing in early hominins. Proceedings of the National Academy of Sciences 106:6567 - 6572.
    Study questions: 
    1. Can you think of other ways that the bones of the feet may reflect behavior?
    2. What are the main differences between human feet and ape feet?
    3. Take some slow footsteps to see how the foot transfers weight during each step. Is your foot rigid like a lever moving the body, or is it soft and pliable?
    Study terms: 
    metatarsal
    Australopithecus afarensis
    Lucy
    tibia
    Tags: 
    A. afarensis
    bipedality
    feet
    functional morphology

  • The Laetoli footprints

    Fri, 2011-09-02 00:53 -- John Hawks
    Synopsis: 
    A lab exercise in making footprints to compare to the Laetoli G footprint track.

    The most striking piece of evidence for bipedality in our earliest hominin relatives is a series of footprint trails at Laetoli, a fossil-bearing site in Tanzania. The longest trail, known as trail G, was made by at least two individuals, one much larger than the other. These individuals were probably members of a species called Australopithecus afarensis, with fossil remains that have been found in other parts of the Laetoli area from nearly the same time, 3.5 million years ago. This species lived long before any that scientists call humans, they are different from us in many, many respects. But the evidence shows that they walked bipedally in a very humanlike way.

    Studying these footprints poses many challenges to scientists. Their shape should give us clues about the shape of the feet, the way they struck the ground, the length and pattern of steps. Probably the most obvious aspect of these footprints are the big toes, which were aligned more or less with the other toes. This is a very different shape than a chimpanzee or gorilla foot, in which the big toe is relatively short and diverges from the foot, and the other toes are long and curving. Nevertheless, the toes of A. afarensis were not quite the same as ours, as you can compare as you make your own footprints.

    A comparison of one of the Laetoli footprints (bottom) with a footprint from a later site attributed two modern humans (top and middle). The human (middle) is walking with a bent-knee, bent-hip (BKBH) gait, not a normal gait for a person. The image shows the depth of different parts of the print. From a research paper by David Raichlen and colleagues [1].

    This lab station has you making footprints, to see how you might study the shape and conditions under which the Laetoli footprints were made. As you make footprints, try to use different styles of gait. Move fast or slow, maybe try to simulate a running step. Can you rule out some patterns of movement for the makers of the Laetoli footprint trail?

    References

    1. Raichlen DA, Gordon AD, Harcourt-Smith WEH, Foster AD, and Haas WR. 2010. Laetoli Footprints Preserve Earliest Direct Evidence of Human-Like Bipedal Biomechanics. PLoS ONE 5:e9769.
    Study questions: 
    1. What kind of locomotion can you imagine would be intermediate between human-like bipedality and ape-like quadrupedality?
    2. One of the main points of contention about the Laetoli footprints is whether they preserve human-like arches in the midfoot. What do your comparisons indicate?
    Study terms: 
    bipedality
    trace fossil
    Australopithecus
    Australopithecus afarensis
    Tags: 
    bipedality
    A. afarensis
    Laetoli
    Tanzania
    East Africa
    laboratory
    Anthropology 105

  • Cutmarked bones from Dikika critiqued

    Wed, 2010-11-17 00:18 -- John Hawks

    Manuel Dominguez-Rodrigo, writing with my University of Wisconsin colleagues Travis Pickering and Henry Bunn, has challenged the interpretation that two bovid bones from Dikika bear cutmarks made by hominins [1]. I wrote about the Dikika cutmark claims earlier this year (Australopithecus afarensis used stone tools). The new paper is a strong critique of that earlier work.

    Our taphonomic configurational approach to assess the claims of A. afarensis butchery at Dikika suggests the claims of unexpectedly early butchering at the site are not warranted. The Dikika research group focused its analysis on the morphology of the marks in question but failed to demonstrate, through recovery of similarly marked in situ fossils, the exact provenience of the pub- lished fossils, and failed to note occurrences of random striae on the cortices of the published fossils (incurred through incidental move- ment of the defleshed specimens across and/or within their abrasive encasing sediments). The occurrence of such random striae (some- times called collectively “trampling” damage) on the two fossils provide the configurational context for rejection of the claimed butchery marks. The earliest best evidence for hominin butchery thus remains at 2.6 to 2.5 Ma, presumably associated with more derived species than A. afarensis.

    These authors are experts on cutmarks, both from their work on Oldowan faunal assemblages and from experimental work where they have controlled the actual circumstances of cutmarking, trampling and weathering. Their critique of the two Dikika bones takes two main paths:

    1. The surfaces of the bones themselves are relatively poorly preserved, with evidence of "trampling" modification and subadult status for one specimen and evidence of "moderate weathering" on the other. The matrix containing the bones was highly abrasive, making spurious marks more likely. This would make it difficult to get clear results even in an experimental context.

    2. The purported cutmarks themselves are similar to marks that occur in bones subject to trampling damage. Dominguez-Rodrigo and colleagues argue that some of these marks are more diagnostic of trampling than of cutting or hammerstone damage.

    The authors do not say they have disproven the hypothesis that A. afarensis cut on these bones with naturally-occurring stones, but they clearly question whether such a hypothesis is credible:

    The Dikika “butchery mark” evidence does not, however, withstand peer scrutiny undertaken from an actualistic perspective and with a configurational approach. Our approach in assessing the Dikika claims was intentionally conservative: the claims are extraordinary because of their singularity and because of the inferred age of the fossils. Thus, natural processes of bone modification need to be eliminated before precluding nonanthropogenic origin(s) for the surficial marks on DIK-55–2 and DIK-55–3. High probability trampling damage on both specimens does not allow for this elimination and, again, taking our contextualized, maximally conservative position, forces us to reject even marks A1 and A2, the two morphologically strongest claims of cutmarks on DIK-55–2.

    Their discussion emphasizes that, in their view, a hypothesis that an unusual tool type was responsible for cutmarks should be accompanied by experimental or actualistic evidence concerning the effects of that tool type. I think that for discoveries as potentially important as this, it is very reasonable for reviewers to expect such evidence will be provided. Also, a full statistical workup of other faunal bones from the site would be worthwhile. If the matrix really is abrasive and readily gives rise to trampling scratches, these should be evident in a wider distribution of bone from the site.

    But for the moment, it looks like we should continue to treat cautiously claims of very early stone tool use. Possibly further comparisons will back up the hypothesis of cutmarks with more evidence. Since it took only three months from the initial publication of the Dikika evidence to this response, maybe we won't have to wait long for more comparisons!

    References

    1. Dominguez-Rodrigo M, Pickering TR, and Bunn HT. 2010. Configurational approach to identifying the earliest hominin butchers. Proceedings of the National Academy of Sciences [Internet] 107:20929–20934. Available from: http://dx.doi.org/10.1073/pnas.1013711107
    Tags: 
    Dikika
    Ethiopia
    stone tools
    Pliocene
    Africa
    Oldowan
    A. afarensis
    Synopsis: 
    The claim of stone tool use by A. africanus comes under fire.

  • Sink Australopithecus!

    Mon, 2010-08-16 14:36 -- John Hawks

    Dennis Etler has been going great guns on his blog, Sinanthropus.

    Last week's article claiming cutmarks on A. afarensis-aged fauna from Dikika (Australopithecus afarensis used stone tools) got Dennis to write a provocative post: "Its time to sink the genus Australopithecus redux."

    Either A. afarensis should be revised to H. afarensis or the possibility must be entertained that the Woranso-Mille individual and the maker of the stone tool cut marks at Dikika represent a new previously unknown species of Homo (perhaps H. antiquus Ferguson 1984) that lived contemporaneously with A. afarensis.

    He mentions the relevance of the Woranso-Mille skeleton (which I haven't yet gotten to here) and A. sediba for this conclusion. Etler's earlier post, "It's time to sink the genus Australopithecus" goes into more detail on these remains.

    To me, the key question is whether Homo as we understand it now (including H. habilis) is polyphyletic. One way to escape this question is to narrow our genus, placing H. habilis and its ilk into Australopithecus. But Australopithecus defined broadly in this way is almost certainly paraphyletic. And that's without considering the issue of robust australopithecines. I can see why one might follow Ernst Mayr and stick them all in Homo.

    Tags: 
    Woranso-Mille
    A. afarensis
    Australopithecus
    Homo
    taxonomy
    A. sediba
    Dikika
    stone tools

  • Mailbag: The capuchin australopithecines

    Thu, 2010-08-12 12:20 -- John Hawks

    Re: australopithecine tools:

    Eh, now that I think about it, your bonus prognostication doesn't seem that outlandish. Capuchins use stone tools. I'll repeat that: capuchins use stone tools. You mention chimp technology, and since we use tools - isn't it logical to assume tool manufacture was a trait of the LCA, therefore anything on the lines from the LCA to both chimps and humans had the capacity to make some sort of tool? Without tools and Isaac-approved butchery sites, the more interesting question remains the same: what happened around Gona's antiquity that made hominins start doing things differently than capuchins and chimps?

    Yeah, the bonus is never all that unlikely. I still think somebody will find a robust australopithecine in Asia.

    It's the mad persistence of Oldowan (and later Acheulean) that gets me. But then maybe it's not really so different from chimpanzees. Honey extraction, bushbaby spearing, and lots of other things are only at one or two field sites. But termite/ant fishing is everywhere. How do they keep that going? I suppose it's partly innate, or they have an innate bias toward learning it. Maybe Oldowan is like that, so there is a biological trigger supporting stone tools in later australopithecines.

    Tags: 
    primates
    tool use
    A. afarensis
    diet
    stone tools
    culture

  • Australopithecus afarensis used stone tools

    Wed, 2010-08-11 15:13 -- John Hawks

    UPDATE (2011-09-06) Note: The conclusions of the research were later critiqued, I posted on that criticism after this post.

    Shannon McPherron, Zeresenay Alemseged and colleagues working at the Dikika field site in Ethiopia have found evidence of stone tool use 3.39 million years ago [1]. That's 800,000 years earlier than the previous first-known tool use, and occurs during the existence of Australopithecus afarensis.

    The evidence is a series of cutmarks and one percussion mark on two bovid bones. One is a piece of rib from a large "cow-sized" animal, the other a femur fragment from a smaller "goat-sized" bovid. The analysis goes through several microscopic comparisons to rule out alternative causes for the cutmarks, such as trampling. The key paragraph of the results:

    The cut marks demonstrate hominin use of sharp-edged stone to remove flesh from the femur and rib. The location and density of the marks on the femur indicate that flesh was rather widely spread on the surface, although it is possible that there could have been isolated patches of flesh. The percussion marks on the femur demonstrate hominin use of a blunt stone to strike the bone, probably to gain access to the marrow. The external surfaces of ribs have thin sheaths of flesh, so the scraping marks on the fossil rib suggest stripping off of these sheaths.

    I have some lingering doubts, none of which are very serious, but that point out the need to look harder at other sites. It sure would have been nice if they'd found an anomalous sharp-edged rock nearby.

    The two bones are compelling, but the study does not give much indication of how representative they are. How many similar-sized bone fragments were left at the site? How many were collected? What fraction of "cutmarked" bones does that make? What fraction show signs of trampling and various kinds of post-depositional damage?

    Those questions are essential to answer the "green car" problem. If you don't know this one, it's fairly simple -- a witness reports a green car leaving the scene, and green cars are very rare -- the police think this is a great lead. But blue cars are very common in the city, and there is a small chance that the witness mistook a blue one for a green one. Whether it actually was a green car depends on the actual proportion of green to blue cars, and the actual probability that the witness was wrong.

    In this case, I think there is a very small chance that the marks on these bones could have been produced by processes other than deliberate cutting by a stone tool. But in a sample of hundreds or thousands of bone fragments, a small chance might well happen a couple of times. It's very difficult to quantify this, because archaeologists don't collect every bone fragment. The only real way to address the problem is to find more cutmarks and do other statistics on them -- do they occur where flesh is attached to bone, etc.

    It does seem odd that nobody's identified clear stone tools, which are in later sites a lot more common than cutmarked bones. A tool-user will make many artifacts during her life. (Why "her"? Well, in chimpanzees, it's the females who dominate technology transmission...) We have a lot of australopithecine bones. If this was a long-lasting tradition, we should have found a lot of stone tools by now.

    Maybe it wasn't a long-lasting tradition. Chimpanzee technology is significantly clustered geographically, some of the most interesting behaviors have been observed only at a single field site. If Australopithecus had a similar pattern of cultural diversity, tool use may have been innovated many times without "catching on" over a wide geographic or temporal extent. Here's what McPherron and colleagues conclude along similar lines:

    Whether A. afarensis also produced stone tools remains to be demonstrated, but the DIK-55 finds may fit with the view that stone tool production pre-dates the earliest known archaeological sites and was initially of low intensity (one-to-a-few flakes removed per nodule) and distributed in extremely low density scatters across the landscape such that its archaeological visibility is quite low (16).

    Or maybe we just haven't noticed. Fluvial contexts may have been bad places for Australopithecus to hang out. McPherron and colleagues allude to this explanation for the local absence of tools at Dikika:

    However, stone tool production and consequently archaeological accumulations are not expected at this locality given the sedimentary environment characterized by the palaeo-Awash River emptying into a nearby lake (3, 4). In this relatively low-energy depositional environment, clasts suitable for stone tool production are not present (few particles larger than fine gravel, 8 mm diameter). Within the exposed SH Member, the distance from DIK-55 to cobble-sized raw materials (>64 mm) is ~6 km (at Gorgore; Fig. 1). Thus, in this instance the absence of evidence for stone tool production in the immediate vicinity of the cut-marked bones may reflect landscape-level raw material constraints.

    The research article is accompanied by an essay by David Braun reviewing the find [2]. He stretches a bit, but I think the interpretations he suggests are worth airing. One -- why are there cutmarked bones 6 km from any good source of stone raw material?

    The meat and marrow of large animals must have been a valued resource, because McPherron et al. conclude that the tool users incurred the cost of transporting stones 6 kilometres from where they occurred naturally to the site where the butchery took place. Further costs that were associated with the consumption of carrion, and were apparently worth the risk, include exposure to parasites and competition with large carnivores.

    Two -- what about the "meat-brain" connection?

    This provides exciting evidence of how A. afarensis behaved. At one time, the species was considered to be a relatively primitive hominin, but this perception is being redefined. For example, it now seems that Lucy's kin had body proportions that were more similar to those of humans than of apes (6). Analyses of the hand of A. afarensis show that it had relatively short fingers that would allow the kind of fine-scale manipulation necessary for tool use (7). A recently discovered skeleton from the Woranso–Mille area of Ethiopia suggests that A. afarensis did not have the ape-like, 'funnel-shaped' thorax usually associated with a large digestive tract and low-quality diet (8). Perhaps the findings that these hominins used tools and had a carnivorous component to their diet should not have been so unexpected.

    A 2.6-million-year-old butchery tradition should already have refuted the hypothesis that meat-eating caused the expansion of brain size in Homo. But it was still possible to maintain that the initial Oldowan was insufficiently dedicated, or that the anatomical specializations (e.g., small guts) allowing brain expansion took time to develop, or that as-yet-undiscovered large-brained hominins would be found. Any of these are still possible, but the observations Braun points out pretty much demolish the 15-year-old story of "expensive tissue." Australopithecus seems to have had a small gut, and a bigger brain than chimpanzees. If there was a tradeoff, A. afarensis had already made it.

    Braun didn't mention A. sediba, which adds another wrinkle. A late species of Australopithecus with human-sized teeth. Or (as some prefer), a pre-habilis species of Homo with an Australopithecus-sized brain. What was its diet like? I have a feeling we'll know before too long.

    Meanwhile, I'll be floating for the rest of the year, since I included this as the far-out "bonus" entry in my 2010 New Year predictions! You know, the one that's so bizarre that it seems like it'll never happen. Heh.

    UPDATE (2010-08-11): John Noble Wilford got ahold of some skeptics for his NY Times story on the discovery:

    Still, the discoverers are already being pressed to defend their interpretation that the cut marks on the bones are evidence of stone-tool butchery. Tim D. White of the University of California, Berkeley, one of the foremost investigators of early human origins, said flatly that their “claims greatly outstrip the evidence,” and noted, “We have been working sites in this area for 40 years, and not a single stone tool has been found in deposits of this antiquity.”

    Sileshi Semaw, a paleoanthropologist at Indiana University who was a discoverer of the oldest confirmed stone tools, from 2.6 million years ago, noted in an e-mail message from Ethiopia that researchers had often been misled by bone markings left by trampling animals and other natural causes. “I am not convinced of the new discovery,” he said.

    UPDATE (2010-08-12): Maybe some are looking for more about australopithecine diets. My post from 2005, "Chemistry and early hominid diets" has a good compilation of stable isotope observations and what may explain them. With the cutmark evidence, you can read through the discussion of C4 plant contributions, and think about the grazers that A. africanus may have been eating.

    UPDATE (2010-08-16): Science Friday with Ira Flatow covered this story last week, including commentary by Alemseged and David DeGusta, who suggests that the marks may be crocodile bite marks. Doesn't look like it to me, but as I wrote above, I'd like to see statistics on a few hundred damaged bones to see the probability that an arbitrary one will look like stone cutmarks.

    References

    1. McPherron SP, Alemseged Z, Marean CW, Wynn JG, Reed D, Geraads D, Bobe R, and Bearat HA. 2010. Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia. Nature [Internet] 466:857–860. Available from: http://dx.doi.org/10.1038/nature09248
    2. Braun DR. 2010. Palaeoanthropology: Australopithecine butchers. Nature [Internet] 466:828. Available from: http://dx.doi.org/10.1038/466828a
    Tags: 
    Dikika
    Lower Paleolithic
    Pliocene
    A. afarensis
    taphonomy
    archaeology
    Ethiopia
    Oldowan
    stone tools
    Africa
    zooarchaeology
    Synopsis: 
    A report finds cutmarks on fauna from Dikika, Ethiopia, 3.4 million years ago.

  • Why didn't they let Kenyanthropus save them?

    Thu, 2009-12-10 00:42 -- John Hawks

    In the fossil record, a species is a hypothesis. We can't test that hypothesis in the way we do with living animals. Even in the dark, after all the paleontologists have left, the fossil bones just won't get it on. No reproduction, no test.

    So, sometimes we have to live with hypotheses that we can't immediately test. Because many hypotheses are wrong, we have to keep juggling in our minds the names of more species than probably existed.

    All the juggling frequently leads to confusion. One may reasonably wonder how we know that species X evolved into species Y, when half the field rejects the hypothesis that X was a real species. Often we don't disagree about the "evolving into", but we do disagree about the boundaries and other relationships of the populations -- which we can understand only indirectly from the fossils. The fossils don't change, but our hypotheses about them do.

    That brings us back to A. anamensis. Here's a hypothesis about an ancient lineage of hominins, based on a certain number of differences from later fossil samples assigned to A. afarensis. That's a precarious place for a hypothesis, because from the beginning, the definition of A. afarensis has encompassed geographic and temporal variability. How hard would it be to recognize a little more temporal variability? Not very.

    So, as I described last week, Haile-Selassie and colleagues (2010) propose sinking A. anamensis ("Woranso-Mille: A ladder not a bush"). I should mention, if I haven't already, that I have great sympathy for this viewpoint. Absent some compelling evidence that the lineage includes a speciation event, I prefer slow gradual anagenesis to be categorized into one evolving species, not an arbitrary set of chronospecies.

    In the same post, I described the work of Kimbel and colleagues (2006), who had argued for anagenesis in the same sample of A. anamensis and A. afarensis-referred fossils, but retained the two distinct names for them. One thing stands out as a mystery to me in that paper. Why didn't they let Kenyanthropus make the argument for them?

    If you want to establish that A. anamensis is taxonomically valid, the simplest way to answer all critics is if it has more than one descendant. You don't have to demonstrate the phylogeny beyond all doubt, I would say, you just have to take the hypothesis seriously.

    In this instance, we seem to have two good candidates for a non-A. afarensis descendant of A. anamensis. The more obvious of them is Kenyanthropus. Why didn't they advance the hypothesis of a A. afarensis-Kenyanthropus clade? Here's what Kimbel and colleagues wrote about the latest Leakey find:

    A more significant concern is the presence of Kenyanthropus at 3.5 Ma. Kenyanthropus may demonstrate cladogenesis prior to this time, but this taxon is only directly relevant to the analysis if any of the samples share derived character-states with it. At present, the Kenyanthropus hypodigm does not match the others in the availability or quality of character data for the mandible and anterior teeth, while the evidence that exists (from the maxilla, for example) does not suggest a close relationship of Kenyanthropus to any of the phena considered here (Leakey et al., 2001).

    So, they took the hypothesis off the table. And again, they've refocused the question solely upon dental and mandibular evidence.

    This is a very large hole. At 3.5 million years, KNM-WT 40000 is earlier than any other comparably complete hominin skull, except for Ardi and Toumaï. It doesn't look like them, that's for sure. Not the same phenum at all. If you're going to insist that KNM-WT 40000 isn't A. afarensis, it's hard to see a better hypothesis than that it's descended from A. anamensis.

    If it doesn't have a close relationship to either A. afarensis or A. anamensis, then I'm at a loss to figure out what they think it is related to!

    I'm willing to believe White's (2003) argument that it just is a member of the A. anamensis-A. afarensis lineage, but I don't see the contrary argument that it's so different from this lineage that it must be derived from some as-yet-undiscovered hominin. Keep in mind that the argument was formed by people, many of whom already knew basically what Ardi's skull was going to look like. I just can't feature why this phylogenetic problem didn't raise itself to a higher profile.

    As I wrote above, we juggle more hypotheses than can be true. In this instance, the null hypothesis is that all these hominins belong to a single evolving species, which would be called A. afarensis. But one alternative, in which A. anamensis existed as the ancestor of A. afarensis and at least one other species, has some utility. It lets us refer clearly to phylogenies with late-diverging sister taxa. As Yoda might have said, there is another possible sister taxon for A. afarensis: A. africanus might be derived from, or might itself be, a South African contemporary of the Hadar-Maka-Laetoli sample. The earliest Sterkfontein dates go up and down; one or more of the remains might be contemporaries of Laetoli or even earlier East African localities.

    Anyway, I'm not so interested in this question of bushes versus ladders, or "Pliocene diversity". I'm more focused on the curiously non-hominin-like features of Ardipithecus. If we suppose a late molecular divergence of hominins and chimpanzees (say after 4.4 million years ago), then Ardipithecus might be an ape or ancestral (stem) hominid, not a hominin. If so, then samples now referred to A. anamensis, including later specimens from Aramis, Asa Issie, and Kanapoi, are in fact the earliest-known members of the human lineage.

    A. anamensis is no afterthought in that case, it may be the stem hominin.

    That is, if it hasn't been sunk into A. afarensis.

    UPDATE (2009-12-10): A reader writes to remind me about A. garhi, which presents itself as very much like A. africanus and is quite a lot later than any known A. afarensis samples. One alternative is that A. africanus is simply the latest element of the single A. anamensis-A. afarensis-A. africanus anagenetic sequence.

    I don't think that's very easy to reject, considering the lack of good cranial remains between 3 and 2.5 million years ago (or for that matter, even later) in East Africa.

    Some of the Sterkfontein specimens, particularly those from Jakovec cavern and from Member 2 (including Little Foot) have been suggested to be older than 3 million years. Partridge and colleagues (2003) put them at older than 4 million years ago, which would make them rivals of A. anamensis as the earliest hominins. But Berger and colleagues (2002) argued (apparently preemptively) that these early dates are not necessary on faunal or magnetostratigraphic grounds.

    A second reader wonders why I didn't mention A. bahrelghazali as a possible sister to A. afarensis. Well, nothing's impossible, but I'd say the case for the Bahr el Ghazal mandible being distinct from the Hadar-Maka-Laetoli sample isn't very strong. Still, we're only talking about hypotheses here, I suppose.

    References:

    Berger LR, Lacruz R, de Ruiter DJ. 2002. Revised age estimates of Australoipithecus-bearing deposits at Sterkfontein, South Africa. Am J Phys Anthropol 119:192-197. doi:10.1002/ajpa.10156

    Haile-Selassie Y, Saylor BZ, Deino A, Alene M, Latimer BM. 2010. New hominid fossils from Woranso-Mille (Central Afar, Ethiopia) and taxonomy of early Australopithecus. Am J Phys Anthropol (in press) doi:10.1002/ajpa.21159

    Kimbel WH, Lockwood CA, Ward CV, Leakey MG, Rak Y, Johanson DC. 2006. Was Australopithecus anamensis ancestral to A. afarensis? A case of anagenesis in the hominin fossil record. J Hum Evol 51:134-152. doi:10.1016/j.jhevol.2006.02.003

    White T. 2003. Early hominids -- diversity or distortion? Science 299:1994-1996.

    Suwa G, Asfaw B, Kono RT, Kubo D, Lovejoy CO, White TD. 2009. The Ardipithecus ramidus skull and its implications for hominid origins. Science 326:68e1-68e7. doi:10.1126/science.1175825

    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.

    Partridge TC, Granger DE, Caffee MW, Clarke RJ. 2003. Lower Pliocene hominid remains from Sterkfontein. Science 300:607-612.

    Tags: 
    Ardipithecus
    Kenya
    Pliocene
    A. afarensis
    Africa
    Ethiopia
    Kenyanthropus
    Woranso-Mille
    A. anamensis
    Synopsis: 
    If A. anamensis evolved into A. afarensis, why do we need two names for them?

  • The teeth that didn't bark

    Sat, 2009-12-05 15:00 -- John Hawks

    Earlier in the week, I wrote about the new interpretation of fossil teeth from Woranso-Mille, Ethiopia ("Woranso-Mille: A ladder not a bush"). There was one aspect of the paper that I wanted to comment at some greater length: Why didn't the paper include comparisons with the Lomekwi sample of teeth and mandibles?

    The Lomekwi sample includes more teeth than the sample reported from Woranso-Mille. At around 3.5 million years old, Lomekwi is a more relevant comparison than Hadar or Kanapoi, closer in age to Woranso-Mille than any sample other than Laetoli. So why didn't they include the comparison?

    The obvious answer is that these teeth belong to Kenyanthropus platyops, a different species off the line leading from A. anamensis to A. afarensis. So they're not relevant to testing hypotheses about evolution within that lineage.

    But...I don't see why that answer is obvious if Kenyanthropus is a fictitious species, based on a distorted skull (e.g., White 2003).

    Suppose that the teeth don't represent Kenyanthropus. Then they ought to belong to the one cosmopolitan species that exists both north and south of Lomekwi, both earlier and later in time. They ought to be A. afarensis. Which makes them directly relevant to the Woranso-Mille hominins. At the very least, they add to the variation of the A. afarensis sample, helping to inform about the temporal trend in that lineage.

    Besides that, even if we thought that Kenyanthropus was a real taxon, most of the Lomekwi teeth might still be A. afarensis. Only two specimens were assigned to K. platyops by Leakey and colleagues (2001): the KNM-WT 40000 skull and the unassociated maxilla fragment KNM-WT 38350. Both those have relatively small molars compared to the known A. afarensis sample, but in both cases the teeth are broken and dimensions are estimated. The rest of the dental sample is unassociated. The only thing keeping them from being A. afarensis is what we're willing to assume about the number of species at Lomekwi.

    So, it seems like a comparison that ought to be done. The information content is not going to be really high -- we're only talking about gross dimensions of the crowns, and the only ones that look informative at all are upper molars. But Leakey and colleagues argued that the KNM-WT 40000 and KNM-WT 38500 molars are at the very lower end of the range of A. afarensis. Guess what? The Woranso-Mille sample extends this range downward, right around the Lomekwi range.

    Something there...

    UPDATE (2009-12-08): I should add that Brown and colleagues (2001) reported additional teeth from Lomekwi and other localities in the Turkana Basin, which they referred to A. afarensis on the basis of their temporal position. They're not in the Woranso-Mille comparisons either.

    References:

    Haile-Selassie Y, Saylor BZ, Deino A, Alene M, Latimer BM. 2010. New hominid fossils from Woranso-Mille (Central Afar, Ethiopia) and taxonomy of early Australopithecus. Am J Phys Anthropol (in press) doi:10.1002/ajpa.21159

    Kimbel WH, Lockwood CA, Ward CV, Leakey MG, Rak Y, Johanson DC. 2006. Was Australopithecus anamensis ancestral to A. afarensis? A case of anagenesis in the hominin fossil record. J Hum Evol 51:134-152. doi:10.1016/j.jhevol.2006.02.003

    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.

    White T. 2003. Early hominids -- diversity or distortion? Science 299:1994-1996.

    Tags: 
    Pliocene
    A. afarensis
    Africa
    Kenyanthropus
    Woranso-Mille
    A. anamensis
    Ethiopia

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