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Early HominidsLearning OutcomesAt the end of this lecture you should be able to:

Describe the various Australopithecines in some detail Discuss possible phylogenetic relationships among the Australopithecines Discuss various theories concerning Australopithecine bipedalism

IntroductionThis lecture is about early hominids. This title needs to be treated as a colloquialism. It iswhat everyone calls the group of animals that will be discussed but it really does not fit withany rational classification. Early Hominins would be a better title but it is such a mouthfulthat no one actually uses it. The group contains the direct human ancestors from the point ofdivergence with the chimpanzee lineage which molecular (and morphological) evidencesuggests occurred approximately 6 million years ago.The period covered is known as the Plio-pleistocene since it covers both the Pliocene and thePleistocene. That is the last 5 million years up to about 10,000 years ago by which time mostspecies on Earth are pretty much as they are today (certainly in terms of morphology). In thislecture I will introduce the species in approximately chronological order and investigate howthey might all be related. I will also look at the evolutionary pressures that may have led tothe two major features of modern humans: habitual bipedalism and huge brains.

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Background

Figure 1. Miocene Evolution Summary [Jurmain & Nelson 1994]

Just to remind you where we have got to here Figure 1 shows a summary of primate evolu-tion in the Miocene. This is only a suggested phylogeny but it shows the starting point withthe African Great Apes and Humans in the top right hand corner.

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Figure 2. Hominid Species [Fleagle 1999]

Since the chimp and human lineages have split we have 4 recognised genera: Ardepithecus,Australopithecus, Paranthropus and Homo as shown in Figure 2 (but see the Stop Pressinformation at the end) Some authors would actually claim there are only two genera: Aus-tralopithecus (containing the first 3) and Homo but this does seems rather restrictive sincethere are very clear anatomical differences between all 4 groups.

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Figure 3. Temporal span [Fleagle 1999]

This slide shows the ages of the fossils that have been divided into these groups (Australo-pithecus bahlrelghazi is missing from the figure - it would sit in the A. afarensis band at 3 to3.5 mya but since we still know very little about the species it adds very little to the overallpicture). These temporal bands have been obtained by working out the age ranges of thefossils so they are underestimates: there could easily be individuals living both before andafter these dates that have not been fossilised. The diagram stops at 1mya because after thatlife becomes yet more complicated!

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Earliest Hominids

Figure 4. Australopithecine Map [Jurmain & Nelson 1994]

This section will discuss the australopithecines. This group is characterised by smaller brainsand larger teeth compared to modern humans. They are relatively short and there seems to bea great deal of sexual dimorphism in some species. Compared to apes, these animals havesmall incisors and canines for their body weight and they do not have a sectorial lower P3.They have thick enamel on their teeth and the position of the foramen magnum suggests thatall were habitually upright.

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Figure 4 shows some of the major finds and their locations. There are several general featuresto note.Firstly, the fossils are all named with a site number (or occasionally a popular name for thereally famous fossils although they have accession numbers too). This is to remove ambigu-ity. Fossils get assigned to a variety of different species as people alter classifications but thesite number will never change. If I talk about WT-17,000 (for West Turkana fossil number17,000) then I can only be talking about a certain fossil whereas if I say the Paranthropusaethipicus cranium it assumes that we all agree that species designation which is certainly notthe case.Secondly, the sites. There are two main areas where almost all the early hominid material isfound. The first site is the East African rift valley system (1,200 miles long) associated withmountain building, faulting and vulcanism over last few million years. Earth movement meansediments get exposed (Plio-Pleistocene 4 to 1 mya), and volcanic activity causes layers ofvolcanic ash (tuffs) which can be dated (potassium argon, or fission tracks) accurately. Hereis a brief description of some of the localities within the Rift Valley:

HadarVery many fossils and artefacts. Fossils from 3.9 to 3 mya. Most famous is Lucy (AfarLocality (AL) 288-1, Australopitheucs afarensis), a 40% complete skeleton (one of only 2hominid skeletons earlier than 100,000 ya). AL 333 is a group of over 13 individuals includ-ing 4 infants: a catastrophic assemblage perhaps.

OmoVery thick continuous sequence (0.5 mile thick). 2.9 to 1 mya. Very rich fauna, so useful forbiostratigraphic dating, but fossil hominids restricted to teeth and bone fragments.

East TurkanaPossibly the richest site. Approx. 1.8 mya, though there are some much older beds (3.3 mya).Complete skulls, jaws and postcrania.

West TurkanaWest side of Lake Turcana. 2 very famous finds: an almost complete, 1.6 mya Homo erectusadolescent (see later) and the so called black skull (Paranthropus aethiopicus), a 2.4 myarobust australopithecine skull that is still causing problems with classification!

Olduvai GorgeMini Grand Canyon. 2 mya to present, providing an excellent sequence of fossils and arte-facts, including the original robust australopithecine cranium (Paranthropus bosei).

Laetoli3.75 to 3.5 mya. Most famous for the Laetoli footprints: thousands of footprints of over 20different species including hominids. One set of hominid tracks are of 2-3 individuals in atrail more than 25 m long - bipedal walking such as this is considered to be characteristic ofhominids

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The other main area is South Africa. Unlike the East African deposits, the South African onesare all cave deposits - piles of mineralised sediment that has fallen into caves. They are notcurrently datable to any great accuracy and the ages are usually inferred by looking at thefaunal context of the fossil and comparing it to faunal remains from better dated sites. How-ever, they have produced many fossils including some good postcranial material (includingpelvises and foot bones: important for investigating bipedalism)

TaungLimestone mine which produced the Taung Child in the 1920s (Australopithecus afri-canus). Before this, it was always thought that the earliest humans would be discovered inEurope, or perhaps the Far East. Consequently, it took quite a while before this discovery wasaccepted for what it was.

Sterkfontein, Kromdrai & SwartkransAnother set of caves in SA. (Sterkfontein was another commercial lime works). Sts-5 is anadult Australopithecus africanus and Sk-48 is a Parathropus robustus cranium.

MakapansgatAnother cave

ArdipithecusArdipithecus ramadusThis is the oldest of the group and is considered to be close to the split point between chim-panzees and humans. We do not have a great deal of fossil material to work on but due to thepresence of certain ape-like features (thin enamel, narrow molars) some people consider it tobe an early chimp ancestor (i.e. on the chimp side of the split). However, it does appear to beupright due to the forward placement of the foramen magnum.

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Australopithecus

Figure 5. Gracile and Robust Australopithecines [Fleagle 1999]

Australopithecus is now reserved for the gracile form of the early Pliocene fossils. As youcan see from Figure 5 this means that the skulls are rather small although there is no realdifference in the brain size of robust and gracile forms. They lack the thick bony buttressesseen in the robusts and have very much smaller teeth (although still large by modern stan-dards). Interestingly gracile and robust forms are found contemporaneously. The differing

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morphology is thought to be diet related so this may have allowed sufficient niche separationto prevent exclusion.

Figure 6. Gracile and Robust Australopithecines [Wood 1992]

Figure 6 shows the features of the two best known gracile australopithecines and comparestheir features with those of chimpanzees and the early Miocene Proconsul africanus.

Australopithecus afarensis

Figure 7. Australopithecus afarensis [Wood 1992]

This is the East African gracile form. Figure 7 shows a list of the distinguishing features ofAustralopithecus afarensis. Interestingly this hominid has an extremely large degree of sexualdimorphism - either that or there is another contemporaneous species. The jury is out on this

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but sexual dimorphism seems the most likely. We have an extremely good postcranial skele-ton (Lucy) and recently a decent cranium has also been found. This animal is assumed to bethe source of the hominid bipedal trackways found at Laetoli.

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Figure 8. Laetoli Footprints (human and hippo) [Behrensmeyer 1992]

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There is some dispute about the nature of A. afarensis bipedality. The trackway shows a veryhumanlike gait with the big toes in line with the other digits. Analysis of A. afarensis footbones has suggested that the pollux may have been divergent and that the digits are long andcurved. This has led to speculation that A. afarensis, whilst able to walk upright, still spent agreat deal of time arboreally.

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Figure 9. Is Australopithecus bipedal? [Fleagle 1999]

Also the pelvis of A. afarensis is very human-like in shape and suggests an upright posture.The human pelvis is bucket shaped to stop the abdominal contents falling out through theperineum. The chimp pelvis does not offer the same degree of support - it is much moresimilar to the pelvises found in animals that maintain a mostly horizontal trunk.

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Figure 10. Pelvis comparison [Conroy 1990]

There is no suggestion that the bipedality of the other early Pliocene hominids was any moreadvanced. We simply do not have as good fossil evidence to allow speculation in the otherspecies.

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Australopithecus africanus

Figure 11. Australopithecus africanus [Wood 1992]

This is the South African gracile form. We have no complete skeleton to go on but there areisolated crania and postcrania which suggest bipedality. There seems to be less sexual dimor-phism and smaller canines which might suggest a less polygynous social system or smallergroups.

Australopithecus anamensisThis is a relative recent addition to the Australopithecus genus. It has been found in severalsites in East Africa and is between 3.9 and 4.2 mya. There is a very small amount of postcra-nial material but what there is has been described as showing evidence of bipedality andsexual dimorphism.

Australopithecus bahrelghazaliWhat can I say? A few bits of upper and lower jaw and teeth from Chad. They are dated at 3to 3.5 mya and show that there were australopithecines further West than just the East Afri-can rift valley.

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Paranthropus

Figure 12. Paranthropus morphology [Jurmain & Nelson 1994]

These are the robust australopithecines. Figure 12 shows some of the cranial features whichare associated with the large increase in size of the bony elements of the skull. This change isassumes to be related to a diet that relies heavily on tough plant material that needs a greatdeal of chewing before swallowing. Some forms were given the name nutcracker manbecause of the size of their molar teeth but it seems more likely that the size is associatedwith a high fibre diet rather than breaking nut shells. Large flat molars are usually associatedwith grinding and shearing. As with the gracile forms there are a number of species although

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all are more recent than the earliest graciles and this suggest that they are a now extinctoffshoot from the direct human lineage.

Figure 13. Paranthropus and Homo skull comparison [Wood 1992]

Paranthropus bosei

Figure 14. Paranthropus bosei [Wood 1992]

The slide shows the characteristic features of the best known East African form. This is theoriginal Zinjanthropus, nutcracker man).

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Paranthropus aethiopicusThis is the oldest robust australopithecine. It has been described as hyper robust, but also asintermediate between A. afarensis and the other robusts. It dates from 2.3 to 2.7 mya.

Paranthropus robustus

Figure 15. Paranthropus robustus [Wood 1992]

This is the South African robust form. Interestingly it seems to have lived sympatrically withthe first member of the genus Homo (H. habilis). Its remains have been associated with theearliest stone tools, but it is unclear whether these were made by P. robustus or H. habilis. H.habilis is the popular choice but it is actually very difficult to be sure.

BipedalismThe main evolutionary question of this period is the one of bipedalism. Australopithecines dohave quite big brains but not really any bigger than a modern chimpanzee and the real push toincrease brain size seems to have occurred later on. However, the evidence suggest thataustralopithecines were fully capable bipeds (even if some species still spent some time intrees). The question is therefore what caused this change to habitual bipedalism.

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Figure 16. Why Bipedalism? [Fleagle 1999]

This slide illustrates some of the theories that have been postulated. Some of these make agreat deal of sense. Others are more or less crackpot. Some seem better able to explain thecausative chain of events - especially since we think that bipedalism probably evolved beforeaustralopithecines moved out from a jungle environment to a savannah one. After the moveto savannah living several of these theories show clear advantages to being bipedal whichwould explain the success of a bipedal primate in that environment but not what would havecaused the adoption of bipedalism in the first place. I favour one of the freeing up the handsoptions. Chimpanzees sometimes walk bipedally when carrying, they are sometimes bipedalfor display and they are bipedal when foraging requires it. These seem like the more probablescenarios.

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Figure 17. Bipedal adaptions [Martin 1992]

This slide shows the skeletal adaptations associated with bipedalism. These are shown to agreater or lesser extent in Australopithecines although the fossil evidence is not good enoughto be entirely certain.

PhylogenySo how do these groups link up to modern humans? We will come back to this when we talkabout modern humans but as you might expect there are plenty of alternatives even when justconsidering the australopithecines.

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Figure 18. Alternative phylogenies [Fleagle 1999]

Here are a couple of very similar trees. These both assume A. afarensis as the ultimatecommon ancestor and differ as to whether A. africanus is merely an offshoot or a directancestor and the closeness of the early robust forms to Homo habilis. Since these trees are afew years old they do not include the newer species.

Figure 19. Really Good Summary tree [Fleagle 1999]

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This one however does and obviously favours the A. africanus sideshoot. This is a good treesince it attempts to go all the way through to modern humans so we will come back to itagain.

Stop PressI wrote this lecture in 2000 and of course Science does not stand still. There have beenseveral major fossil finds since then. These do not effect the basic conclusions but they havehad a great impact on the details especially in terms of timing the chimp/human commonancestor and the numbers of species involved.

Figure 20. Orrorin tugenensis fossils [Senut et al. 2001]

First of all at the Orrorin tugenensis was reported in 2001 (Figure 20). This fossil wasreliably dated and considerably older than other early hominid finds. In fact it probablypredated the previously considered chimp/human split time that had been slowly creepingdown towards 5mya. However the description of the fossils certainly suggests a fully bipedalhominid and that led to a lot of tree revision as in Figure 21.

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Figure 21. 2001 tree [Aiello & Collard 2001]

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Figure 22. Sahelanthropus tchadensis [Brunet et al. 2002]

The in 2002 another new hominid was found: Sahelanthropus tchadensis (Figure 22). Thisfossil is, if anything, slightly older that Orrorin and very reliably dated. Once again we needto rewrite our family tree and some authors have given up on trying to link the variousmembers altogether (Figure 23).

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Figure 23. 2002 tree [Wood 2002]

This fossil again has some highly derived features that suggest it is firmly on the humanbranch of the tree once again pushing back the earliest time of the chimp/human commonancestor. Perhaps its a good job that a very recent article has found that the DNA differencebetween humans and chimps is more like 5% rather than the 2% previously suggested(Britten 2002).

BibliographyAiello, L.C., Collard, M. Our newest oldest ancestor? Nature 410: 526-527.Behrensmeyer, A.K. 1992. Fossil deposits and their investigation. In: The Cambridge Ency-clopedia of Human Evolution, eds. Jones, S., Martin, R., Pilbeam, D. pp. 187-190. Cam-bridge University Press: Cambridge.Britten, R.J. 2002. Divergence between samples of chimpanzee and human DNA sequencesis 5%, counting indels. Proc. Natl. Acad. Sci. Online at http://www.pnas.org/Brunet, M., Guy, F., Pilbeam, D., Mackaye, H.T., Likius, A., Ahounta, D., Beauvilain, A.,Blondel, C., Bocherens, H., Boisserie, J.R., De Bonis, L., Coppens, Y., Dejax, J., Denys, C.,Duringer, P., Eisenmann, V.R., Fanone, G., Fronty, P., Geraads, D,. Lehmann, T., Lihoreau,F., Louchart, A., Mahamat, A., Merceron, G., Mouchelin, G., Otero, O., Campomanes, P.P.,De Leon, M.P., Rage, J.C., Sapanet, M., Schuster, M., Sudre, J., Tassy, P., Valentin, X.,Vignaud, P., Viriot, L., Zazzo, A., Zollikofer, C. A new hominid from the Upper Miocene ofChad, central Africa. Nature 418 (6894): 145-151.Conroy, G. C. 1990. Primate Evolution. London: Norton.Fleagle, J.G. 1999. Primate Adaptation and Evolution. London: Academic Press.

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Jurmain, R., Nelson, H. 1994. Introduction to Physical Anthropology. Minneapolis: WestPublishing Company. Wood, B.A. 1992. Evolution of Australopithecines. In: The CambridgeEncyclopedia of Human Evolution, eds. Jones, S., Martin, R., Pilbeam, D. pp. 231-240.Cambridge University Press: Cambridge.Martin, R. 1992. Walking on two legs. In: The Cambridge Encyclopedia of Human Evolution,eds. Jones, S., Martin, R., Pilbeam, D. pp. 76. Cambridge University Press: Cambridge.Senut, B., Pickford, M., Gommery, D., Mein, P., Cheboi, K., Coppens, Y. (2001). Firsthominid from the Miocene (Lukeino Formation, Kenya). Comptes Rendus de lAcademie desSciences Serie II Fascicule A-Sciences de la Terre et des Planetes 332 (2): 137-144Wood, B.A. 1992. Evolution of Australopithecines. In: The Cambridge Encyclopedia ofHuman Evolution, eds. Jones, S., Martin, R., Pilbeam, D. pp. 231-240. Cambridge UniversityPress: Cambridge.Wood B.A. 2002. Hominid revelations from Chad. Nature 418:133-135.