11/22/20 1711Probable Hominins (various Genera); Gracile Hominins (Australopithecus)Robust Hominins (Paranthropus)P. robustusP. boiseiP.aethiopicus2Australopithecus anamensis: “southern ape from the lake”• Kanapoi & Allia Bay, Kenya (East Africa)• 4.2-3.9 Ma211/22/20 1723Australopithecus anamensis• Morphology:– Reduced canines– Loss of CP3homing complex– thicker enamel than chimp, thinner than later Australopiths– strongly built tibia, both knee & ankle showing bipedality– BUT! long, curved fingers3Earliest biped with certainty4?Orrorin tugenensis?Chimpanzee Modern humanAustralopithecus anamensis411/22/20 1735Australopithecus afarensis: “southern ape from the Afar”• Afar depression, Ethiopia; also in Laetoli, Tanzania, and Turkana basin, Kenya (East Africa)• 3.9-2.9 Ma56The Hadar find AL 288-1, called “Lucy,” an Australopithecus afarensis.11/22/20 1747Cranial shape and brain size78Dental comparison of Chimp, A. afarensis, & Human• Intermediate canines.• U-shaped dental arcade more like chimp.• Thick enamel like human (even thicker).• Much larger molars than either.811/22/20 1759Comparison of Modern human and A. afarensisChimpA. afarensisNotice the chimp-like intermembralindex10Human valgus knee with bicondylar or carrying angle11/22/20 176Broad femoro-tibial (knee) jointChimpanzee “Lucy” Modern HumanChimpanzee“Lucy”Modern Human11Distal FemurProximal Tibia12Figure 05.21d11/22/20 17713Mary Leakey at Laetoli, Tanzania3.5 mya14Analysis of the Laetoli Footprints• Russel Tuttle’s comparison with Machigenga people of Peru: a bipedal striding gait.• Analysis of cadence suggests a shorter stride than modern.• Note slightly abducted big toe (relative to foot) on the left.11/22/20 17815Figure 05.23Humerus• Quadrupeds have more robust humeri than bipeds (weight bearing)• Bicipital grove– Deep = very muscular– Shallow = limited upper body strength16A. afarensis’ bicipital groove is deeper than that of modern humans, if not as deep as African great apes11/22/20 179Thorax Shape and Scapula Placement18Scapular Adaptations for Suspensory LocomotionHuman Chimpanzee11/22/20 171019A. afarensis fossil from Dikika, Ethiopia (3.3 ma, 3 year old)Alemseged et al. 200620Dikika-1-1H. sapiensG. gorillaChimpP. trog.11/22/20 171121Australopithecus afarensis• Morphology: general– 375-500 c.c.– large procumbent incisors– prognathic face– megadonty (very large molars and premolars, with thick enamel)• Morphology: arboreal– cranially-oriented scapular glenoid– curved phalanges– long toes– high intermembral index• Morphology: terrestrial– anteriorly placed, downward directed foramen magnum– distinct foot arch– distinct valgus knee angle2122Chimpanzee Modern humanAustralopithecus anamensis22?Orrorin tugenensis?Australopithecus afarensis11/22/20 1712Topics• Hypotheses for the origins of bipedality• Last of the Gracile Australopiths• Robust Australopiths (Genus Paranthropus)• Dietary reconstructions of the Australopiths• Australopiths– Gracile (genus Australopithecus)– Robust (genus Paranthropus)2324• Why bipedalism?• 1950s: Bipedalism evolved to allow hunting of grassland game.• Now: There is no evidence for hunting until much later in the Pliocene, while bipedalism began in the Late Miocene.11/22/20 171325Raymond Dart’s Osteodonto-keratic Industry26Hunter or Prey?C.K. Brain’s “The Hunters or the Hunted”A. africanus11/22/20 171427The Savanna Hypothesis: opening of the forest terrain into grasslands at the beginning of the Pliocene offered apes a new niche, which then produced selection for more bipedalism28The Late Miocene Problem with the SavannaHypothesis• Woodland fauna (like the kudu & red colobus) were associated with the earliest evidence for bipedal traits.11/22/20 1715A Modified Savanna HypothesisProblems• “Habitat” reconstructions for individual sites can run foul of the “Drunk under the Lamp Post” Problem.• Forest habitats were decreasing due to climate change from mid-Miocene.• Late Miocene apes were very diverse.• Selection for different ways to adapt to the new “open landscape” niche for being an ape.• This produced a mosaic pattern to the evolution of bipedal apes into the Pliocene, generalist adaptation• The Late Miocene record is so patchy that it is hard to identify which fossils are really on the hominin lineage until the Pliocene’s A. anamensis.2930Hypotheses currently seen as UNLIKELY to explain evolution of bipedalism11/22/20 171631• Bipedalism is good for vigilance against predators and threats.• IF SO, why are baboons not now bipeds?UNLIKELY 32Terrestrial feeding hypothesis• Gelada monkeys feed on grasses & herbs while sitting in an orthograde position.• IF SO, why are geladas not now bipeds?UNLIKELY11/22/20 171733Threat Display hypothesis• Being bipedal makes it easier to display. • Chimp males frequently display bipedally (frequently quadrupedally as well!)• This is a fairly temporary behavior to cause such large anatomical changes.UNLIKELY 34Carrying Hypothesis•Provisioning monogamy.•BUT sexual dimorphism is HIGH when bipedalism beginsUNLIKELY11/22/20 171835Hypotheses currently seen as LIKELY to explain evolution of bipedalism36A Likely Carrying HypothesisKnuckle-walking constrains chimp tool use.LIKELY11/22/20 171937Still Another Carrying HypothesisOpen grassland is rich in food. Remember central place foraging.LIKELY38The USO Hypothesis: Underground Storage Organs (tubers) as Fallback Foods • Thicker enamel and larger chewing surfaces of Australopithecus & Paranthropus dentition (compared to chimps & gorillas).• Dental adaptations for USOs increase during Pliocene.• USOs developed in abundance with the end of the Miocene, as seen by evolution of USO-eating mole-rats.• Correlation between mole-rats and hominin sites suggests USO-rich habitats occupied by hominins.11/22/20 172039The Thermoregulatory Hypothesis: Erect posture keeps you cool.LIKELYNo Single Cause is Probable; Multiple causal factors are more likely.All successful hypotheses relate to the increase in distance between trees (as climate got cooler and drier from the end of the Miocene through the Pliocene)4011/22/20 172141Chimpanzee Modern humanAustralopithecus anamensis41?Orrorin tugenensis?Australopithecus afarensisKenyanthropus platyops, 3.5 mya• Unknown phylogenetic relationships due to poor
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