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Chapter 6

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Francois Larose

CHAPTER 6 THE FIRST HOMINIDS AND THE EMERGENCE OF HOMO - defining feature of hominids: BIPEDALISM -genus:AUSTRALOPITHECUS - undisputed bipedal hominids lived in east africa 4 mya HOMINOIDS to HOMINIDS important physical changes did not occur until: 1. Bipedalism *further changes occured 2 million years after bipedalism: 2. expansion of brain 3. modification of female pelvis to allow for bigger brained babies 4. reduction of the face, teeth, and jaws 5. extended period of infant and child dependency 6. increased meat eating - we don't know if bipedalism developed quickly or gradually. - climate changes 11-16 mya turned the tropical landscape into a savanna (grasslands) and scattered deciduous woodlands became common. these conditions probably favoured bipedal creatures THEORIES FOR BIPEDALISM - what favoured bipedalism? one early idea: adaptive for life amid tall grasses of savannas so that you can spot predators and prey above grasses. baboons and some catarrhines live in savannahs though, and they are not fully bipedal. evidence emerging that that area may not have been predominantly savanna but rather a mix of woodland and open country. - other theories stress importance of freeing the hands to perform other activities. ● hewes says for carrying hunted/scavanged meat, or any food really to somewhere safe from predators. ● Lovejoy proposed that if males carried food back to females and offspring, females could focus more energy on child rearing/reproduction. ● Jolly argues that feeding their very self more efficiently with the ability to harvest small seeds and nuts, which may have been plentiful in the changing atmosphere. ● allow for making tools, allow for carrying tools place to place. tools may be used to kill and butcher animals (primates make for lousy predators with what little they already have) ● washburn says tools could have been used to dig for edible roots as some contemporary primates do. ● pilbeam suggests tools may have allowed a variety of vegitation to be chopped, crushed or prepared that would otherwise not have been edible or worth the trouble ● tools may have been used as weapons against predators ● wolpoff thinks it was the advantage of carrying weapons continuously that selected for bipedalism. savage-rumbaugh adds to that that the ability to snap back the rist (abduct) permitted early humans to perfect both throwing and rock-striking skills (for toolmaking) - some question idea of tool/toolmaking as cause of bipedalism selection because stone tools don't appear for another 2 mya after emergence of bipedalism. Wolpoff says that protohominids may have been using wood or bone tools and some stone tools may not be recognizable as tools. - researchers have found that bipedalism was MORE EFFICIENT for savannah locomotion than quadrapedal locomotion (especially over long distances). this seems most convincing to me. conserve energy and you don't need to eat as much to survive. - bipedalism also might have been favoured by NS as a way of regulating body temp. in the increasingly hot and dry enviroments of east africa at the end of the Miocene / beginning of the Pliocene. wheeler argues that bipedal posture limits the body area directly exposed to sun, especially at mid day when sun is hottest. also, evaporation of sweat would be facilitated by bipedal posture, heat not trapped underneath body, more skin area exposed to cooling winds, etc. ALLTHESE THEORIES ARE SPECULATIVE THE "COSTS" OF BIPEDALISM - makes it harder to overcome gravity to supply brain with sufficient blood - weight of body above the pelvis and lower limbs puts stress on hips and lower back and knees and feet. these are even greater for pregnant or infant carrying females. - primary skeletal changes to make bipedalism possible are in the ● skull ● pelvis ● knees ● feet - in skull, spinal column enters skull toward the back for modern and ancient apes since they normally walk on all fours and need their heads facing forward. in bipedal humans the spinal column enters through a hole called the FORAMEN MAGNUM, located underneath the skull closer to the jaw so that the eyes are perpendicular the spine. - whereas ape pelvises are long and flat, forming a bony plate in the lower back to which the leg muscles attach, bipedal hominids have a bowl shaped pelvis which supports the internal organs and lowers the body's center of gravity for better balance. it also has a different set of muscle attachments and shifts the orientation of the femurs from the side of pelvis to the front allowing for legs to swing forward. (apes move their legs forward not by kicking like we do, but by shifting their pelvis side to side) - when apes hang their legs fall in a straight down from their pelvis but ours hang inward in a knock-kneed posture that helps us maintain our center of gravity. - hominid feet do not have opposable thumbsAND the ankle bones are substantially enlarged to withstand the force placed on them. hominid feet also have an arch that absobs forces endured by feet while walking (flat-footed people have chronic back, knee, feet and ankle problems) THE TRANSITION FROM HOMINOIDS TO HOMINIDS - SAHELANTHROPUS TCHADENSIS lived in the Sahel desert of Chad (which used to be an ancient lake). had hominoid skull formation but hominid teeth. we await evidence as to whether or not it was bipedal. - ORRORIN TUGENENSIS may have been bipedal. Discovered in western Kenya. top of femur shows adaptations to bipedalism. orrorin tugenensis dates between 5.8 and 6 million years, so it may be the earliest hominid. - ARDIPITHECUS RAMIDUS was discovered in the middle awash region of Ethiopia, 5.8 million years old. unique in that ardipithecus has apelike dentition along with evidence of bipedalism and an overall hominidlike skeleton. apelike small cheek teeth with thin enamel and large canines, however arm bones seem homindlike and base of skull shows foramen magnum positioned underneeath the skull! may be the earliest hominid found yet. AUSTRALOPITHECUS: THE FIRST DEFINITE HOMINID - some doubt still remains aboutArdipithecus but no doubt about Australopithecines, dating to 4 mya, these are east african hominids. hominid dentition with parabolic dental arch. unambiguous evidence of full bipedalism. they still seem fully capable, including later ones, of climbing and moving in trees. scholars divide the australopithecines into two groups: GRACILE AUSTRALOPITHECINES - the gracile include A. anamensis, A afarensis, & A. africanus. all have smaller dentiiton and lighter facial and dental musculature than the ROBUSTA.s 1. AUSTRALOPITHECUS ANAMENSIS ● earliest, 3.9 - 4.2 mya. ● it is said thatA. Anamensis is "A. afarensis-like from the neck up and homo-like from the neck down" teeth like afarensis, long bones suggesting well developed bipedalism, homo like knees and elbows. 2. AUSTRALOPITHECUS AFARENSIS ● dates from 3-4 mya. ● at least two dozen dug up at Laetoli, Tanzania by famous trail of footprints, two individuals side by side, guaranteeing us they were bipedal. 35 more found at another site, Hadar, Ethiopia. ● Females perhaps 65 pounds and 3 feet tall, males weighed more than 90 pounds, standing about five feet. ● teeth large compared to body. thick molar enamel. large ape-like canines but these did not project into diastema or prevent horizontal chewing. ● face juts out with large teeth and jaw ● base of skull flares out to provide attachment areas for large neck muscles necessary for holding up the heavy head. ● brain 400cc, small for human, big for creature of this size. ● arms and legs same length. finger and toe bones curved suggesting great strength. ● pelvis wide and flaring but does have bowl shape of later hominids. bipedal. ● legs angle inwards and feet have arch and strong ankles like later humans. ● analysis of footprints shows a. afarensis may have had shorter and less efficient stride than modern humans 3. AUSTRALOPITHECUS AFRICANUS ● 3 - 2 mya ● africanus what the firstAustralopithecine discovery. 1925 by Raymond Dart. Found in SOUTH AFRICA. he discovered the skull of an infant who came to be known as the TAUNG CHILD. ● first evidence that an erect bipedal hominid existed in the Pliocene epoch. ● lived in semiarid climate not too different from today. ● rounded braincase, fairly well developed forehead, moderate brow ridges, and projecting face. ● large chinless jaw resembles A.Afarensis but some dental features are similar to modern man: broad incisors, small short canines. premolars and molars larger though, but similarly formed. ● brain estimated at 428-485 cc ● adults 3.5 - 4.5 feet tall, weighing 60-90 pounds, sexually dimorphic. 4. AUSTRALOPITHECUS BAHRELGHAZALI ● only a single fragmentary jaw represents BAHRELGHAZALI! ● found further west in present day central Chad, far from the EastAfrican Rift Valley. Most scholars interpret theAustralopithecus adaptations as directly influenced by this environment, not that of central Chad. Bahrenghazali calls this assumption into question. ● seems very similar to A. Agarensis but differs in some distinct ways (its premolars have thinner enamel and more well defined roots) 5. AUSTRALOPITHECUS GARHI ● a handful of skull and limb fragments represent represent Garhi! ● 2.5 mya ● larger molars than Afarensis but doesn't have the huge face and jaws of the robust australopithecines. ● also lacks enlarged brain of early Homo. ● Garhi basically looks likeAfarensis with enlarged molars. GARHI may just represent one end of the range of variation inAfarensis or it may represnet an adaptation to eating tough foods, simlar to that made by the robust australopithecines (described below) ● remains of butchered animals found in same rock layer as a. garhi remains. ROBUSTAUSTRALOPITHECINES - Lived in East and South Africa from 2.7-1 mya - some paleoanthropologists thing these fossils are so different that they should be placed in a different genus (which they call PARANTHROPUS). There are three main robust australopithecines: 1. AUSTRALOPITHECUSAETHIOPICUS 2. AUSTRALOPITHECUS ROBUSTUS 3. AUSTRALOPITHECUS BOISEI - in contrast to the GRACILEA's, the ROBUSTA's have thicket jaws, larger molars and premolars, smaller incisors, more massive muscle attachments for chewing, and well developed sagittal crests and ridges to support heavy chewing. larger cranial capacities (490-530 cc) in robustus and boisei. - brain larger than A. africanus but similar body forms. Robustness refers to the jaws and teeth and skull. 1. AUSTRALOPITHECUS AETHIOPICUS ● EARLIESTAND LEAST KNOWN OF THE ROBUSTA'S. ● found in northern kenya and southern ethioia ● 2.7-2.3 mya ● contemporary ofA.Afarensus and lived in same area, had similar bodies but had much larger dentition and jaw and saggital crests along with aprojecting and dis-shaped (round and flat) face. Basically: resemblesAfarensus but with scaled up dental apparatus. 2. AUSTRALOPITHECUS ROBUSTUS ● many scholars believe robustus to be different enough to warrant classification in a separate PARANTHROPUS genus. ● how did two strikingly different hominid species evolve in the same environment? (robustus contemporary of Africanus). ● conjecture is that Robustus was vegetarian andAfricanus what not but this is being called into question. ● Most paleoanthropologists think a. robustus died out shortly after 1 million year
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