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Chapter 11

Chapter 11 How Humans Evolved - Bio 1M03.docx

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McMaster University
James S Quinn

Bio 1M03 How Humans Evolved Part 3: The History Of The Human Lineage Chapter 11: From Hominoid to Hominin Late Miocene/Pliocene Primates  Global cooling was associated with decreased rainfall and seasonally, including dry seasons  Tropical rainforests shrank in size; dry woodland and grassland habitat expanded  Ancestors of humans – the hominins – moved into the grassland habitat  Hominins – classified with humans in the tribe “Hominini”; taxonomic unit between the family and the genus  Humans beong to the family Hominidae – includes all of the great apes and the genus Homo  5 Human Characteristics: 1. Bipedalism 2. Differences in dental anatomy from other apes 3. Very large brains relative to body size 4. Long period of juvenile development 5. Language  Spoken  Dependence on a material and symbolic culture  Ancestral hominins shared some of the features of humans and some features of contemporary chimpanzees At The Beginning: Chimp-Human Split (5-7 mya)  Genetic data suggest that the most recent common ancestor (MRCA) of chimps and humans lived about 5-7 mya  3 fossils have illuminated what this ancestor was like; Ardipithecus ramidus, Orrorin tugenensis, and Sahelanthropus tchadensis o Ardipithecus Ramidus  Ardipithecus genus has two species: A. ramidus and A. kadabba Bio 1M03  Foramen Magnum – opening on the bottom of the skull through which the spinal cord passes  Associated with bipedal locomotion  Smaller, more incisor-like canine teeth that are not sharpened by the lower premolar, unlike apes (relatively large canine teeth that are sharpened by the lower first premolar)  Ape Like Traits  Molars are smaller in relation to body size than in other early hominins  Thinner enamel  Canines are smaller than in chimpanzees and gorillas, but larger than in later hominins  Deciduous (baby) molars and jaw joint are very similar to those of apes  Base of skull is more pneumatized (filled with air pockets) than in later hominins  Numerous fossils of wood and seeds found at in forested environments and come from plant species that grow in woodland habitats  Earliest stage of hominin evolution  Related more closely to chimpanzees than to modern humans o Orrorin Tugenensis  Genus name means “original man”  Similar to both chimps and humans  Chimp Traits  Incisors, canines and one of the premolars  Arm and finger bones have features that are believed to be adaptations for climbing  Human-Like Traits  Thick enamel  Thigh bones  Evidence of bipedalism o Sahelanthropus Tchadensis  “Sahel” – the vast dry region south of the Sahara  “Tchadensis” – Chad, the country where the fossil was found  Nicknamed Toumai – hope of life  Comes from an unexpected place  From the middle of the African continent – most work on human evolution is focuses on East Africa and South Africa  Fossil found is surprisingly old  Between 7 – 6 mya  Oldest hominin cranium  Possesses a surprising mix of anatomical features  Face relatively flat  Massive brow ridge over the eyes  Foramen magnum located under the skull – bipedal locomotion  Teeth are primitive  Back of skull is ape-like  Ancestral features of these fossils included: o Small molars o Thin enamel Bio 1M03 o Canines still larger than humans o Large brow ridge o Small braincase relative to modern humans  Derives features of these fossils included: o Forward location of the foramen magnum; this is a signature of bipedalism o Relatively small canine teeth that are not sharpened by first lower premolar o Changes in the femur and pelvis, knee and ankle o Flattening of the face The Hominin (2-4 mya) Community Diversity  Hominins lineage is the lineage leading to humans after divergence from (chimps + bonobos)  Hominin lineage diversified ~4 mya and there were 4-7 lineages in Africa for the next 2 million years  Divide the species into three genera 1. Australopithecus o Small bipeds with small teeth 2. Paranthropus o Small bipeds with big teeth, probably ate plants 3. Kenyanthropus o Small teeth and a flat face Australopithecus  A. Anamensis o Large molars with thick, enamel and smaller canines o Shape of knee and ankle joints strongly indicate bipedalism o Arm bones suggest that it retained adaptations for life in trees o Small ear holes and shaped like ellipses (as they are in living apes)  Later australopithecines have larger and rounded ear holes o Dental arcade is a U shape (seen in chimps and gorillas)  Later australopithecines have V-shaped dental arcade o Chin recedes more sharply than that of other australopithecines do o Larger canines than those in later hominins  A. Afarensis o Found at sites in East Africa that date from 4-3 mya o Hadar, Afar Depression in Ethiopia, 1970’s, Don Johanson  Lucy  3 million year old knee joint that showed striking resemblance to that of a modern human  sizeable fraction of the skeleton of a single individual  Afar Locality 333/”The First Family”  Remains of 13 other individuals o Woodlands to Dry Savanna o Cranium quite ape-like  Endocranial volume – less than a pint, 404 cc  Same size brain as modern chimpanzee  Base of cranium is flared at the bottom and the bone is pneumatized  Front of the face below the nose is pushed out – subnasal prognathism  Jaw joint is shallow Bio 1M03 o Teeth and jaw are intermediate between apes and humans  Dental arcade has an intermediate V shape, the ccanines are medium sized, and the diastema (the space between the teeth and the jaw) is modest  Displays less sexual dimorphism than chimps but more than humans in its canines  The first premolar has a small inner cusp and a larger outer cusp  Chimpanzees have a single cusp on their lower premolars  Humans have two cusps of equal size o The Pelvis  A. Afarensis pelvis is more like a human pelvis – infer that it was bipedal  When walking, a large amount of time is spent on one foot  Each step, the body swings over the foot that is on the ground and weight is balanced over that foot (on the ground)  The weight of the body pulls down on the center of the pelvis, inward from the hip joint – creates torque (twisting force) that acts to rotate the torso down and away from the weighted leg  Torso does not tip because the torque is opposed by the abductors (muscles that run from the outer side of the pelvis to the femur – muscles tighten to keep you upright  The abductors are attached to the ilium (a flaring blade of bone on the upper end of the pelvis) – the widening and thickening of the ilium and the lengthening of the neck of the femur (thigh bone) add to the leverage that the abductors can exert and make bipedal walking for efficient Bio 1M03 o Knee Joint  Bipedal locomotion requires that the knees lie close to the center of the body line  Human femur slants down and inward, and its lower end is angled at the knee joint to make proper contact with the bones of the lower leg  Chimpanzee femur descends vertically from the pelvis and the end of the femur at the knee joint is not slanted  A. afarensis has the angled shape (like human) and feet show a number of derived features associated with bipedal locomotion (longitudinal arch and humanlike ankle) o Believed that A. afarensis walked with an inefficient, bent-legged gait  Ilium is oriented more toward the back  Abductors would be less efficient  Legs were shorter in relationship to body size  Reduce the efficiency and speed of walking  Pelvis is wider in relationship to body  Minimized vertical motion of the body during walking  Equivalent to the effect of humans long legs o Laetoli Footprints  3.5 million years old  Illustrates that Australopithecus afaransis (or some other hominin that lived there at this time) was bipedal  Found by Mary Leakey and Coworkers at Laetoli Trail in Tanzania  Trail 30 m long  3 bipedal individuals who had crossed a thick bed of wet volcanic ash – wet ash solidified as it dried o Probably spent time in trees  Nonhuman primates (except some gorillas) spent night perched in trees to protect themselves from predators  Traits of the feet, hands, wrists and shoulder joints suggest that A. afarensis are partly arboreal  Bones of fingers and toes are slender and curved like those of modern apes – good for grasping branches  Shoulder blades are well suited for supporting the body in a hanging position o Sexually Dimorphic in Body Size  Largest individuals – 1.51 m tall, weighed 45 kg  Smallest individuals – 1.05 m tall, weighed 30 kg  1.5x difference  Two explanations  Sexual Dimorphism – largest males, smallest female  Two different species o Larger individuals were found in Laetoli – then a savanna o Smaller individuals were found in forested environments Bio 1M03  Australopithecus africanus o Relative to A. afarensis, A. africanus was characterized by shared derived humanlike traits  Fewer air pockets in the skull base  Shorter and less-protruding face  Less sexually dimorphic canines  Base of the cranium is bent further upward or flexed o But this species also had some derived non-human like traits  Large molars and jaws o A. africanus developed rapidly, like chimpanzees, rather than slowly, like humans  Infants were not as dependent as human infants are o Date from 3 -2.2 mya o Found in several sites
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