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Chapter 11

Chapter 11- From Hominoid to Hominin.docx

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Jon Stone

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Chapter 11: From Hominoid to Hominin
-during Miocene, Earth’s temperature began to fall because:
ototal amount of rain declined each year
orainfall became more seasonal, so there were several months each year
when no rain fell
-some animals failed to adapt and became extinct
-spread of woodland and savanna led to evolution of first hominins about 6 mya
-hominins were different from any of the Miocene apes in two ways:
1. they walk upright
bipedal locomotion led to major morphological changes in bodies
2. in new savanna and woodland habitats, new kinds of food became
caused changes in teeth, jaws, and skulls
-5 categories distinguish modern humans from contemporary apes:
1. We habitually walk bipedally.
2. Our dentition and jaw musculature are different from those of apes
in a number of ways.
3. We have much larger brains in relation to our body size.
4. We develop slowly, with long juvenile period.
5. We depend on an elaborate, highly variable material and symbolic
culture, transmitted in part through spoken language.
-hominins are not included in the same genus (Homo) as modern humans
At the Beginning
-last common ancestor of humans and chimpanzees lived between 7 and 5 mya
-Ardipithecus ramidus, Orrorin tugenensis, and Sahelanthropus tchadensis have
begun to shed light in history of human lineage
-genus Ardipithecus include two species (A. ramidus and A. kadabba) and both
have similarities to both humans and chimpanzees
-several features suggest that A. ramidus was a hominin
oopening on bottom of skull through which spinal cord passes (foramen
magnum) is located forward under the skull, as it is in humans
oforward placement of foramen magnum is associated with bipedal
owere smaller, more incisorlike canine teeth that are not sharpened by the
lower premolar, unlike apes, which have relatively large canines
oenamel is thinner than in other early hominins, and the canines are
smaller than in chimpanzees and gorillas but larger than in later hominins
oseemed to have lived in a forested environment
based on fossils of wood and seeds found at site
-unclear whether Ardipithecus is related more closely to humans or to

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Orrorin tugenensis
-second early fossil with similarities to humans
-incisors, canines, and one of the premolars are more like with the teeth of
chimpanzees than of later hominins
-arm and finger bones have features that are believed to be adaptations for
-thigh bones are more similar to those of later hominins than to those of apes
-habitat was a mix of woodland and savanna
Sahelanthropus tchadensis
-fossil consists of a nearly complete cranium, two fragments of lower jaw, and
several teeth
-fossil shook paleontological community because:
1. it came from an unexpected place
most work on human evolution has focused on East Africa and
South Africa, but this specimen comes from the middle of Africa
suggests that hominins had a larger range than believed
2. this fossil is surprisingly old
geology of site does not allow radiometric or paleomagnetic dating
oldest hominin cranium
3. possesses a very surprising mix of anatomical features
face is relatively flat and massive browridge over eyes
The Hominin Community Diversifies
-a number of hominin species lived in Africa between 4 to 2 mya
othey are divided into three genera: Australopithecus, Paranthropus, and
1. Australopithecus
oincludes 6 species: A. anamensis, A. afarensis, A. africanus, A. garhi, A.
habilis, and A. rudolfensis
othe australopithecines had small bipeds with teeth, skull, and jaws
adapted to a generalized diet
2. Paranthropus
oincludes 3 species: P. aethiopicus, P. robustus, and P. boisei
osimilar to the australopithecines from neck down, but had massive teeth
and jaws adapted to heavy chewing of tough plant materials, and a skull
modified to carry enormous muscles necessary to power chewing
3. Kenyanthropus
oincludes 1 species: K. platyops
odistinguished by a flattened face and small teeth
-A. anamensis
obipedal, but had a more apelike skull than later australopithecines had
ofound near Kenya
ofound parts of upper and lower jaw (part of tibia- larger of two bones in
lower leg) and numerous teeth

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olarge molars with thick enamel and smaller canines, and the shapes of
knee and ankle joints strongly indicate it was bipedal
oarm bones suggest that it retained adaptations for life in trees
oear holes are small and shaped like ellipses, as they are in living apes, but
ear holes of later australopithecines are larger and more rounded
odental arcade is more like U shape (seen in chimpanzees and gorillas),
unlike the V-shaped dental arcade seen in later australopithecines
olived in a mixture of habitats including dry woodlands and gallery forest
along rivers
-A. afarensis
omost extensive fossil collections come from several sites in Ethiopia
ofound bones of a 3 million year old knee joint that showed striking
similarities to a modern human knee
oalso discovered fossils in Tanzania
ohabitats ranging from woodland to dry savanna
ocranium (skull minus lower jawbone) is quite apelike
oits endocranial volume (capacity of brain cavity) is small
about size of modern chimpanzee
ofront of case below the nose is pushed out- a condition known as
subnasal prognathism
ojaw point is shallow
oteeth and jaws are intermediate between those of apes and humans
1. dental arcade of A. afarensishas an intermediate V shape,
the canines are medium-sized, and the diastema (space between
teeth and jaw) is modest
2. in its canines, A. afarensis displays less sexual dimorphism
than chimpanzees but more than modern humans
3. first lower premolar has a small inner cusp and a larger outer
chimpanzees have a single cusp, whereas humans have two
cusps of approximately equal size
owas bipedal
Cranium of A. afarensis
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