Cheatography
https://cheatography.com
Still cramming for my achaeology exam wish me luck. Here's some more biological anthropology, this time human evolution.
This is a draft cheat sheet. It is a work in progress and is not finished yet.
The beginnings
Miocene (~ 23-5 mya) environmental changes leading to much less rainfall and much more seasonal in African tropics - end of Green Sahara. |
Distinction between early hominins and modern humans through bipedal locomotion, brain size, slowed development, dental morphology and cultural adaptations. |
Sahelanthropus tchadensis
Earliest known hominin. |
6.8-7.2 mya |
Fossil record consists of partial cranium, partial mandibles, some teeth, part of a femur and lower arm bones. |
Mixture of ancestral + derived features -> transitional biped? |
Foramen magnum in skull - aligned under skull, would allow bipedalism. Morphology of the femur = also under discussion regarding terrestrial bipedalism.
Lower arms morphology = spent time in trees. |
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Brain size = much smaller than modern human, closer to ape size. Face also = very flat with large brow ridge. |
Ardipithecus
Ar. kadabba and Ar. ramidus from Ethiopia |
5.8-5.2 mya |
Mixture of primitive and derived dental traits e.g. canine sharpens itself on the first premolar like in chimps. |
Ardi |
Nearly complete skeleton of a female Ar. ramidus individual. Limb proportions were similar to Miocene quadrupedal monkeys.
Plant and animal fossils around Ardi paint picture of habitat: woodland areas with dense patches of trees and open grasslands.
Skull: ape-sized brain, flat head but prognathic. Foramen magnum suggests head = upright on spine. |
Bipedalism |
Postcranial skeletal evidence for locomotion suggests bipedalism in Ardipithecus e.g. pelvis, feet and hands.
- Feet show mixture of modern ape and human morphology: opposable toe like apes, but other 4 toes adapted to bipedalism.
- Pelvis shows shorter + broader ilium than in chimps. Lower part of pelvis = more ape-like.
So in conclusion, most likely bipedal but perhaps also transitional? Gait would have been different to that of modern humans. |
Ardi
Partial skeleton of Ardi (Ar. ramidus)
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Bipedalism
Key hominin feature. Deduced from associated morphological traits. |
Pelvis |
Chimp pelvis (quadrupedal) = much taller and slimmer. |
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Modern human and australopithecine pelvis = wider and shorter. |
Torque and abductors |
Stepping involves rotation of torso, facilitated by torque (twisting force). Torso is kept from tipping by abductor muscles attached to the ilium. |
Cortical bone in the femur |
Add leverage for the abductors. Femur neck also lengthens. |
Knee joint |
Bipedalism requires knees to be close to centre of the body (i.e. curve inwards), so femur slants downwards and inwards. |
Why was bipedalism selected for? |
Savannah mosaic hypothesis |
Arboreal hominins faced with environmental changes and retreat of forests. More grasslands with some trees and shrubs. Bipedalism as a means of escaping fast-running predators. |
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(Note: Bipedalism would have evolved slowly and transitionally. This would therefore have been a bad time to evolve a new type of locomotion which would have at first been inefficient as well as being less energetically efficient for running than quadrupedalism. Bipedalism is also rare in nature, so if this had been an adaptation for fast running, we would expect for it to be more common.) |
Adaptation to arboreal life |
Bipedalism could have alternatively evolved in an arboreal setting before being used terrestrially. Morphology of teeth in early hominins show arboreal diet to support this. This also expands the savannah mosaic hypothesis as it suggests the possibility for a semi-arboreal lifestyle - this matches evidence of transitional bipedalism. |
Feeding adaptation |
Developed in an arboreal setting to facilitate feeding? Not mutually exclusive with above theory but is contested. Bipedalism would have allowed for effective harvest of fruit in small fruit trees. |
Keeping cool
(Wheeler) |
Heat stress becomes more important as tree cover retreats and environment becomes more exposed. Standing upright reduces this stress.
Doesn't fit evidence regarding bipedalism evolving in semi-arboreal hominins, but could have been a factor in mosaic environments? |
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