Bipedalism Flashcards
Types of relative dating of fossils
- Stratigraphy - layers of sediment
- Biostratigraphy - changes in animal communities e.g. presence/absence of species, changes in morphology of teeth in rodents
- Palaeomagnetism - evidence of the Earth’s magnetic field in rocks
Types of absolute dating of fossils
- Dating the sediment surrounding fossils e.g. K-Ar, 40Ar-10-Ar, uranium series, fission track, thermoluminescence, electron spin resonance
- Dating fossil bone e.g. 14C, uranium series, amino acid racemisation, electron spin resonance
Types of bipedalism
- Optional - extant apes
- Habitual - Ardipithecus
- Obligate non-modern - Australopithecus
- Obligate modern - Homo
Anatomical modifications of an obligate biped
- Body plan and body proportions
- Position of the foramen magnum
- Basicranial flexion
- Vertebral column shape and flexibility
- Pelvis shape
- Femoral neck morphology
- Knee/distal femur shape
- Foot morphology
Basicranial flexion
Brings the foramen magnum forward
Correlated with a reduction in facial size and increase in brain size
Striding gait
Type of locomotion that allows to walk with long strides
High mechanical efficiency
Far more efficient than bent-hip bent-knee bipedalism seen in chimps
Chimpanzee brain size
~350cc
Human brain size
~1300cc
Candidates for ape-human LCA
Morotopithecus and Proconsul, 20mya, east and south Africa.
Quadrupedal, lacked derived features for suspensory locomotion
Decline of Miocene apes
Climate changes in the late Miocene and slow generation time/developmental period to deal with climate changes
Same climatic conditions favoured the evolution of the earliest ancestors of the human lineage
Sahelanthropus tchadensis
6-7mya, Chad (central Africa)
Primitve features: ape-shaped brain, U-shaped palate
Derived traits: reduced prognathism, more anterior foramen magnum, small canines, no diastema, intermediate enamel thickness
Basicranium indicades that S. tchadensis may have been a biped - this is controversial
Orrorin tugenensis
6.2-5.65mya, Tugen Hills, Kenya
Woodland/lake environment.
Primitive traits: ape-like teeth in size and morphology, curved proximal phalanges
Derived traits: thicker enamel than chimps, derived aspects of the femur (elongated femoral neck, linea aspera, increased bone density in femoral neck)
Derived traits suggest bipedalism, but ipper limb morphology suggest arboreality
Ardipithecus kadabba
5.2-5.8mya, Ethiopia
Intermediate canine size between apes and later hominins
Curved foot phalanx - weak evidence for bipedalism, based on single bone
Ardipithecus ramidus
4.4mya, Ethiopia
Combination of primitive and derived traits:
Relatively small brain
Prognathism as in apes
More anterior foramen magnum - consisten with bipedalism
Reduced canine size
Minimal sexual dimorphism
Short and broad ilium
Divergent hallux
Lacked other features for suspension, vertical climbing, and knuckle walking seen in apes
ARA-VP-6/500
“Ardi”
Ardipithecus ramidus
Exceptionally complete skeleton for a hominin of this age
Probably female based on canine size
Phylogenetic placement of Ardipithecus ramidus
Hypothesis 1: all known evidence represents a species lineage evolving phyletically across its entire range.
Hypothesis 2: an Ardipithecus-to-Australopithecus transition occurring between 4.5mya and 4.2mya in a local group.
Hypothesis 3: an earlier allopatric speciation event with Ar. ramidus representing the end point of the lineage.
Models for the origins of bipedalism
- Savannah hypothesis
- Postural feeding model
- Travel efficiency
- Thermoregulation
- Carrying hypothesis/male provisioning model
- Complex topography
Savannah hypothesis
Raymond Dart
Environmentally driven
Shift to grasslands as forests receded - lack of trees forced hominins to adapt to terrestriality, freed hands
Make and use tools to scavenge - eventually leads to hunting
HOWEVER hominins known to be bipedal evolved in non-savannah conditions
Postural feeding model
Based on the feeding ecology of chimpanzees and other primates
Hominins spending much time feeding while standing
Preadaption to obligate bipedalism seen in later hominins
Travel efficiency model
Climate driven model
Patchy food resources with forest fragmentation
Need to travel long distances
Walking bipedally consumes less energy
Thermoregulatory model
Reduced heat gain and better heat dissipation
BUT earliest bipeds evolved in closed wooded environments
Carrying hypothesis/male provisioning model
Various iterations since the 1960s - food, infants
Owen Lovejoy - linked to monogamy, allows for a reduction in interbirth interval and increases infant survivorship - linked to infant dependence on mother
Evidence against male provisioning:
body size dimorphism is high in most Au. species; more likely that early hominins lived in polygynous groups
Complex typography model
Rough terrain provided an ecological niche that afforded protection from predators and access to food resources
Explains retention of climbing anatomy in Pliocene hominins
Difficult to test
Australopithecus anamensis
4.1-3.9mya, Kenya and Ethiopia
Australopithecus afarensis
3.9-3.0mya, Ethiopia and Tanzania
Australopithecus africanus
3.0-2.3mya, South Africa
Paranthropus aethiopicus
2.6-2.4mya, Ethiopia and Kenya
Paranthropus robustus
1.9.-1.0mya, South Africa
Paranthropus boisei
2.3-1.4mya, East Africa
AL 288-1
“Lucy”
Australopithecus afarensis
STS 14
Australopithecus africanus
Axial skeleton
Head and trunk
Appendicular skeleton
Limbs
MH1 and MH2
Two partial skeletons of Australopithecus sediba
